Target Independent Opportunities: //===---------------------------------------------------------------------===// Dead argument elimination should be enhanced to handle cases when an argument is dead to an externally visible function. Though the argument can't be removed from the externally visible function, the caller doesn't need to pass it in. For example in this testcase: void foo(int X) __attribute__((noinline)); void foo(int X) { sideeffect(); } void bar(int A) { foo(A+1); } We compile bar to: define void @bar(i32 %A) nounwind ssp { %0 = add nsw i32 %A, 1 ; [#uses=1] tail call void @foo(i32 %0) nounwind noinline ssp ret void } The add is dead, we could pass in 'i32 undef' instead. This occurs for C++ templates etc, which usually have linkonce_odr/weak_odr linkage, not internal linkage. //===---------------------------------------------------------------------===// With the recent changes to make the implicit def/use set explicit in machineinstrs, we should change the target descriptions for 'call' instructions so that the .td files don't list all the call-clobbered registers as implicit defs. Instead, these should be added by the code generator (e.g. on the dag). This has a number of uses: 1. PPC32/64 and X86 32/64 can avoid having multiple copies of call instructions for their different impdef sets. 2. Targets with multiple calling convs (e.g. x86) which have different clobber sets don't need copies of call instructions. 3. 'Interprocedural register allocation' can be done to reduce the clobber sets of calls. //===---------------------------------------------------------------------===// Make the PPC branch selector target independant //===---------------------------------------------------------------------===// Get the C front-end to expand hypot(x,y) -> llvm.sqrt(x*x+y*y) when errno and precision don't matter (ffastmath). Misc/mandel will like this. :) This isn't safe in general, even on darwin. See the libm implementation of hypot for examples (which special case when x/y are exactly zero to get signed zeros etc right). //===---------------------------------------------------------------------===// Solve this DAG isel folding deficiency: int X, Y; void fn1(void) { X = X | (Y << 3); } compiles to fn1: movl Y, %eax shll $3, %eax orl X, %eax movl %eax, X ret The problem is the store's chain operand is not the load X but rather a TokenFactor of the load X and load Y, which prevents the folding. There are two ways to fix this: 1. The dag combiner can start using alias analysis to realize that y/x don't alias, making the store to X not dependent on the load from Y. 2. The generated isel could be made smarter in the case it can't disambiguate the pointers. Number 1 is the preferred solution. This has been "fixed" by a TableGen hack. But that is a short term workaround which will be removed once the proper fix is made. //===---------------------------------------------------------------------===// On targets with expensive 64-bit multiply, we could LSR this: for (i = ...; ++i) { x = 1ULL << i; into: long long tmp = 1; for (i = ...; ++i, tmp+=tmp) x = tmp; This would be a win on ppc32, but not x86 or ppc64. //===---------------------------------------------------------------------===// Shrink: (setlt (loadi32 P), 0) -> (setlt (loadi8 Phi), 0) //===---------------------------------------------------------------------===// Reassociate should turn things like: int factorial(int X) { return X*X*X*X*X*X*X*X; } into llvm.powi calls, allowing the code generator to produce balanced multiplication trees. First, the intrinsic needs to be extended to support integers, and second the code generator needs to be enhanced to lower these to multiplication trees. //===---------------------------------------------------------------------===// Interesting? testcase for add/shift/mul reassoc: int bar(int x, int y) { return x*x*x+y+x*x*x*x*x*y*y*y*y; } int foo(int z, int n) { return bar(z, n) + bar(2*z, 2*n); } This is blocked on not handling X*X*X -> powi(X, 3) (see note above). The issue is that we end up getting t = 2*X s = t*t and don't turn this into 4*X*X, which is the same number of multiplies and is canonical, because the 2*X has multiple uses. Here's a simple example: define i32 @test15(i32 %X1) { %B = mul i32 %X1, 47 ; X1*47 %C = mul i32 %B, %B ret i32 %C } //===---------------------------------------------------------------------===// Reassociate should handle the example in GCC PR16157: extern int a0, a1, a2, a3, a4; extern int b0, b1, b2, b3, b4; void f () { /* this can be optimized to four additions... */ b4 = a4 + a3 + a2 + a1 + a0; b3 = a3 + a2 + a1 + a0; b2 = a2 + a1 + a0; b1 = a1 + a0; } This requires reassociating to forms of expressions that are already available, something that reassoc doesn't think about yet. //===---------------------------------------------------------------------===// This function: (derived from GCC PR19988) double foo(double x, double y) { return ((x + 0.1234 * y) * (x + -0.1234 * y)); } compiles to: _foo: movapd %xmm1, %xmm2 mulsd LCPI1_1(%rip), %xmm1 mulsd LCPI1_0(%rip), %xmm2 addsd %xmm0, %xmm1 addsd %xmm0, %xmm2 movapd %xmm1, %xmm0 mulsd %xmm2, %xmm0 ret Reassociate should be able to turn it into: double foo(double x, double y) { return ((x + 0.1234 * y) * (x - 0.1234 * y)); } Which allows the multiply by constant to be CSE'd, producing: _foo: mulsd LCPI1_0(%rip), %xmm1 movapd %xmm1, %xmm2 addsd %xmm0, %xmm2 subsd %xmm1, %xmm0 mulsd %xmm2, %xmm0 ret This doesn't need -ffast-math support at all. This is particularly bad because the llvm-gcc frontend is canonicalizing the later into the former, but clang doesn't have this problem. //===---------------------------------------------------------------------===// These two functions should generate the same code on big-endian systems: int g(int *j,int *l) { return memcmp(j,l,4); } int h(int *j, int *l) { return *j - *l; } this could be done in SelectionDAGISel.cpp, along with other special cases, for 1,2,4,8 bytes. //===---------------------------------------------------------------------===// It would be nice to revert this patch: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20060213/031986.html And teach the dag combiner enough to simplify the code expanded before legalize. It seems plausible that this knowledge would let it simplify other stuff too. //===---------------------------------------------------------------------===// For vector types, TargetData.cpp::getTypeInfo() returns alignment that is equal to the