llvm-6502/lib/Target/ARM
Eric Christopher 7a56f33583 Move to thumb2 loads, fixes a problem with incoming registers
as thumb1.

Fixes lencod.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@116027 91177308-0d34-0410-b5e6-96231b3b80d8
2010-10-08 01:13:17 +00:00
..
AsmParser implement rdar://8456378 and PR7557 - support for the fstsw, 2010-09-29 01:50:45 +00:00
Disassembler Add ARM Disassembler to the CMake build. 2010-09-28 11:48:19 +00:00
InstPrinter Kill of the vestiges of the 'call' Modifier (no longer needed for PLT). 2010-10-06 21:36:43 +00:00
TargetInfo make -fno-rtti the default unless a directory builds with REQUIRES_RTTI. 2010-01-24 20:43:08 +00:00
ARM.h I added a new file ARMAsmBackend which stubs out in similar ways to 2010-09-30 02:17:26 +00:00
ARM.td Nuke it from orbit. It's the only way to be sure. 2010-09-30 01:57:53 +00:00
ARMAddressingModes.h Change ARM VFP VLDM/VSTM instructions to use addressing mode #4, just like 2010-08-27 23:18:17 +00:00
ARMAsmBackend.cpp Move getPointerSize() to the base class since it's not dependent on MachO 2010-09-30 17:45:51 +00:00
ARMAsmPrinter.cpp remove trailing whitespace 2010-10-06 22:46:47 +00:00
ARMBaseInfo.h Add support for ELF PLT references for ARM MC asm printing. Adding a 2010-09-22 23:27:36 +00:00
ARMBaseInstrInfo.cpp Code refactoring. 2010-10-07 23:12:15 +00:00
ARMBaseInstrInfo.h Code refactoring. 2010-10-07 23:12:15 +00:00
ARMBaseRegisterInfo.cpp Simplify ARM callee-saved register handling by removing the distinction 2010-09-20 19:32:20 +00:00
ARMBaseRegisterInfo.h move getRegisterNumbering() to out of ARMBaseRegisterInfo into the helper 2010-09-15 20:26:25 +00:00
ARMBuildAttrs.h First in a sequence of ARM/MC/*ELF* specific work. 2010-10-06 22:36:46 +00:00
ARMCallingConv.h Newline at end of file. 2010-09-10 22:46:03 +00:00
ARMCallingConv.td fix emacs language spec's, patch by Edmund Grimley-Evans! 2010-08-17 16:20:04 +00:00
ARMCodeEmitter.cpp Make <target>CodeEmitter::getBinaryCodeForInstr() a const method. 2010-10-08 00:21:28 +00:00
ARMConstantIslandPass.cpp Remove extra header. 2010-08-18 23:38:16 +00:00
ARMConstantPoolValue.cpp Fix a potential null dereference bug. 2010-09-24 05:18:35 +00:00
ARMConstantPoolValue.h Add <cstddef> include to get ptrdiff_t, for gcc-4.6; patch by Dimitry Andric. 2010-06-15 14:50:42 +00:00
ARMELFWriterInfo.cpp Jim Asked us to move DataLayout on ARM back to the most specialized classes. Do 2010-10-03 18:59:45 +00:00
ARMELFWriterInfo.h Odd additional stub framework for the ARM MC ELF emission. 2010-09-27 18:31:37 +00:00
ARMExpandPseudoInsts.cpp Change the NEON VDUPfdf and VDUPfqf pseudo-instructions to actually be 2010-10-06 21:16:16 +00:00
ARMFastISel.cpp Move to thumb2 loads, fixes a problem with incoming registers 2010-10-08 01:13:17 +00:00
ARMFrameInfo.h
ARMGlobalMerge.cpp 80-col fixups. 2010-09-28 04:18:29 +00:00
ARMInstrFormats.td Add a 'pattern' arg to the ARM PseudoNeonI class. 2010-10-06 20:36:55 +00:00
