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llvm-6502/lib/Target/X86/README-SSE.txt
Chris Lattner dbec2d9dcf a note 
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34670 91177308-0d34-0410-b5e6-96231b3b80d8
2007-02-27 17:21:09 +00:00

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//===---------------------------------------------------------------------===//
// Random ideas for the X86 backend: SSE-specific stuff.
//===---------------------------------------------------------------------===//
//===---------------------------------------------------------------------===//
Expand libm rounding functions inline: Significant speedups possible.
http://gcc.gnu.org/ml/gcc-patches/2006-10/msg00909.html
//===---------------------------------------------------------------------===//
When compiled with unsafemath enabled, "main" should enable SSE DAZ mode and
other fast SSE modes.
//===---------------------------------------------------------------------===//
Think about doing i64 math in SSE regs.
//===---------------------------------------------------------------------===//
This testcase should have no SSE instructions in it, and only one load from
a constant pool:
double %test3(bool %B) {
%C = select bool %B, double 123.412, double 523.01123123
ret double %C
}
Currently, the select is being lowered, which prevents the dag combiner from
turning 'select (load CPI1), (load CPI2)' -> 'load (select CPI1, CPI2)'
The pattern isel got this one right.
//===---------------------------------------------------------------------===//
SSE doesn't have [mem] op= reg instructions. If we have an SSE instruction
like this:
X += y
and the register allocator decides to spill X, it is cheaper to emit this as:
Y += [xslot]
store Y -> [xslot]
than as:
tmp = [xslot]
tmp += y
store tmp -> [xslot]
..and this uses one fewer register (so this should be done at load folding
time, not at spiller time). *Note* however that this can only be done
if Y is dead. Here's a testcase:
%.str_3 = external global [15 x sbyte] ; <[15 x sbyte]*> [#uses=0]
implementation ; Functions:
declare void %printf(int, ...)
void %main() {
build_tree.exit:
br label %no_exit.i7
no_exit.i7: ; preds = %no_exit.i7, %build_tree.exit
%tmp.0.1.0.i9 = phi double [ 0.000000e+00, %build_tree.exit ], [ %tmp.34.i18, %no_exit.i7 ] ; <double> [#uses=1]
%tmp.0.0.0.i10 = phi double [ 0.000000e+00, %build_tree.exit ], [ %tmp.28.i16, %no_exit.i7 ] ; <double> [#uses=1]
%tmp.28.i16 = add double %tmp.0.0.0.i10, 0.000000e+00
%tmp.34.i18 = add double %tmp.0.1.0.i9, 0.000000e+00
br bool false, label %Compute_Tree.exit23, label %no_exit.i7
Compute_Tree.exit23: ; preds = %no_exit.i7
tail call void (int, ...)* %printf( int 0 )
store double %tmp.34.i18, double* null
ret void
}
We currently emit:
.BBmain_1:
xorpd %XMM1, %XMM1
addsd %XMM0, %XMM1
*** movsd %XMM2, QWORD PTR [%ESP + 8]
*** addsd %XMM2, %XMM1
*** movsd QWORD PTR [%ESP + 8], %XMM2
jmp .BBmain_1 # no_exit.i7
This is a bugpoint reduced testcase, which is why the testcase doesn't make
much sense (e.g. its an infinite loop). :)
//===---------------------------------------------------------------------===//
SSE should implement 'select_cc' using 'emulated conditional moves' that use
pcmp/pand/pandn/por to do a selection instead of a conditional branch:
double %X(double %Y, double %Z, double %A, double %B) {
%C = setlt double %A, %B
%z = add double %Z, 0.0 ;; select operand is not a load
%D = select bool %C, double %Y, double %z
ret double %D
}
We currently emit:
_X:
subl $12, %esp
xorpd %xmm0, %xmm0
addsd 24(%esp), %xmm0
movsd 32(%esp), %xmm1
movsd 16(%esp), %xmm2
ucomisd 40(%esp), %xmm1
jb LBB_X_2
LBB_X_1:
movsd %xmm0, %xmm2
LBB_X_2:
movsd %xmm2, (%esp)
fldl (%esp)
addl $12, %esp
ret
//===---------------------------------------------------------------------===//
It's not clear whether we should use pxor or xorps / xorpd to clear XMM
registers. The choice may depend on subtarget information. We should do some
more experiments on different x86 machines.
//===---------------------------------------------------------------------===//
Currently the x86 codegen isn't very good at mixing SSE and FPStack
code:
unsigned int foo(double x) { return x; }
foo:
subl $20, %esp
movsd 24(%esp), %xmm0
movsd %xmm0, 8(%esp)
fldl 8(%esp)
fisttpll (%esp)
movl (%esp), %eax
addl $20, %esp
ret
This will be solved when we go to a dynamic programming based isel.