type size. It works but can be overly conservative as the alignment of specific vector types are target dependent. //===---------------------------------------------------------------------===// We should produce an unaligned load from code like this: v4sf example(float *P) { return (v4sf){P[0], P[1], P[2], P[3] }; } //===---------------------------------------------------------------------===// Add support for conditional increments, and other related patterns. Instead of: movl 136(%esp), %eax cmpl $0, %eax je LBB16_2 #cond_next LBB16_1: #cond_true incl _foo LBB16_2: #cond_next emit: movl _foo, %eax cmpl $1, %edi sbbl $-1, %eax movl %eax, _foo //===---------------------------------------------------------------------===// Combine: a = sin(x), b = cos(x) into a,b = sincos(x). Expand these to calls of sin/cos and stores: double sincos(double x, double *sin, double *cos); float sincosf(float x, float *sin, float *cos); long double sincosl(long double x, long double *sin, long double *cos); Doing so could allow SROA of the destination pointers. See also: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17687 This is now easily doable with MRVs. We could even make an intrinsic for this if anyone cared enough about sincos. //===---------------------------------------------------------------------===// quantum_sigma_x in 462.libquantum contains the following loop: for(i=0; isize; i++) { /* Flip the target bit of each basis state */ reg->node[i].state ^= ((MAX_UNSIGNED) 1 << target); } Where MAX_UNSIGNED/state is a 64-bit int. On a 32-bit platform it would be just so cool to turn it into something like: long long Res = ((MAX_UNSIGNED) 1 << target); if (target < 32) { for(i=0; isize; i++) reg->node[i].state ^= Res & 0xFFFFFFFFULL; } else { for(i=0; isize; i++) reg->node[i].state ^= Res & 0xFFFFFFFF00000000ULL } ... which would only do one 32-bit XOR per loop iteration instead of two. It would also be nice to recognize the reg->size doesn't alias reg->node[i], but this requires TBAA. //===---------------------------------------------------------------------===// This isn't recognized as bswap by instcombine (yes, it really is bswap): unsigned long reverse(unsigned v) { unsigned t; t = v ^ ((v << 16) | (v >> 16)); t &= ~0xff0000; v = (v << 24) | (v >> 8); return v ^ (t >> 8); } //===---------------------------------------------------------------------===// [LOOP RECOGNITION] These idioms should be recognized as popcount (see PR1488): unsigned countbits_slow(unsigned v) { unsigned c; for (c = 0; v; v >>= 1) c += v & 1; return c; } unsigned countbits_fast(unsigned v){ unsigned c; for (c = 0; v; c++) v &= v - 1; // clear the least significant bit set return c; } BITBOARD = unsigned long long int PopCnt(register BITBOARD a) { register int c=0; while(a) { c++; a &= a - 1; } return c; } unsigned int popcount(unsigned int input) { unsigned int count = 0; for (unsigned int i = 0; i < 4 * 8; i++) count += (input >> i) & i; return count; } This is a form of idiom recognition for loops, the same thing that could be useful for recognizing memset/memcpy. //===---------------------------------------------------------------------===// These should turn into single 16-bit (unaligned?) loads on little/big endian processors. unsigned short read_16_le(const unsigned char *adr) { return adr[0] | (adr[1] << 8); } unsigned short read_16_be(const unsigned char *adr) { return (adr[0] << 8) | adr[1]; } //===---------------------------------------------------------------------===// -instcombine should handle this transform: icmp pred (sdiv X / C1 ), C2 when X, C1, and C2 are unsigned. Similarly for udiv and signed operands. Currently InstCombine avoids this transform but will do it when the signs of the operands and the sign of the divide match. See the FIXME in InstructionCombining.cpp in the visitSetCondInst method after the switch case for Instruction::UDiv (around line 4447) for more details. The SingleSource/Benchmarks/Shootout-C++/hash and hash2 tests have examples of this construct. //===---------------------------------------------------------------------===// [LOOP RECOGNITION] viterbi speeds up *significantly* if the various "history" related copy loops are turned into memcpy calls at the source level. We need a "loops to memcpy" pass. //===---------------------------------------------------------------------===// [LOOP OPTIMIZATION] SingleSource/Benchmarks/Misc/dt.c shows several interesting optimization opportunities in its double_array_divs_variable function: it needs loop interchange, memory promotion (which LICM already does), vectorization and variable trip count loop unrolling (since it has a constant trip count). ICC apparently produces this very nice code with -ffast-math: ..B1.70: # Preds ..B1.70 ..B1.69 mulpd %xmm0, %xmm1 #108.2 mulpd %xmm0, %xmm1 #108.2 mulpd %xmm0, %xmm1 #108.2 mulpd %xmm0, %xmm1 #108.2 addl $8, %edx # cmpl $131072, %edx #108.2 jb ..B1.70 # Prob 99% #108.2 It would be better to count down to zero, but this is a lot better than what we do. //===---------------------------------------------------------------------===// Consider: typedef unsigned U32; typedef unsigned long long U64; int test (U32 *inst, U64 *regs) { U64 effective_addr2; U32 temp = *inst; int r1 = (temp >> 20) & 0xf; int b2 = (temp >> 16) & 0xf; effective_addr2 = temp & 0xfff; if (b2) effective_addr2 += regs[b2]; b2 = (temp >> 12) & 0xf; if (b2) effective_addr2 += regs[b2]; effective_addr2 &= regs[4]; if ((effective_addr2 & 3) == 0) return 1; return 0; } Note that only the low 2 bits of effective_addr2 are used. On 32-bit systems, we don't eliminate the computation of the top half of effective_addr2 because we don't have whole-function selection dags. On x86, this means we use one extra register for the function when effective_addr2 is declared as U64 than when it is declared U32. PHI Slicing could be extended to do this. //===---------------------------------------------------------------------===// LSR should know what GPR types a target has from TargetData. This code: volatile short X, Y; // globals void foo(int N) { int i; for (i = 0; i < N; i++) { X = i; Y = i*4; } } produces two near identical IV's (after promotion) on PPC/ARM: LBB1_2: ldr r3, LCPI1_0 ldr r3, [r3] strh r2, [r3] ldr r3, LCPI1_1 ldr r3, [r3] strh r1, [r3] add r1, r1, #4 add r2, r2, #1 <- [0,+,1] sub r0, r0, #1 <- [0,-,1] cmp r0, #0 bne LBB1_2 LSR should reuse the "+" IV for the exit test. //===---------------------------------------------------------------------===// Tail