ARMInstrInfo.cpp Slightly change the meaning of the reMaterialize target hook when the original 2010-06-02 22:47:25 +00:00
ARMInstrInfo.h Slightly change the meaning of the reMaterialize target hook when the original 2010-06-02 22:47:25 +00:00
ARMInstrInfo.td Allow use of the 16-bit literal move instruction in CMOVs for ARM mode. 2010-10-07 00:42:42 +00:00
ARMInstrNEON.td Model operand cycles of vldm / vstm; also fixes scheduling itineraries of vldr / vstr, etc. 2010-10-07 01:50:48 +00:00
ARMInstrThumb2.td Allow use of the 16-bit literal move instruction in CMOVs for Thumb2 mode. 2010-10-07 00:53:56 +00:00
ARMInstrThumb.td Kill of the vestiges of the 'call' Modifier (no longer needed for PLT). 2010-10-06 21:36:43 +00:00
ARMInstrVFP.td Model operand cycles of vldm / vstm; also fixes scheduling itineraries of vldr / vstr, etc. 2010-10-07 01:50:48 +00:00
ARMISelDAGToDAG.cpp Allow use of the 16-bit literal move instruction in CMOVs for Thumb2 mode. 2010-10-07 00:53:56 +00:00
ARMISelLowering.cpp User proper libcall names & condcodes while compiling for ARM EABI. 2010-09-28 21:39:26 +00:00
ARMISelLowering.h Define the TargetLowering::getTgtMemIntrinsic hook for ARM so that NEON load 2010-09-21 17:56:22 +00:00
ARMJITInfo.cpp move getRegisterNumbering() to out of ARMBaseRegisterInfo into the helper 2010-09-15 20:26:25 +00:00
ARMJITInfo.h Clean up 80 column violations. No functional change. 2010-06-02 21:53:11 +00:00
ARMLoadStoreOptimizer.cpp Increase ARM APCS preferred alignment for i64 and f64 from 32 bits to 64 bits. 2010-09-29 17:54:10 +00:00
ARMMachineFunctionInfo.h Simplify ARM callee-saved register handling by removing the distinction 2010-09-20 19:32:20 +00:00
ARMMCAsmInfo.cpp eliminate the magic AbsoluteDebugSectionOffsets MAI hook, 2010-04-04 23:22:29 +00:00
ARMMCAsmInfo.h
ARMMCCodeEmitter.cpp Enable binary encoding of some simple instructions. 2010-10-08 00:39:21 +00:00
ARMMCInstLower.cpp Add support for ELF PLT references for ARM MC asm printing. Adding a 2010-09-22 23:27:36 +00:00
ARMMCInstLower.h Add support for ELF PLT references for ARM MC asm printing. Adding a 2010-09-22 23:27:36 +00:00
ARMPerfectShuffle.h
ARMRegisterInfo.cpp
ARMRegisterInfo.h Switch ARMRegisterInfo.td to use SubRegIndex and eliminate the parallel enums 2010-05-24 16:54:32 +00:00
ARMRegisterInfo.td Now that register allocation properly considers reserved regs, simplify the 2010-09-02 18:14:29 +00:00
ARMRelocations.h Add support to MOVimm32 using movt/movw for ARM JIT 2010-05-25 08:42:45 +00:00
ARMSchedule.td Model operand cycles of vldm / vstm; also fixes scheduling itineraries of vldr / vstr, etc. 2010-10-07 01:50:48 +00:00
ARMScheduleA8.td Model operand cycles of vldm / vstm; also fixes scheduling itineraries of vldr / vstr, etc. 2010-10-07 01:50:48 +00:00
ARMScheduleA9.td Model operand cycles of vldm / vstm; also fixes scheduling itineraries of vldr / vstr, etc. 2010-10-07 01:50:48 +00:00