//===---------------------------------------------------------------------===//
Lower memcpy / memset to a series of SSE 128 bit move instructions when it's
feasible.
//===---------------------------------------------------------------------===//
Teach the coalescer to commute 2-addr instructions, allowing us to eliminate
the reg-reg copy in this example:
float foo(int *x, float *y, unsigned c) {
float res = 0.0;
unsigned i;
for (i = 0; i < c; i++) {
float xx = (float)x[i];
xx = xx * y[i];
xx += res;
res = xx;
}
return res;
}
LBB_foo_3: # no_exit
cvtsi2ss %XMM0, DWORD PTR [%EDX + 4*%ESI]
mulss %XMM0, DWORD PTR [%EAX + 4*%ESI]
addss %XMM0, %XMM1
inc %ESI
cmp %ESI, %ECX
**** movaps %XMM1, %XMM0
jb LBB_foo_3 # no_exit
//===---------------------------------------------------------------------===//
Codegen:
if (copysign(1.0, x) == copysign(1.0, y))
into:
if (x^y & mask)
when using SSE.
//===---------------------------------------------------------------------===//
Use movhps to update upper 64-bits of a v4sf value. Also movlps on lower half
of a v4sf value.
//===---------------------------------------------------------------------===//
Better codegen for vector_shuffles like this { x, 0, 0, 0 } or { x, 0, x, 0}.
Perhaps use pxor / xorp* to clear a XMM register first?
//===---------------------------------------------------------------------===//
How to decide when to use the "floating point version" of logical ops? Here are
some code fragments:
movaps LCPI5_5, %xmm2
divps %xmm1, %xmm2
mulps %xmm2, %xmm3
mulps 8656(%ecx), %xmm3
addps 8672(%ecx), %xmm3
andps LCPI5_6, %xmm2
andps LCPI5_1, %xmm3
por %xmm2, %xmm3
movdqa %xmm3, (%edi)
movaps LCPI5_5, %xmm1
divps %xmm0, %xmm1
mulps %xmm1, %xmm3
mulps 8656(%ecx), %xmm3
addps 8672(%ecx), %xmm3
andps LCPI5_6, %xmm1
andps LCPI5_1, %xmm3
orps %xmm1, %xmm3
movaps %xmm3, 112(%esp)
movaps %xmm3, (%ebx)
Due to some minor source change, the later case ended up using orps and movaps
instead of por and movdqa. Does it matter?
//===---------------------------------------------------------------------===//
X86RegisterInfo::copyRegToReg() returns X86::MOVAPSrr for VR128. Is it possible
to choose between movaps, movapd, and movdqa based on types of source and
destination?
How about andps, andpd, and pand? Do we really care about the type of the packed
elements? If not, why not always use the "ps" variants which are likely to be
shorter.
//===---------------------------------------------------------------------===//
External test Nurbs exposed some problems. Look for
__ZN15Nurbs_SSE_Cubic17TessellateSurfaceE, bb cond_next140. This is what icc
emits:
movaps (%edx), %xmm2 #59.21
movaps (%edx), %xmm5 #60.21
movaps (%edx), %xmm4 #61.21
movaps (%edx), %xmm3 #62.21
movl 40(%ecx), %ebp #69.49
shufps $0, %xmm2, %xmm5 #60.21
movl 100(%esp), %ebx #69.20
movl (%ebx), %edi #69.20
imull %ebp, %edi #69.49
addl (%eax), %edi #70.33
shufps $85, %xmm2, %xmm4 #61.21
shufps $170, %xmm2, %xmm3 #62.21
shufps $255, %xmm2, %xmm2 #63.21
lea (%ebp,%ebp,2), %ebx #69.49
negl %ebx #69.49
lea -3(%edi,%ebx), %ebx #70.33
shll $4, %ebx #68.37
addl 32(%ecx), %ebx #68.37
testb $15, %bl #91.13
jne L_B1.24 # Prob 5% #91.13
This is the llvm code after instruction scheduling:
cond_next140 (0xa910740, LLVM BB @0xa90beb0):
%reg1078 = MOV32ri -3
%reg1079 = ADD32rm %reg1078, %reg1068, 1, %NOREG, 0
%reg1037 = MOV32rm %reg1024, 1, %NOREG, 40
%reg1080 = IMUL32rr %reg1079, %reg1037
%reg1081 = MOV32rm %reg1058, 1, %NOREG, 0
%reg1038 = LEA32r %reg1081, 1, %reg1080, -3
%reg1036 = MOV32rm %reg1024, 1, %NOREG, 32
%reg1082 = SHL32ri %reg1038, 4
%reg1039 = ADD32rr %reg1036, %reg1082