call elim should be more aggressive, checking to see if the call is followed by an uncond branch to an exit block. ; This testcase is due to tail-duplication not wanting to copy the return ; instruction into the terminating blocks because there was other code ; optimized out of the function after the taildup happened. ; RUN: llvm-as < %s | opt -tailcallelim | llvm-dis | not grep call define i32 @t4(i32 %a) { entry: %tmp.1 = and i32 %a, 1 ; [#uses=1] %tmp.2 = icmp ne i32 %tmp.1, 0 ; [#uses=1] br i1 %tmp.2, label %then.0, label %else.0 then.0: ; preds = %entry %tmp.5 = add i32 %a, -1 ; [#uses=1] %tmp.3 = call i32 @t4( i32 %tmp.5 ) ; [#uses=1] br label %return else.0: ; preds = %entry %tmp.7 = icmp ne i32 %a, 0 ; [#uses=1] br i1 %tmp.7, label %then.1, label %return then.1: ; preds = %else.0 %tmp.11 = add i32 %a, -2 ; [#uses=1] %tmp.9 = call i32 @t4( i32 %tmp.11 ) ; [#uses=1] br label %return return: ; preds = %then.1, %else.0, %then.0 %result.0 = phi i32 [ 0, %else.0 ], [ %tmp.3, %then.0 ], [ %tmp.9, %then.1 ] ret i32 %result.0 } //===---------------------------------------------------------------------===// Tail recursion elimination should handle: int pow2m1(int n) { if (n == 0) return 0; return 2 * pow2m1 (n - 1) + 1; } Also, multiplies can be turned into SHL's, so they should be handled as if they were associative. "return foo() << 1" can be tail recursion eliminated. //===---------------------------------------------------------------------===// Argument promotion should promote arguments for recursive functions, like this: ; RUN: llvm-as < %s | opt -argpromotion | llvm-dis | grep x.val define internal i32 @foo(i32* %x) { entry: %tmp = load i32* %x ; [#uses=0] %tmp.foo = call i32 @foo( i32* %x ) ; [#uses=1] ret i32 %tmp.foo } define i32 @bar(i32* %x) { entry: %tmp3 = call i32 @foo( i32* %x ) ; [#uses=1] ret i32 %tmp3 } //===---------------------------------------------------------------------===// We should investigate an instruction sinking pass. Consider this silly example in pic mode: #include void foo(int x) { assert(x); //... } we compile this to: _foo: subl $28, %esp call "L1$pb" "L1$pb": popl %eax cmpl $0, 32(%esp) je LBB1_2 # cond_true LBB1_1: # return # ... addl $28, %esp ret LBB1_2: # cond_true ... The PIC base computation (call+popl) is only used on one path through the code, but is currently always computed in the entry block. It would be better to sink the picbase computation down into the block for the assertion, as it is the only one that uses it. This happens for a lot of code with early outs. Another example is loads of arguments, which are usually emitted into the entry block on targets like x86. If not used in all paths through a function, they should be sunk into the ones that do. In this case, whole-function-isel would also handle this. //===---------------------------------------------------------------------===// Investigate lowering of sparse switch statements into perfect hash tables: http://burtleburtle.net/bob/hash/perfect.html //===---------------------------------------------------------------------===// We should turn things like "load+fabs+store" and "load+fneg+store" into the corresponding integer operations. On a yonah, this loop: double a[256]; void foo() { int i, b; for (b = 0; b < 10000000; b++) for (i = 0; i < 256; i++) a[i] = -a[i]; } is twice as slow as this loop: long long a[256]; void foo() { int i, b; for (b = 0; b < 10000000; b++) for (i = 0; i < 256; i++) a[i] ^= (1ULL << 63); } and I suspect other processors are similar. On X86 in particular this is a big win because doing this with integers allows the use of read/modify/write instructions. //===---------------------------------------------------------------------===// DAG Combiner should try to combine small loads into larger loads when profitable. For example, we compile this C++ example: struct THotKey { short Key; bool Control; bool Shift; bool Alt; }; extern THotKey m_HotKey; THotKey GetHotKey () { return m_HotKey; } into (-O3 -fno-exceptions -static -fomit-frame-pointer): __Z9GetHotKeyv: pushl %esi movl 8(%esp), %eax movb _m_HotKey+3, %cl movb _m_HotKey+4, %dl movb _m_HotKey+2, %ch movw _m_HotKey, %si movw %si, (%eax) movb %ch, 2(%eax) movb %cl, 3(%eax) movb %dl, 4(%eax) popl %esi ret $4 GCC produces: __Z9GetHotKeyv: movl _m_HotKey, %edx movl 4(%esp), %eax movl %edx, (%eax) movzwl _m_HotKey+4, %edx movw %dx, 4(%eax) ret $4 The LLVM IR contains the needed alignment info, so we should be able to merge the loads and stores into 4-byte loads: %struct.THotKey = type { i16, i8, i8, i8 } define void @_Z9GetHotKeyv(%struct.THotKey* sret %agg.result) nounwind { ... %tmp2 = load i16* getelementptr (@m_HotKey, i32 0, i32 0), align 8 %tmp5 = load i8* getelementptr (@m_HotKey, i32 0, i32 1), align 2 %tmp8 = load i8* getelementptr (@m_HotKey, i32 0, i32 2), align 1 %tmp11 = load i8* getelementptr (@m_HotKey, i32 0, i32 3), align 2 Alternatively, we should use a small amount of base-offset alias analysis to make it so the scheduler doesn't need to hold all the loads in regs at once. //===---------------------------------------------------------------------===// We should add an FRINT node to the DAG to model targets that have legal implementations of ceil/floor/rint. //===---------------------------------------------------------------------===// Consider: int test() { long long input[8] = {1,1,1,1,1,1,1,1}; foo(input); } We currently compile this into a memcpy from a global array since the initializer is fairly large and not memset'able. This is good, but the memcpy gets lowered to load/stores in the code generator. This is also ok, except that the codegen lowering for memcpy doesn't handle the case when the source is a constant global. This gives us atrocious code like this: call "L1$pb" "L1$pb": popl %eax movl _C.0.1444-"L1$pb"+32(%eax), %ecx movl %ecx, 40(%esp) movl _C.0.1444-"L1$pb"+20(%eax), %ecx movl %ecx, 28(%esp) movl _C.0.1444-"L1$pb"+36(%eax), %ecx movl %ecx, 44(%esp) movl _C.0.1444-"L1$pb"+44(%eax), %ecx movl %ecx, 52(%esp) movl _C.0.1444-"L1$pb"+40(%eax), %ecx movl %ecx, 48(%esp) movl _C.0.1444-"L1$pb"+12(%eax), %ecx movl %ecx, 20(%esp) movl _C.0.1444-"L1$pb"+4(%eax), %ecx ... instead of: movl $1, 16(%esp) movl $0, 20(%esp) movl $1, 24(%esp) movl $0, 28(%esp) movl $1, 32(%esp) movl $0, 36(%esp) ... //===---------------------------------------------------------------------===// http://llvm.org/PR717: The following code should compile into "ret int undef". Instead, LLVM produces "ret