ARMScheduleV6.td Model operand cycles of vldm / vstm; also fixes scheduling itineraries of vldr / vstr, etc. 2010-10-07 01:50:48 +00:00
ARMSelectionDAGInfo.cpp reimplement memcpy/memmove/memset lowering to use MachinePointerInfo 2010-09-21 05:40:29 +00:00
ARMSelectionDAGInfo.h reimplement memcpy/memmove/memset lowering to use MachinePointerInfo 2010-09-21 05:40:29 +00:00
ARMSubtarget.cpp Add a subtarget hook for reporting the misprediction penalty. Use this to provide more precise 2010-09-28 21:57:50 +00:00
ARMSubtarget.h Jim Asked us to move DataLayout on ARM back to the most specialized classes. Do 2010-10-03 18:59:45 +00:00
ARMTargetMachine.cpp Jim Asked us to move DataLayout on ARM back to the most specialized classes. Do 2010-10-03 18:59:45 +00:00
ARMTargetMachine.h Jim Asked us to move DataLayout on ARM back to the most specialized classes. Do 2010-10-03 18:59:45 +00:00
ARMTargetObjectFile.cpp delete a forwarding function. 2010-04-08 21:34:17 +00:00
ARMTargetObjectFile.h Now that the default for Darwin platforms is to place the LSDA into the TEXT 2010-03-15 21:09:38 +00:00
CMakeLists.txt grammar 2010-10-01 14:57:48 +00:00
Makefile Rename the AsmPrinter directory to InstPrinter for those targets that have 2010-10-01 22:39:28 +00:00
NEONMoveFix.cpp Reapply r110396, with fixes to appease the Linux buildbot gods. 2010-08-06 18:33:48 +00:00
README-Thumb2.txt
README-Thumb.txt Use Unified Assembly Syntax for the ARM backend. 2009-11-09 00:11:35 +00:00
README.txt And a bit more non-ASCII stuff. 2010-07-26 22:28:18 +00:00
Thumb1InstrInfo.cpp convert targets to the new MF.getMachineMemOperand interface. 2010-09-21 04:39:43 +00:00
Thumb1InstrInfo.h RISC architectures get their memory operand folding for free. 2010-07-11 19:19:13 +00:00
Thumb1RegisterInfo.cpp Simplify ARM callee-saved register handling by removing the distinction 2010-09-20 19:32:20 +00:00
Thumb1RegisterInfo.h Simplify eliminateFrameIndex() interface back down now that PEI doesn't need 2010-08-26 23:32:16 +00:00
Thumb2HazardRecognizer.cpp tidy up style. no functional change. 2010-06-28 21:29:17 +00:00
Thumb2HazardRecognizer.h Teach if-converter to be more careful with predicating instructions that would 2010-09-10 01:29:16 +00:00
Thumb2InstrInfo.cpp Thread the determination of branch prediction hit rates back through the if-conversion heuristic APIs. For now, 2010-10-01 22:45:50 +00:00
Thumb2InstrInfo.h Thread the determination of branch prediction hit rates back through the if-conversion heuristic APIs. For now, 2010-10-01 22:45:50 +00:00
Thumb2ITBlockPass.cpp Reapply r110396, with fixes to appease the Linux buildbot gods. 2010-08-06 18:33:48 +00:00
Thumb2RegisterInfo.cpp Add const qualifiers to CodeGen's use of LLVM IR constructs. 2010-04-15 01:51:59 +00:00
Thumb2RegisterInfo.h 80-column cleanup of file header comments 2009-11-07 22:00:39 +00:00
Thumb2SizeReduction.cpp Clarify comment 2010-09-14 20:35:46 +00:00

