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Added notes about a x86 isel deficiency.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25706 91177308-0d34-0410-b5e6-96231b3b80d8
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Evan Cheng 2006-01-27 22:11:01 +00:00
parent 038c2a1292
commit e826a018b9
1 changed files with 22 additions and 0 deletions
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lib/Target/X86/README.txt
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@ -166,3 +166,25 @@ http://gcc.gnu.org/ml/gcc-patches/2004-08/msg02011.html
Combine: a = sin(x), b = cos(x) into a,b = sincos(x).
//===---------------------------------------------------------------------===//
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. This prevents the folding.