int 0": int f() { int x = 4; int y; if (x == 3) y = 0; return y; } //===---------------------------------------------------------------------===// The loop unroller should partially unroll loops (instead of peeling them) when code growth isn't too bad and when an unroll count allows simplification of some code within the loop. One trivial example is: #include int main() { int nRet = 17; int nLoop; for ( nLoop = 0; nLoop < 1000; nLoop++ ) { if ( nLoop & 1 ) nRet += 2; else nRet -= 1; } return nRet; } Unrolling by 2 would eliminate the '&1' in both copies, leading to a net reduction in code size. The resultant code would then also be suitable for exit value computation. //===---------------------------------------------------------------------===// We miss a bunch of rotate opportunities on various targets, including ppc, x86, etc. On X86, we miss a bunch of 'rotate by variable' cases because the rotate matching code in dag combine doesn't look through truncates aggressively enough. Here are some testcases reduces from GCC PR17886: unsigned long long f(unsigned long long x, int y) { return (x << y) | (x >> 64-y); } unsigned f2(unsigned x, int y){ return (x << y) | (x >> 32-y); } unsigned long long f3(unsigned long long x){ int y = 9; return (x << y) | (x >> 64-y); } unsigned f4(unsigned x){ int y = 10; return (x << y) | (x >> 32-y); } unsigned long long f5(unsigned long long x, unsigned long long y) { return (x << 8) | ((y >> 48) & 0xffull); } unsigned long long f6(unsigned long long x, unsigned long long y, int z) { switch(z) { case 1: return (x << 8) | ((y >> 48) & 0xffull); case 2: return (x << 16) | ((y >> 40) & 0xffffull); case 3: return (x << 24) | ((y >> 32) & 0xffffffull); case 4: return (x << 32) | ((y >> 24) & 0xffffffffull); default: return (x << 40) | ((y >> 16) & 0xffffffffffull); } } On X86-64, we only handle f2/f3/f4 right. On x86-32, a few of these generate truly horrible code, instead of using shld and friends. On ARM, we end up with calls to L___lshrdi3/L___ashldi3 in f, which is badness. PPC64 misses f, f5 and f6. CellSPU aborts in isel. //===---------------------------------------------------------------------===// We do a number of simplifications in simplify libcalls to strength reduce standard library functions, but we don't currently merge them together. For example, it is useful to merge memcpy(a,b,strlen(b)) -> strcpy. This can only be done safely if "b" isn't modified between the strlen and memcpy of course. //===---------------------------------------------------------------------===// We compile this program: (from GCC PR11680) http://gcc.gnu.org/bugzilla/attachment.cgi?id=4487 Into code that runs the same speed in fast/slow modes, but both modes run 2x slower than when compile with GCC (either 4.0 or 4.2): $ llvm-g++ perf.cpp -O3 -fno-exceptions $ time ./a.out fast 1.821u 0.003s 0:01.82 100.0% 0+0k 0+0io 0pf+0w $ g++ perf.cpp -O3 -fno-exceptions $ time ./a.out fast 0.821u 0.001s 0:00.82 100.0% 0+0k 0+0io 0pf+0w It looks like we are making the same inlining decisions, so this may be raw codegen badness or something else (haven't investigated). //===---------------------------------------------------------------------===// We miss some instcombines for stuff like this: void bar (void); void foo (unsigned int a) { /* This one is equivalent to a >= (3 << 2). */ if ((a >> 2) >= 3) bar (); } A few other related ones are in GCC PR14753. //===---------------------------------------------------------------------===// Divisibility by constant can be simplified (according to GCC PR12849) from being a mulhi to being a mul lo (cheaper). Testcase: void bar(unsigned n) { if (n % 3 == 0) true(); } This is equivalent to the following, where 2863311531 is the multiplicative inverse of 3, and 1431655766 is ((2^32)-1)/3+1: void bar(unsigned n) { if (n * 2863311531U < 1431655766U) true(); } The same transformation can work with an even modulo with the addition of a rotate: rotate the result of the multiply to the right by the number of bits which need to be zero for the condition to be true, and shrink the compare RHS by the same amount. Unless the target supports rotates, though, that transformation probably isn't worthwhile. The transformation can also easily be made to work with non-zero equality comparisons: just transform, for example, "n % 3 == 1" to "(n-1) % 3 == 0". //===---------------------------------------------------------------------===// Better mod/ref analysis for scanf would allow us to eliminate the vtable and a bunch of other stuff from this example (see PR1604): #include struct test { int val; virtual ~test() {} }; int main() { test t; std::scanf("%d", &t.val); std::printf("%d\n", t.val); } //===---------------------------------------------------------------------===// These functions perform the same computation, but produce different assembly. define i8 @select(i8 %x) readnone nounwind { %A = icmp ult i8 %x, 250 %B = select i1 %A, i8 0, i8 1 ret i8 %B } define i8 @addshr(i8 %x) readnone nounwind { %A = zext i8 %x to i9 %B = add i9 %A, 6 ;; 256 - 250 == 6 %C = lshr i9 %B, 8 %D = trunc i9 %C to i8 ret i8 %D } //===---------------------------------------------------------------------===// From gcc bug 24696: int f (unsigned long a, unsigned long b, unsigned long c) { return ((a & (c - 1)) != 0) || ((b & (c - 1)) != 0); } int f (unsigned long a, unsigned long b, unsigned long c) { return ((a & (c - 1)) != 0) | ((b & (c - 1)) != 0); } Both should combine to ((a|b) & (c-1)) != 0. Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// From GCC Bug 20192: #define PMD_MASK (~((1UL << 23) - 1)) void clear_pmd_range(unsigned long start, unsigned long end) { if (!(start & ~PMD_MASK) && !(end & ~PMD_MASK)) f(); } The expression should optimize to something like "!((start|end)&~PMD_MASK). Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// From GCC Bug 3756: int pn (int n) { return (n >= 0 ? 1 : -1); } Should combine to (n >> 31) | 1. Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts | llc". //===---------------------------------------------------------------------===// void a(int variable) { if (variable == 4 || variable == 6) bar(); } This should optimize to "if ((variable | 2) == 6)". Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts | llc". //===---------------------------------------------------------------------===// unsigned int f(unsigned int i, unsigned int n) {++i; if (i == n) ++i; return i;} unsigned int f2(unsigned int i, unsigned int n) {++i; i += i == n; return i;} These should combine to the same thing. Currently, the first function produces better code on X86. //===---------------------------------------------------------------------===// From GCC Bug 15784: #define abs(x) x>0?x:-x int f(int x, int y) { return (abs(x)) >= 0; } This should optimize to x == INT_MIN. (With -fwrapv.) Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// From GCC Bug 14753: void rotate_cst (unsigned int a) { a = (a << 10) | (a >> 22); if (a == 123) bar (); } void minus_cst (unsigned int a) { unsigned int tem; tem = 20 - a; if (tem == 5) bar (); } void mask_gt (unsigned int a) { /* This is equivalent to a > 15. */ if ((a & ~7) > 8) bar (); } void rshift_gt (unsigned int a) { /* This is equivalent to a > 23. */ if ((a >> 2) > 5) bar (); } All should simplify to a single comparison. All of these are currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// From GCC Bug 32605: int c(int* x) {return (char*)x+2 == (char*)x;} Should combine to 0. Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts" (although llc can optimize it). //===---------------------------------------------------------------------===// int a(unsigned b) {return ((b << 31) | (b << 30)) >> 31;} Should be combined to "((b >> 1) | b) & 1". Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// unsigned a(unsigned x, unsigned y) { return x | (y & 1) | (y & 2);} Should combine to "x | (y & 3)". Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// int a(int a, int b, int c) {return (~a & c) | ((c|a) & b);} Should fold to "(~a & c) | (a & b)". Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// int a(int a,int b) {return (~(a|b))|a;} Should fold to "a|~b". Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// int a(int a, int b) {return (a&&b) || (a&&!b);} Should fold to "a". Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// int a(int a, int b, int c) {return (a&&b) || (!a&&c);} Should fold to "a ? b : c", or at least something sane. Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// int a(int a, int b, int c) {return (a&&b) || (a&&c) || (a&&b&&c);} Should fold to a && (b || c). Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// int a(int x) {return x | ((x & 8) ^ 8);} Should combine to x | 8. Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// int a(int x) {return x ^ ((x & 8) ^ 8);} Should also combine to x | 8. Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// int a(int x) {return (x & 8) == 0 ? -1 : -9;} Should combine to (x | -9) ^ 8. Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// int a(int x) {return (x & 8) == 0 ? -9 : -1;} Should combine to x | -9. Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// int a(int x) {return ((x | -9) ^ 8) & x;} Should combine to x & -9. Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// unsigned a(unsigned a) {return a * 0x11111111 >> 28 & 1;} Should combine to "a * 0x88888888 >> 31". Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// unsigned a(char* x) {if ((*x & 32) == 0) return b();} There's an unnecessary zext in the generated code with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// unsigned a(unsigned long long x) {return 40 * (x >> 1);} Should combine to "20 * (((unsigned)x) & -2)". Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// This was noticed in the entryblock for grokdeclarator in 403.gcc: %tmp = icmp eq i32 %decl_context, 4 %decl_context_addr.0 = select i1 %tmp, i32 3, i32 %decl_context %tmp1 = icmp eq i32 %decl_context_addr.0, 1 %decl_context_addr.1 = select i1 %tmp1, i32 0, i32 %decl_context_addr.0 tmp1 should be simplified to something like: (!tmp || decl_context == 1) This allows recursive simplifications, tmp1 is used all over the place in the function, e.g. by: %tmp23 = icmp eq i32 %decl_context_addr.1, 0 ; [#uses=1] %tmp24 = xor i1 %tmp1, true ; [#uses=1] %or.cond8 = and i1 %tmp23, %tmp24 ; [#uses=1] later. //===---------------------------------------------------------------------===// [STORE SINKING] Store sinking: This code: void f (int n, int *cond, int *res) { int i; *res = 0; for (i = 0; i < n; i++) if (*cond) *res ^= 234; /* (*) */ } On this function GVN hoists the fully redundant value of *res, but nothing moves the store out. This gives us this code: bb: ; preds = %bb2, %entry %.rle = phi i32 [ 0, %entry ], [ %.rle6, %bb2 ] %i.05 = phi i32 [ 0, %entry ], [ %indvar.next, %bb2 ] %1 = load i32* %cond, align 4 %2 = icmp eq i32 %1, 0 br i1 %2, label %bb2, label %bb1 bb1: ; preds = %bb %3 = xor i32 %.rle, 234 store i32 %3, i32* %res, align 4 br label %bb2 bb2: ; preds = %bb, %bb1 %.rle6 = phi i32 [ %3, %bb1 ], [ %.rle, %bb ] %indvar.next = add i32 %i.05, 1 %exitcond = icmp eq i32 %indvar.next, %n br i1 %exitcond, label %return, label %bb DSE should sink partially dead stores to get the store out of the loop. Here's another partial dead case: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12395 //===---------------------------------------------------------------------===// Scalar PRE hoists the mul in the common block up to the else: int test (int a, int b, int c, int g) { int d, e; if (a) d = b * c; else d = b - c; e = b * c + g; return d + e; } It would be better to do the mul once to reduce codesize above the if. This is GCC PR38204. //===---------------------------------------------------------------------===// [STORE SINKING] GCC PR37810 is an interesting case where we should sink load/store reload into the if block and outside the loop, so we don't reload/store it on the non-call path. for () { *P += 1; if () call(); else ... -> tmp = *P for () { tmp += 1; if () { *P = tmp; call(); tmp = *P; } else ... } *P = tmp; We now hoist the reload after the call (Transforms/GVN/lpre-call-wrap.ll), but we don't sink the store. We need partially dead store sinking. //===---------------------------------------------------------------------===// [LOAD PRE CRIT EDGE SPLITTING] GCC PR37166: Sinking of loads prevents SROA'ing the "g" struct on the stack leading to excess stack traffic. This could be handled by GVN with some crazy symbolic phi translation. The code we get looks like (g is on the stack): bb2: ; preds = %bb1 .. %9 = getelementptr %struct.f* %g, i32 0, i32 0 store i32 %8, i32* %9, align bel %bb3 bb3: ; preds = %bb1, %bb2, %bb %c_addr.0 = phi %struct.f* [ %g, %bb2 ], [ %c, %bb ], [ %c, %bb1 ] %b_addr.0 = phi %struct.f* [ %b, %bb2 ], [ %g, %bb ], [ %b, %bb1 ] %10 = getelementptr %struct.f* %c_addr.0, i32 0, i32 0 %11 = load i32* %10, align 4 %11 is partially redundant, an in BB2 it