%reg1083 = MOVAPSrm %reg1059, 1, %NOREG, 0
%reg1034 = SHUFPSrr %reg1083, %reg1083, 170
%reg1032 = SHUFPSrr %reg1083, %reg1083, 0
%reg1035 = SHUFPSrr %reg1083, %reg1083, 255
%reg1033 = SHUFPSrr %reg1083, %reg1083, 85
%reg1040 = MOV32rr %reg1039
%reg1084 = AND32ri8 %reg1039, 15
CMP32ri8 %reg1084, 0
JE mbb<cond_next204,0xa914d30>
Still ok. After register allocation:
cond_next140 (0xa910740, LLVM BB @0xa90beb0):
%EAX = MOV32ri -3
%EDX = MOV32rm <fi#3>, 1, %NOREG, 0
ADD32rm %EAX<def&use>, %EDX, 1, %NOREG, 0
%EDX = MOV32rm <fi#7>, 1, %NOREG, 0
%EDX = MOV32rm %EDX, 1, %NOREG, 40
IMUL32rr %EAX<def&use>, %EDX
%ESI = MOV32rm <fi#5>, 1, %NOREG, 0
%ESI = MOV32rm %ESI, 1, %NOREG, 0
MOV32mr <fi#4>, 1, %NOREG, 0, %ESI
%EAX = LEA32r %ESI, 1, %EAX, -3
%ESI = MOV32rm <fi#7>, 1, %NOREG, 0
%ESI = MOV32rm %ESI, 1, %NOREG, 32
%EDI = MOV32rr %EAX
SHL32ri %EDI<def&use>, 4
ADD32rr %EDI<def&use>, %ESI
%XMM0 = MOVAPSrm %ECX, 1, %NOREG, 0
%XMM1 = MOVAPSrr %XMM0
SHUFPSrr %XMM1<def&use>, %XMM1, 170
%XMM2 = MOVAPSrr %XMM0
SHUFPSrr %XMM2<def&use>, %XMM2, 0
%XMM3 = MOVAPSrr %XMM0
SHUFPSrr %XMM3<def&use>, %XMM3, 255
SHUFPSrr %XMM0<def&use>, %XMM0, 85
%EBX = MOV32rr %EDI
AND32ri8 %EBX<def&use>, 15
CMP32ri8 %EBX, 0
JE mbb<cond_next204,0xa914d30>
This looks really bad. The problem is shufps is a destructive opcode. Since it
appears as operand two in more than one shufps ops. It resulted in a number of
copies. Note icc also suffers from the same problem. Either the instruction
selector should select pshufd or The register allocator can made the two-address
to three-address transformation.
It also exposes some other problems. See MOV32ri -3 and the spills.
//===---------------------------------------------------------------------===//
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25500
LLVM is producing bad code.
LBB_main_4: # cond_true44
addps %xmm1, %xmm2
subps %xmm3, %xmm2
movaps (%ecx), %xmm4
movaps %xmm2, %xmm1
addps %xmm4, %xmm1
addl $16, %ecx
incl %edx
cmpl $262144, %edx
movaps %xmm3, %xmm2
movaps %xmm4, %xmm3
jne LBB_main_4 # cond_true44
There are two problems. 1) No need to two loop induction variables. We can
compare against 262144 * 16. 2) Known register coalescer issue. We should
be able eliminate one of the movaps:
addps %xmm2, %xmm1 <=== Commute!
subps %xmm3, %xmm1
movaps (%ecx), %xmm4
movaps %xmm1, %xmm1 <=== Eliminate!
addps %xmm4, %xmm1
addl $16, %ecx
incl %edx
cmpl $262144, %edx
movaps %xmm3, %xmm2
movaps %xmm4, %xmm3
jne LBB_main_4 # cond_true44
//===---------------------------------------------------------------------===//
Consider:
__m128 test(float a) {
return _mm_set_ps(0.0, 0.0, 0.0, a*a);
}
This compiles into:
movss 4(%esp), %xmm1
mulss %xmm1, %xmm1
xorps %xmm0, %xmm0
movss %xmm1, %xmm0
ret
Because mulss doesn't modify the top 3 elements, the top elements of
xmm1 are already zero'd. We could compile this to:
movss 4(%esp), %xmm0
mulss %xmm0, %xmm0
ret
//===---------------------------------------------------------------------===//
Here's a sick and twisted idea. Consider code like this:
__m128 test(__m128 a) {
float b = *(float*)&A;
...
return _mm_set_ps(0.0, 0.0, 0.0, b);
}
This might compile to this code:
movaps c(%esp), %xmm1
xorps %xmm0, %xmm0
movss %xmm1, %xmm0
ret
Now consider if the ... code caused xmm1 to get spilled. This might produce
this code:
movaps c(%esp), %xmm1
movaps %xmm1, c2(%esp)
...
xorps %xmm0, %xmm0
movaps c2(%esp), %xmm1
movss %xmm1, %xmm0
ret
However, since the reload is only used by these instructions, we could
"fold" it into the uses, producing something like this:
movaps c(%esp), %xmm1
movaps %xmm1, c2(%esp)
...
movss c2(%esp), %xmm0
ret
... saving two instructions.