//===---------------------------------------------------------------------===//
// Random ideas for the ARM backend.
//===---------------------------------------------------------------------===//

Reimplement 'select' in terms of 'SEL'.

* We would really like to support UXTAB16, but we need to prove that the
  add doesn't need to overflow between the two 16-bit chunks.

* Implement pre/post increment support.  (e.g. PR935)
* Implement smarter constant generation for binops with large immediates.

A few ARMv6T2 ops should be pattern matched: BFI, SBFX, and UBFX

Interesting optimization for PIC codegen on arm-linux:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43129

//===---------------------------------------------------------------------===//

Crazy idea:  Consider code that uses lots of 8-bit or 16-bit values.  By the
time regalloc happens, these values are now in a 32-bit register, usually with
the top-bits known to be sign or zero extended.  If spilled, we should be able
to spill these to a 8-bit or 16-bit stack slot, zero or sign extending as part
of the reload.

Doing this reduces the size of the stack frame (important for thumb etc), and
also increases the likelihood that we will be able to reload multiple values
from the stack with a single load.

//===---------------------------------------------------------------------===//

The constant island pass is in good shape.  Some cleanups might be desirable,
but there is unlikely to be much improvement in the generated code.

1.  There may be some advantage to trying to be smarter about the initial
placement, rather than putting everything at the end.

2.  There might be some compile-time efficiency to be had by representing
consecutive islands as a single block rather than multiple blocks.

3.  Use a priority queue to sort constant pool users in inverse order of
    position so we always process the one closed to the end of functions
    first. This may simply CreateNewWater.

//===---------------------------------------------------------------------===//

Eliminate copysign custom expansion. We are still generating crappy code with
default expansion + if-conversion.

//===---------------------------------------------------------------------===//

Eliminate one instruction from:

define i32 @_Z6slow4bii(i32 %x, i32 %y) {
        %tmp = icmp sgt i32 %x, %y
        %retval = select i1 %tmp, i32 %x, i32 %y
        ret i32 %retval
}

__Z6slow4bii:
        cmp r0, r1
        movgt r1, r0
        mov r0, r1
        bx lr
=>

__Z6slow4bii:
        cmp r0, r1
        movle r0, r1
        bx lr

//===---------------------------------------------------------------------===//

Implement long long "X-3" with instructions that fold the immediate in.  These
were disabled due to badness with the ARM carry flag on subtracts.

//===---------------------------------------------------------------------===//

More load / store optimizations:
1) Better representation for block transfer? This is from Olden/power:

	fldd d0, [r4]
	fstd d0, [r4, #+32]
	fldd d0, [r4, #+8]
	fstd d0, [r4, #+40]
	fldd d0, [r4, #+16]
	fstd d0, [r4, #+48]
	fldd d0, [r4, #+24]
	fstd d0, [r4, #+56]

If we can spare the registers, it would be better to use fldm and fstm here.
Need major register allocator enhancement though.

2) Can we recognize the relative position of constantpool entries? i.e. Treat

	ldr r0, LCPI17_3
	ldr r1, LCPI17_4
	ldr r2, LCPI17_5

   as
	ldr r0, LCPI17
	ldr r1, LCPI17+4
	ldr r2, LCPI17+8

   Then the ldr's can be combined into a single ldm. See Olden/power.

Note for ARM v4 gcc uses ldmia to load a pair of 32-bit values to represent a
double 64-bit FP constant:

	adr	r0, L6
	ldmia	r0, {r0-r1}

	.align 2
L6:
	.long	-858993459
	.long	1074318540

3) struct copies appear to be done field by field 
instead of by words, at least sometimes:

struct foo { int x; short s; char c1; char c2; };
void cpy(struct foo*a, struct foo*b) { *a = *b; }

llvm code (-O2)
        ldrb r3, [r1, #+6]
        ldr r2, [r1]
        ldrb r12, [r1, #+7]
        ldrh r1, [r1, #+4]
        str r2, [r0]
        strh r1, [r0, #+4]
        strb r3, [r0, #+6]
        strb r12, [r0, #+7]
gcc code (-O2)
        ldmia   r1, {r1-r2}
        stmia   r0, {r1-r2}

In this benchmark poor handling of aggregate copies has shown up as
having a large effect on size, and possibly speed as well (we don't have
a good way to measure on ARM).