should have the value %8. GCC PR33344 and PR35287 are similar cases. //===---------------------------------------------------------------------===// [LOAD PRE] There are many load PRE testcases in testsuite/gcc.dg/tree-ssa/loadpre* in the GCC testsuite, ones we don't get yet are (checked through loadpre25): [CRIT EDGE BREAKING] loadpre3.c predcom-4.c [PRE OF READONLY CALL] loadpre5.c [TURN SELECT INTO BRANCH] loadpre14.c loadpre15.c actually a conditional increment: loadpre18.c loadpre19.c //===---------------------------------------------------------------------===// [SCALAR PRE] There are many PRE testcases in testsuite/gcc.dg/tree-ssa/ssa-pre-*.c in the GCC testsuite. //===---------------------------------------------------------------------===// There are some interesting cases in testsuite/gcc.dg/tree-ssa/pred-comm* in the GCC testsuite. For example, we get the first example in predcom-1.c, but miss the second one: unsigned fib[1000]; unsigned avg[1000]; __attribute__ ((noinline)) void count_averages(int n) { int i; for (i = 1; i < n; i++) avg[i] = (((unsigned long) fib[i - 1] + fib[i] + fib[i + 1]) / 3) & 0xffff; } which compiles into two loads instead of one in the loop. predcom-2.c is the same as predcom-1.c predcom-3.c is very similar but needs loads feeding each other instead of store->load. //===---------------------------------------------------------------------===// [ALIAS ANALYSIS] Type based alias analysis: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14705 We should do better analysis of posix_memalign. At the least it should no-capture its pointer argument, at best, we should know that the out-value result doesn't point to anything (like malloc). One example of this is in SingleSource/Benchmarks/Misc/dt.c //===---------------------------------------------------------------------===// A/B get pinned to the stack because we turn an if/then into a select instead of PRE'ing the load/store. This may be fixable in instcombine: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37892 struct X { int i; }; int foo (int x) { struct X a; struct X b; struct X *p; a.i = 1; b.i = 2; if (x) p = &a; else p = &b; return p->i; } //===---------------------------------------------------------------------===// Interesting missed case because of control flow flattening (should be 2 loads): http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26629 With: llvm-gcc t2.c -S -o - -O0 -emit-llvm | llvm-as | opt -mem2reg -gvn -instcombine | llvm-dis we miss it because we need 1) CRIT EDGE 2) MULTIPLE DIFFERENT VALS PRODUCED BY ONE BLOCK OVER DIFFERENT PATHS //===---------------------------------------------------------------------===// http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19633 We could eliminate the branch condition here, loading from null is undefined: struct S { int w, x, y, z; }; struct T { int r; struct S s; }; void bar (struct S, int); void foo (int a, struct T b) { struct S *c = 0; if (a) c = &b.s; bar (*c, a); } //===---------------------------------------------------------------------===// simplifylibcalls should do several optimizations for strspn/strcspn: strcspn(x, "") -> strlen(x) strcspn("", x) -> 0 strspn("", x) -> 0 strspn(x, "") -> strlen(x) strspn(x, "a") -> strchr(x, 'a')-x strcspn(x, "a") -> inlined loop for up to 3 letters (similarly for strspn): size_t __strcspn_c3 (__const char *__s, int __reject1, int __reject2, int __reject3) { register size_t __result = 0; while (__s[__result] != '\0' && __s[__result] != __reject1 && __s[__result] != __reject2 && __s[__result] != __reject3) ++__result; return __result; } This should turn into a switch on the character. See PR3253 for some notes on codegen. 456.hmmer apparently uses strcspn and strspn a lot. 471.omnetpp uses strspn. //===---------------------------------------------------------------------===// "gas" uses this idiom: else if (strchr ("+-/*%|&^:[]()~", *intel_parser.op_string)) .. else if (strchr ("<>", *intel_parser.op_string) Those should be turned into a switch. //===---------------------------------------------------------------------===// 252.eon contains this interesting code: %3072 = getelementptr [100 x i8]* %tempString, i32 0, i32 0 %3073 = call i8* @strcpy(i8* %3072, i8* %3071) nounwind %strlen = call i32 @strlen(i8* %3072) ; uses = 1 %endptr = getelementptr [100 x i8]* %tempString, i32 0, i32 %strlen call void @llvm.memcpy.i32(i8* %endptr, i8* getelementptr ([5 x i8]* @"\01LC42", i32 0, i32 0), i32 5, i32 1) %3074 = call i32 @strlen(i8* %endptr) nounwind readonly This is interesting for a couple reasons. First, in this: %3073 = call i8* @strcpy(i8* %3072, i8* %3071) nounwind %strlen = call i32 @strlen(i8* %3072) The strlen could be replaced with: %strlen = sub %3072, %3073, because the strcpy call returns a pointer to the end of the string. Based on that, the endptr GEP just becomes equal to 3073, which eliminates a strlen call and GEP. Second, the memcpy+strlen strlen can be replaced with: %3074 = call i32 @strlen([5 x i8]* @"\01LC42") nounwind readonly Because the destination was just copied into the specified memory buffer. This, in turn, can be constant folded to "4". In other code, it contains: %endptr6978 = bitcast i8* %endptr69 to i32* store i32 7107374, i32* %endptr6978, align 1 %3167 = call i32 @strlen(i8* %endptr69) nounwind readonly Which could also be constant folded. Whatever is producing this should probably be fixed to leave this as a memcpy from a string. Further, eon also has an interesting partially redundant strlen call: bb8: ; preds = %_ZN18eonImageCalculatorC1Ev.exit %682 = getelementptr i8** %argv, i32 6 ; [#uses=2] %683 = load i8** %682, align 4 ; [#uses=4] %684 = load i8* %683, align 1 ; [#uses=1] %685 = icmp eq i8 %684, 0 ; [#uses=1] br i1 %685, label %bb10, label %bb9 bb9: ; preds = %bb8 %686 = call i32 @strlen(i8* %683) nounwind readonly %687 = icmp ugt i32 %686, 254 ; [#uses=1] br i1 %687, label %bb10, label %bb11 bb10: ; preds = %bb9, %bb8 %688 = call i32 @strlen(i8* %683) nounwind readonly This could be eliminated by doing the strlen once in bb8, saving code size and improving perf on the bb8->9->10 path. //===---------------------------------------------------------------------===// I see an interesting fully redundant call to strlen left in 186.crafty:InputMove which looks like: %movetext11 = getelementptr [128 x i8]* %movetext, i32 0, i32 0 bb62: ; preds = %bb55, %bb53 %promote.0 = phi i32 [ %169, %bb55 ], [ 0, %bb53 ] %171 = call i32 @strlen(i8* %movetext11) nounwind