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add a horrible note 

llvm-svn: 64719
This commit is contained in:
Chris Lattner 2009-02-17 01:16:14 +00:00
parent 31180bbfb4
commit cba4b6f83d
1 changed files with 41 additions and 0 deletions
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41
llvm/lib/Target/X86/README.txt
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@ -1777,3 +1777,44 @@ LBB1_2: ## overflow
it would be nice to produce "into" someday.
//===---------------------------------------------------------------------===//
This code:
void vec_mpys1(int y[], const int x[], int scaler) {
int i;
for (i = 0; i < 150; i++)
y[i] += (((long long)scaler * (long long)x[i]) >> 31);
}
Compiles to this loop with GCC 3.x:
.L5:
movl %ebx, %eax
imull (%edi,%ecx,4)
shrdl $31, %edx, %eax
addl %eax, (%esi,%ecx,4)
incl %ecx
cmpl $149, %ecx
jle .L5
llvm-gcc compiles it to the much uglier:
LBB1_1: ## bb1
movl 24(%esp), %eax
movl (%eax,%edi,4), %ebx
movl %ebx, %ebp
imull %esi, %ebp
movl %ebx, %eax
mull %ecx
addl %ebp, %edx
sarl $31, %ebx
imull %ecx, %ebx
addl %edx, %ebx
shldl $1, %eax, %ebx
movl 20(%esp), %eax
addl %ebx, (%eax,%edi,4)
incl %edi
cmpl $150, %edi
jne LBB1_1 ## bb1
//===---------------------------------------------------------------------===//