//===---------------------------------------------------------------------===//

* Consider this silly example:

double bar(double x) {  
  double r = foo(3.1);
  return x+r;
}

_bar:
        stmfd sp!, {r4, r5, r7, lr}
        add r7, sp, #8
        mov r4, r0
        mov r5, r1
        fldd d0, LCPI1_0
        fmrrd r0, r1, d0
        bl _foo
        fmdrr d0, r4, r5
        fmsr s2, r0
        fsitod d1, s2
        faddd d0, d1, d0
        fmrrd r0, r1, d0
        ldmfd sp!, {r4, r5, r7, pc}

Ignore the prologue and epilogue stuff for a second. Note 
	mov r4, r0
	mov r5, r1
the copys to callee-save registers and the fact they are only being used by the
fmdrr instruction. It would have been better had the fmdrr been scheduled
before the call and place the result in a callee-save DPR register. The two
mov ops would not have been necessary.

//===---------------------------------------------------------------------===//

Calling convention related stuff:

* gcc's parameter passing implementation is terrible and we suffer as a result:

e.g.
struct s {
  double d1;
  int s1;
};

void foo(struct s S) {
  printf("%g, %d\n", S.d1, S.s1);
}

'S' is passed via registers r0, r1, r2. But gcc stores them to the stack, and
then reload them to r1, r2, and r3 before issuing the call (r0 contains the
address of the format string):

	stmfd	sp!, {r7, lr}
	add	r7, sp, #0
	sub	sp, sp, #12
	stmia	sp, {r0, r1, r2}
	ldmia	sp, {r1-r2}
	ldr	r0, L5
	ldr	r3, [sp, #8]
L2:
	add	r0, pc, r0
	bl	L_printf$stub

Instead of a stmia, ldmia, and a ldr, wouldn't it be better to do three moves?

* Return an aggregate type is even worse:

e.g.
struct s foo(void) {
  struct s S = {1.1, 2};
  return S;
}

	mov	ip, r0
	ldr	r0, L5
	sub	sp, sp, #12
L2:
	add	r0, pc, r0
	@ lr needed for prologue
	ldmia	r0, {r0, r1, r2}
	stmia	sp, {r0, r1, r2}
	stmia	ip, {r0, r1, r2}
	mov	r0, ip
	add	sp, sp, #12
	bx	lr

r0 (and later ip) is the hidden parameter from caller to store the value in. The
first ldmia loads the constants into r0, r1, r2. The last stmia stores r0, r1,
r2 into the address passed in. However, there is one additional stmia that
stores r0, r1, and r2 to some stack location. The store is dead.

The llvm-gcc generated code looks like this:

csretcc void %foo(%struct.s* %agg.result) {
entry:
	%S = alloca %struct.s, align 4		; <%struct.s*> [#uses=1]
	%memtmp = alloca %struct.s		; <%struct.s*> [#uses=1]
	cast %struct.s* %S to sbyte*		; <sbyte*>:0 [#uses=2]
	call void %llvm.memcpy.i32( sbyte* %0, sbyte* cast ({ double, int }* %C.0.904 to sbyte*), uint 12, uint 4 )
	cast %struct.s* %agg.result to sbyte*		; <sbyte*>:1 [#uses=2]
	call void %llvm.memcpy.i32( sbyte* %1, sbyte* %0, uint 12, uint 0 )
	cast %struct.s* %memtmp to sbyte*		; <sbyte*>:2 [#uses=1]
	call void %llvm.memcpy.i32( sbyte* %2, sbyte* %1, uint 12, uint 0 )
	ret void
}

llc ends up issuing two memcpy's (the first memcpy becomes 3 loads from
constantpool). Perhaps we should 1) fix llvm-gcc so the memcpy is translated
into a number of load and stores, or 2) custom lower memcpy (of small size) to
be ldmia / stmia. I think option 2 is better but the current register
allocator cannot allocate a chunk of registers at a time.