readonly align 1 %172 = add i32 %171, -1 ; [#uses=1] %173 = getelementptr [128 x i8]* %movetext, i32 0, i32 %172 ... no stores ... br i1 %or.cond, label %bb65, label %bb72 bb65: ; preds = %bb62 store i8 0, i8* %173, align 1 br label %bb72 bb72: ; preds = %bb65, %bb62 %trank.1 = phi i32 [ %176, %bb65 ], [ -1, %bb62 ] %177 = call i32 @strlen(i8* %movetext11) nounwind readonly align 1 Note that on the bb62->bb72 path, that the %177 strlen call is partially redundant with the %171 call. At worst, we could shove the %177 strlen call up into the bb65 block moving it out of the bb62->bb72 path. However, note that bb65 stores to the string, zeroing out the last byte. This means that on that path the value of %177 is actually just %171-1. A sub is cheaper than a strlen! This pattern repeats several times, basically doing: A = strlen(P); P[A-1] = 0; B = strlen(P); where it is "obvious" that B = A-1. //===---------------------------------------------------------------------===// 186.crafty also contains this code: %1906 = call i32 @strlen(i8* getelementptr ([32 x i8]* @pgn_event, i32 0,i32 0)) %1907 = getelementptr [32 x i8]* @pgn_event, i32 0, i32 %1906 %1908 = call i8* @strcpy(i8* %1907, i8* %1905) nounwind align 1 %1909 = call i32 @strlen(i8* getelementptr ([32 x i8]* @pgn_event, i32 0,i32 0)) %1910 = getelementptr [32 x i8]* @pgn_event, i32 0, i32 %1909 The last strlen is computable as 1908-@pgn_event, which means 1910=1908. //===---------------------------------------------------------------------===// 186.crafty has this interesting pattern with the "out.4543" variable: call void @llvm.memcpy.i32( i8* getelementptr ([10 x i8]* @out.4543, i32 0, i32 0), i8* getelementptr ([7 x i8]* @"\01LC28700", i32 0, i32 0), i32 7, i32 1) %101 = call@printf(i8* ... @out.4543, i32 0, i32 0)) nounwind It is basically doing: memcpy(globalarray, "string"); printf(..., globalarray); Anyway, by knowing that printf just reads the memory and forward substituting the string directly into the printf, this eliminates reads from globalarray. Since this pattern occurs frequently in crafty (due to the "DisplayTime" and other similar functions) there are many stores to "out". Once all the printfs stop using "out", all that is left is the memcpy's into it. This should allow globalopt to remove the "stored only" global. //===---------------------------------------------------------------------===// This code: define inreg i32 @foo(i8* inreg %p) nounwind { %tmp0 = load i8* %p %tmp1 = ashr i8 %tmp0, 5 %tmp2 = sext i8 %tmp1 to i32 ret i32 %tmp2 } could be dagcombine'd to a sign-extending load with a shift. For example, on x86 this currently gets this: movb (%eax), %al sarb $5, %al movsbl %al, %eax while it could get this: movsbl (%eax), %eax sarl $5, %eax //===---------------------------------------------------------------------===// GCC PR31029: int test(int x) { return 1-x == x; } // --> return false int test2(int x) { return 2-x == x; } // --> return x == 1 ? Always foldable for odd constants, what is the rule for even? //===---------------------------------------------------------------------===// PR 3381: GEP to field of size 0 inside a struct could be turned into GEP for next field in struct (which is at same address). For example: store of float into { {{}}, float } could be turned into a store to the float directly. //===---------------------------------------------------------------------===// #include double foo(double a) { return sin(a); } This compiles into this on x86-64 Linux: foo: subq $8, %rsp call sin addq $8, %rsp ret vs: foo: jmp sin //===---------------------------------------------------------------------===// The arg promotion pass should make use of nocapture to make its alias analysis stuff much more precise. //===---------------------------------------------------------------------===// The following functions should be optimized to use a select instead of a branch (from gcc PR40072): char char_int(int m) {if(m>7) return 0; return m;} int int_char(char m) {if(m>7) return 0; return m;} //===---------------------------------------------------------------------===// int func(int a, int b) { if (a & 0x80) b |= 0x80; else b &= ~0x80; return b; } Generates this: define i32 @func(i32 %a, i32 %b) nounwind readnone ssp { entry: %0 = and i32 %a, 128 ; [#uses=1] %1 = icmp eq i32 %0, 0 ; [#uses=1] %2 = or i32 %b, 128 ; [#uses=1] %3 = and i32 %b, -129 ; [#uses=1] %b_addr.0 = select i1 %1, i32 %3, i32 %2 ; [#uses=1] ret i32 %b_addr.0 } However, it's functionally equivalent to: b = (b & ~0x80) | (a & 0x80); Which generates this: define i32 @func(i32 %a, i32 %b) nounwind readnone ssp { entry: %0 = and i32 %b, -129 ; [#uses=1] %1 = and i32 %a, 128 ; [#uses=1] %2 = or i32 %0, %1 ; [#uses=1] ret i32 %2 } This can be generalized for other forms: b = (b & ~0x80) | (a & 0x40) << 1; //===---------------------------------------------------------------------===// These two functions produce different code. They shouldn't: #include uint8_t p1(uint8_t b, uint8_t a) { b = (b & ~0xc0) | (a & 0xc0); return (b); } uint8_t p2(uint8_t b, uint8_t a) { b = (b & ~0x40) | (a & 0x40); b = (b & ~0x80) | (a & 0x80); return (b); } define zeroext i8 @p1(i8 zeroext %b, i8 zeroext %a) nounwind readnone ssp { entry: %0 = and i8 %b, 63 ; [#uses=1] %1 = and i8 %a, -64 ; [#uses=1] %2 = or i8 %1, %0 ; [#uses=1] ret i8 %2 } define zeroext i8 @p2(i8 zeroext %b, i8 zeroext %a) nounwind readnone ssp { entry: %0 = and i8 %b, 63 ; [#uses=1] %.masked = and i8 %a, 64 ; [#uses=1] %1 = and i8 %a, -128 ; [#uses=1] %2 = or i8 %1, %0 ; [#uses=1] %3 = or i8 %2, %.masked ; [#uses=1] ret i8 %3 } //===---------------------------------------------------------------------===// IPSCCP does not currently propagate argument dependent constants through functions where it does not not all of the callers. This includes functions with normal external linkage as well as templates, C99 inline functions etc. Specifically, it does nothing to: define i32 @test(i32 %x, i32 %y, i32 %z) nounwind { entry: %0 = add nsw i32 %y, %z %1 = mul i32 %0, %x %2 = mul i32 %y, %z %3 = add nsw i32 %1, %2 ret i32 %3 } define i32 @test2() nounwind { entry: %0 = call i32 @test(i32 1, i32 2, i32 4) nounwind ret i32 %0 } It would be interesting extend IPSCCP to be able to handle simple cases like this, where all of the arguments to a call are constant. Because IPSCCP runs before inlining, trivial templates and inline functions are not yet inlined. The results for a function + set of constant arguments