The basic idea is that a reload from a spill slot, can, if only one 4-byte
chunk is used, bring in 3 zeros the the one element instead of 4 elements.
This can be used to simplify a variety of shuffle operations, where the
elements are fixed zeros.
//===---------------------------------------------------------------------===//
For this:
#include <emmintrin.h>
void test(__m128d *r, __m128d *A, double B) {
*r = _mm_loadl_pd(*A, &B);
}
We generates:
subl $12, %esp
movsd 24(%esp), %xmm0
movsd %xmm0, (%esp)
movl 20(%esp), %eax
movapd (%eax), %xmm0
movlpd (%esp), %xmm0
movl 16(%esp), %eax
movapd %xmm0, (%eax)
addl $12, %esp
ret
icc generates:
movl 4(%esp), %edx #3.6
movl 8(%esp), %eax #3.6
movapd (%eax), %xmm0 #4.22
movlpd 12(%esp), %xmm0 #4.8
movapd %xmm0, (%edx) #4.3
ret #5.1
So icc is smart enough to know that B is in memory so it doesn't load it and
store it back to stack.
//===---------------------------------------------------------------------===//
__m128d test1( __m128d A, __m128d B) {
return _mm_shuffle_pd(A, B, 0x3);
}
compiles to
shufpd $3, %xmm1, %xmm0
Perhaps it's better to use unpckhpd instead?
unpckhpd %xmm1, %xmm0
Don't know if unpckhpd is faster. But it is shorter.
//===---------------------------------------------------------------------===//
This code generates ugly code, probably due to costs being off or something:
void %test(float* %P, <4 x float>* %P2 ) {
%xFloat0.688 = load float* %P
%loadVector37.712 = load <4 x float>* %P2
%inFloat3.713 = insertelement <4 x float> %loadVector37.712, float 0.000000e+00, uint 3
store <4 x float> %inFloat3.713, <4 x float>* %P2
ret void
}
Generates:
_test:
pxor %xmm0, %xmm0
movd %xmm0, %eax ;; EAX = 0!
movl 8(%esp), %ecx
movaps (%ecx), %xmm0
pinsrw $6, %eax, %xmm0
shrl $16, %eax ;; EAX = 0 again!
pinsrw $7, %eax, %xmm0
movaps %xmm0, (%ecx)
ret
It would be better to generate:
_test:
movl 8(%esp), %ecx
movaps (%ecx), %xmm0
xor %eax, %eax
pinsrw $6, %eax, %xmm0
pinsrw $7, %eax, %xmm0
movaps %xmm0, (%ecx)
ret
or use pxor (to make a zero vector) and shuffle (to insert it).
//===---------------------------------------------------------------------===//
Some useful information in the Apple Altivec / SSE Migration Guide:
http://developer.apple.com/documentation/Performance/Conceptual/
Accelerate_sse_migration/index.html
e.g. SSE select using and, andnot, or. Various SSE compare translations.
//===---------------------------------------------------------------------===//
Add hooks to commute some CMPP operations.
//===---------------------------------------------------------------------===//
Apply the same transformation that merged four float into a single 128-bit load
to loads from constant pool.
//===---------------------------------------------------------------------===//
Floating point max / min are commutable when -enable-unsafe-fp-path is
specified. We should turn int_x86_sse_max_ss and X86ISD::FMIN etc. into other
nodes which are selected to max / min instructions that are marked commutable.
//===---------------------------------------------------------------------===//
We should compile this:
#include <xmmintrin.h>
typedef union {
int i[4];
float f[4];
__m128 v;
} vector4_t;
void swizzle (const void *a, vector4_t * b, vector4_t * c) {
b->v = _mm_loadl_pi (b->v, (__m64 *) a);
c->v = _mm_loadl_pi (c->v, ((__m64 *) a) + 1);
}
to:
_swizzle:
movl 4(%esp), %eax
movl 8(%esp), %edx
movl 12(%esp), %ecx
movlps (%eax), %xmm0
movlps %xmm0, (%edx)
movlps 8(%eax), %xmm0
movlps %xmm0, (%ecx)
ret
not:
swizzle:
movl 8(%esp), %eax
movaps (%eax), %xmm0
movl 4(%esp), %ecx
movlps (%ecx), %xmm0
movaps %xmm0, (%eax)
movl 12(%esp), %eax
movaps (%eax), %xmm0
movlps 8(%ecx), %xmm0
movaps %xmm0, (%eax)
ret
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