A feasible temporary solution is to use specific physical registers at the
lowering time for small (<= 4 words?) transfer size.

* ARM CSRet calling convention requires the hidden argument to be returned by
the callee.

//===---------------------------------------------------------------------===//

We can definitely do a better job on BB placements to eliminate some branches.
It's very common to see llvm generated assembly code that looks like this:

LBB3:
 ...
LBB4:
...
  beq LBB3
  b LBB2

If BB4 is the only predecessor of BB3, then we can emit BB3 after BB4. We can
then eliminate beq and and turn the unconditional branch to LBB2 to a bne.

See McCat/18-imp/ComputeBoundingBoxes for an example.

//===---------------------------------------------------------------------===//

Pre-/post- indexed load / stores:

1) We should not make the pre/post- indexed load/store transform if the base ptr
is guaranteed to be live beyond the load/store. This can happen if the base
ptr is live out of the block we are performing the optimization. e.g.

mov r1, r2
ldr r3, [r1], #4
...

vs.

ldr r3, [r2]
add r1, r2, #4
...

In most cases, this is just a wasted optimization. However, sometimes it can
negatively impact the performance because two-address code is more restrictive
when it comes to scheduling.

Unfortunately, liveout information is currently unavailable during DAG combine
time.

2) Consider spliting a indexed load / store into a pair of add/sub + load/store
   to solve #1 (in TwoAddressInstructionPass.cpp).

3) Enhance LSR to generate more opportunities for indexed ops.

4) Once we added support for multiple result patterns, write indexed loads
   patterns instead of C++ instruction selection code.

5) Use VLDM / VSTM to emulate indexed FP load / store.

//===---------------------------------------------------------------------===//

Implement support for some more tricky ways to materialize immediates.  For
example, to get 0xffff8000, we can use:

mov r9, #&3f8000
sub r9, r9, #&400000

//===---------------------------------------------------------------------===//

We sometimes generate multiple add / sub instructions to update sp in prologue
and epilogue if the inc / dec value is too large to fit in a single immediate
operand. In some cases, perhaps it might be better to load the value from a
constantpool instead.

//===---------------------------------------------------------------------===//

GCC generates significantly better code for this function.

int foo(int StackPtr, unsigned char *Line, unsigned char *Stack, int LineLen) {
    int i = 0;

    if (StackPtr != 0) {
       while (StackPtr != 0 && i < (((LineLen) < (32768))? (LineLen) : (32768)))
          Line[i++] = Stack[--StackPtr];
        if (LineLen > 32768)
        {
            while (StackPtr != 0 && i < LineLen)
            {
                i++;
                --StackPtr;
            }
        }
    }
    return StackPtr;
}

//===---------------------------------------------------------------------===//

This should compile to the mlas instruction:
int mlas(int x, int y, int z) { return ((x * y + z) < 0) ? 7 : 13; }

//===---------------------------------------------------------------------===//

At some point, we should triage these to see if they still apply to us:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19598
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18560
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27016

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11831
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11826
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11825
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11824
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11823
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11820
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10982

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10242
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9831
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9760
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9759
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9703
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9702
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9663

http://www.inf.u-szeged.hu/gcc-arm/
http://citeseer.ist.psu.edu/debus04linktime.html

//===---------------------------------------------------------------------===//

gcc generates smaller code for this function at -O2 or -Os:

void foo(signed char* p) {
  if (*p == 3)
     bar();
   else if (*p == 4)
    baz();
  else if (*p == 5)
    quux();
}

llvm decides it's a good idea to turn the repeated if...else into a
binary tree, as if it were a switch; the resulting code requires -1 
compare-and-branches when *p<=2 or *p==5, the same number if *p==4
or *p>6, and +1 if *p==3.  So it should be a speed win
(on balance).  However, the revised code is larger, with 4 conditional 
branches instead of 3.