should be memoized in a map. //===---------------------------------------------------------------------===// The libcall constant folding stuff should be moved out of SimplifyLibcalls into libanalysis' constantfolding logic. This would allow IPSCCP to be able to handle simple things like this: static int foo(const char *X) { return strlen(X); } int bar() { return foo("abcd"); } //===---------------------------------------------------------------------===// InstCombine should use SimplifyDemandedBits to remove the or instruction: define i1 @test(i8 %x, i8 %y) { %A = or i8 %x, 1 %B = icmp ugt i8 %A, 3 ret i1 %B } Currently instcombine calls SimplifyDemandedBits with either all bits or just the sign bit, if the comparison is obviously a sign test. In this case, we only need all but the bottom two bits from %A, and if we gave that mask to SDB it would delete the or instruction for us. //===---------------------------------------------------------------------===// functionattrs doesn't know much about memcpy/memset. This function should be marked readnone rather than readonly, since it only twiddles local memory, but functionattrs doesn't handle memset/memcpy/memmove aggressively: struct X { int *p; int *q; }; int foo() { int i = 0, j = 1; struct X x, y; int **p; y.p = &i; x.q = &j; p = __builtin_memcpy (&x, &y, sizeof (int *)); return **p; } //===---------------------------------------------------------------------===// Missed instcombine transformation: define i1 @a(i32 %x) nounwind readnone { entry: %cmp = icmp eq i32 %x, 30 %sub = add i32 %x, -30 %cmp2 = icmp ugt i32 %sub, 9 %or = or i1 %cmp, %cmp2 ret i1 %or } This should be optimized to a single compare. Testcase derived from gcc. //===---------------------------------------------------------------------===// Missed instcombine transformation: void b(); void a(int x) { if (((1<47)&(b<58); } The sgt and slt should be combined into a single comparison. Testcase derived from gcc. //===---------------------------------------------------------------------===// Missed instcombine transformation: define i32 @a(i32 %x) nounwind readnone { entry: %rem = srem i32 %x, 32 %shl = shl i32 1, %rem ret i32 %shl } The srem can be transformed to an and because if x is negative, the shift is undefined. Testcase derived from gcc. //===---------------------------------------------------------------------===// Missed instcombine/dagcombine transformation: define i32 @a(i32 %x, i32 %y) nounwind readnone { entry: %mul = mul i32 %y, -8 %sub = sub i32 %x, %mul ret i32 %sub } Should compile to something like x+y*8, but currently compiles to an inefficient result. Testcase derived from gcc. //===---------------------------------------------------------------------===// Missed instcombine/dagcombine transformation: define void @lshift_lt(i8 zeroext %a) nounwind { entry: %conv = zext i8 %a to i32 %shl = shl i32 %conv, 3 %cmp = icmp ult i32 %shl, 33 br i1 %cmp, label %if.then, label %if.end if.then: tail call void @bar() nounwind ret void if.end: ret void } declare void @bar() nounwind The shift should be eliminated. Testcase derived from gcc. //===---------------------------------------------------------------------===// These compile into different code, one gets recognized as a switch and the other doesn't due to phase ordering issues (PR6212): int test1(int mainType, int subType) { if (mainType == 7) subType = 4; else if (mainType == 9) subType = 6; else if (mainType == 11) subType = 9; return subType; } int test2(int mainType, int subType) { if (mainType == 7) subType = 4; if (mainType == 9) subType = 6; if (mainType == 11) subType = 9; return subType; } //===---------------------------------------------------------------------===// The following test case (from PR6576): define i32 @mul(i32 %a, i32 %b) nounwind readnone { entry: %cond1 = icmp eq i32 %b, 0 ; [#uses=1] br i1 %cond1, label %exit, label %bb.nph bb.nph: ; preds = %entry %tmp = mul i32 %b, %a ; [#uses=1] ret i32 %tmp exit: ; preds = %entry ret i32 0 } could be reduced to: define i32 @mul(i32 %a, i32 %b) nounwind readnone { entry: %tmp = mul i32 %b, %a ret i32 %tmp } //===---------------------------------------------------------------------===// We should use DSE + llvm.lifetime.end to delete dead vtable pointer updates. See GCC PR34949 Another interesting case is that something related could be used for variables that go const after their ctor has finished. In these cases, globalopt (which can statically run the constructor) could mark the global const (so it gets put in the readonly section). A testcase would be: #include using namespace std; const complex should_be_in_rodata (42,-42); complex should_be_in_data (42,-42); complex should_be_in_bss; Where we currently evaluate the ctors but the globals don't become const because the optimizer doesn't know they "become const" after the ctor is done. See GCC PR4131 for more examples. //===---------------------------------------------------------------------===// In this code: long foo(long x) { return x > 1 ? x : 1; } LLVM emits a comparison with 1 instead of 0. 0 would be equivalent and cheaper on most targets. LLVM prefers comparisons with zero over non-zero in general, but in this case it choses instead to keep the max operation obvious. //===---------------------------------------------------------------------===// Take the following testcase on x86-64 (similar testcases exist for all targets with addc/adde): define void @a(i64* nocapture %s, i64* nocapture %t, i64 %a, i64 %b, i64 %c) nounwind { entry: %0 = zext i64 %a to i128 ; [#uses=1] %1 = zext i64 %b to i128 ; [#uses=1] %2 = add i128 %1, %0 ; [#uses=2] %3 = zext i64 %c to i128 ; [#uses=1] %4 = shl i128 %3, 64 ; [#uses=1] %5 = add i128 %4, %2 ; [#uses=1] %6 = lshr i128 %5, 64 ; [#uses=1] %7 = trunc i128 %6 to i64 ; [#uses=1] store i64 %7, i64* %s, align 8 %8 = trunc i128 %2 to i64 ; [#uses=1] store i64 %8, i64* %t, align 8 ret void } Generated code: addq %rcx, %rdx movl $0, %eax adcq $0, %rax addq %r8, %rax movq %rax, (%rdi) movq %rdx, (%rsi) ret Expected code: addq %rcx, %rdx adcq $0, %r8 movq %r8, (%rdi) movq %rdx, (%rsi) ret The generated SelectionDAG has an ADD of an ADDE, where both operands of the ADDE are zero. Replacing one of the operands of the ADDE with the other operand of the ADD, and replacing the ADD with the ADDE, should give the desired result. (That said, we are doing a lot better than gcc on this testcase. :) ) //===---------------------------------------------------------------------===//