More seriously, there is a byte->word extend before
each comparison, where there should be only one, and the condition codes
are not remembered when the same two values are compared twice.

//===---------------------------------------------------------------------===//

More LSR enhancements possible:

1. Teach LSR about pre- and post- indexed ops to allow iv increment be merged
   in a load / store.
2. Allow iv reuse even when a type conversion is required. For example, i8
   and i32 load / store addressing modes are identical.


//===---------------------------------------------------------------------===//

This:

int foo(int a, int b, int c, int d) {
  long long acc = (long long)a * (long long)b;
  acc += (long long)c * (long long)d;
  return (int)(acc >> 32);
}

Should compile to use SMLAL (Signed Multiply Accumulate Long) which multiplies 
two signed 32-bit values to produce a 64-bit value, and accumulates this with 
a 64-bit value.

We currently get this with both v4 and v6:

_foo:
        smull r1, r0, r1, r0
        smull r3, r2, r3, r2
        adds r3, r3, r1
        adc r0, r2, r0
        bx lr

//===---------------------------------------------------------------------===//

This:
        #include <algorithm>
        std::pair<unsigned, bool> full_add(unsigned a, unsigned b)
        { return std::make_pair(a + b, a + b < a); }
        bool no_overflow(unsigned a, unsigned b)
        { return !full_add(a, b).second; }

Should compile to:

_Z8full_addjj:
	adds	r2, r1, r2
	movcc	r1, #0
	movcs	r1, #1
	str	r2, [r0, #0]
	strb	r1, [r0, #4]
	mov	pc, lr

_Z11no_overflowjj:
	cmn	r0, r1
	movcs	r0, #0
	movcc	r0, #1
	mov	pc, lr

not:

__Z8full_addjj:
        add r3, r2, r1
        str r3, [r0]
        mov r2, #1
        mov r12, #0
        cmp r3, r1
        movlo r12, r2
        str r12, [r0, #+4]
        bx lr
__Z11no_overflowjj:
        add r3, r1, r0
        mov r2, #1
        mov r1, #0
        cmp r3, r0
        movhs r1, r2
        mov r0, r1
        bx lr

//===---------------------------------------------------------------------===//

Some of the NEON intrinsics may be appropriate for more general use, either
as target-independent intrinsics or perhaps elsewhere in the ARM backend.
Some of them may also be lowered to target-independent SDNodes, and perhaps
some new SDNodes could be added.

For example, maximum, minimum, and absolute value operations are well-defined
and standard operations, both for vector and scalar types.

The current NEON-specific intrinsics for count leading zeros and count one
bits could perhaps be replaced by the target-independent ctlz and ctpop
intrinsics.  It may also make sense to add a target-independent "ctls"
intrinsic for "count leading sign bits".  Likewise, the backend could use
the target-independent SDNodes for these operations.

ARMv6 has scalar saturating and halving adds and subtracts.  The same
intrinsics could possibly be used for both NEON's vector implementations of
those operations and the ARMv6 scalar versions.

//===---------------------------------------------------------------------===//

ARM::MOVCCr is commutable (by flipping the condition). But we need to implement
ARMInstrInfo::commuteInstruction() to support it.

//===---------------------------------------------------------------------===//

Split out LDR (literal) from normal ARM LDR instruction. Also consider spliting
LDR into imm12 and so_reg forms. This allows us to clean up some code. e.g.
ARMLoadStoreOptimizer does not need to look at LDR (literal) and LDR (so_reg)
while ARMConstantIslandPass only need to worry about LDR (literal).

//===---------------------------------------------------------------------===//

Constant island pass should make use of full range SoImm values for LEApcrel.
Be careful though as the last attempt caused infinite looping on lencod.

//===---------------------------------------------------------------------===//

Predication issue. This function:   

extern unsigned array[ 128 ];
int     foo( int x ) {
  int     y;
  y = array[ x & 127 ];
  if ( x & 128 )
     y = 123456789 & ( y >> 2 );
  else
     y = 123456789 & y;
  return y;
}

compiles to:

_foo:
	and r1, r0, #127
	ldr r2, LCPI1_0
	ldr r2, [r2]
	ldr r1, [r2, +r1, lsl #2]
	mov r2, r1, lsr #2
	tst r0, #128
	moveq r2, r1
	ldr r0, LCPI1_1
	and r0, r2, r0
	bx lr

It would be better to do something like this, to fold the shift into the
conditional move:

	and r1, r0, #127
	ldr r2, LCPI1_0
	ldr r2, [r2]
	ldr r1, [r2, +r1, lsl #2]
	tst r0, #128
	movne r1, r1, lsr #2
	ldr r0, LCPI1_1
	and r0, r1, r0
	bx lr

it saves an instruction and a register.

//===---------------------------------------------------------------------===//

It might be profitable to cse MOVi16 if there are lots of 32-bit immediates
with the same bottom half.

//===---------------------------------------------------------------------===//

Robert Muth started working on an alternate jump table implementation that
does not put the tables in-line in the text.  This is more like the llvm
default jump table implementation.  This might be useful sometime.  Several
revisions of patches are on the mailing list, beginning at:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2009-June/022763.html

//===---------------------------------------------------------------------===//

Make use of the "rbit" instruction.

//===---------------------------------------------------------------------===//

Take a look at test/CodeGen/Thumb2/machine-licm.ll. ARM should be taught how
to licm and cse the unnecessary load from cp#1.

//===---------------------------------------------------------------------===//

The CMN instruction sets the flags like an ADD instruction, while CMP sets
them like a subtract. Therefore to be able to use CMN for comparisons other
than the Z bit, we'll need additional logic to reverse the conditionals
associated with the comparison. Perhaps a pseudo-instruction for the comparison,
with a post-codegen pass to clean up and handle the condition codes?
See PR5694 for testcase.

//===---------------------------------------------------------------------===//

Given the following on armv5:
int test1(int A, int B) {
  return (A&-8388481)|(B&8388480);
}

We currently generate:
	ldr	r2, .LCPI0_0
	and	r0, r0, r2
	ldr	r2, .LCPI0_1
	and	r1, r1, r2
	orr	r0, r1, r0
	bx	lr

We should be able to replace the second ldr+and with a bic (i.e. reuse the
constant which was already loaded).  Not sure what's necessary to do that.

//===---------------------------------------------------------------------===//

The code generated for bswap on armv4/5 (CPUs without rev) is less than ideal:

int a(int x) { return __builtin_bswap32(x); }

a:
	mov	r1, #255, 24
	mov	r2, #255, 16
	and	r1, r1, r0, lsr #8
	and	r2, r2, r0, lsl #8
	orr	r1, r1, r0, lsr #24
	orr	r0, r2, r0, lsl #24
	orr	r0, r0, r1
	bx	lr

Something like the following would be better (fewer instructions/registers):
	eor     r1, r0, r0, ror #16
	bic     r1, r1, #0xff0000
	mov     r1, r1, lsr #8
	eor     r0, r1, r0, ror #8
	bx	lr

A custom Thumb version would also be a slight improvement over the generic
version.

//===---------------------------------------------------------------------===//

Consider the following simple C code:

void foo(unsigned char *a, unsigned char *b, int *c) {
 if ((*a | *b) == 0) *c = 0;
}

currently llvm-gcc generates something like this (nice branchless code I'd say):

       ldrb    r0, [r0]
       ldrb    r1, [r1]
       orr     r0, r1, r0
       tst     r0, #255
       moveq   r0, #0
       streq   r0, [r2]
       bx      lr

Note that both "tst" and "moveq" are redundant.

//===---------------------------------------------------------------------===//