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add a note, I have no idea how important this is.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25907 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2006-02-02 19:16:34 +00:00
parent c8c0bb00a3
commit 9acddcd07e
1 changed files with 24 additions and 0 deletions
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24
lib/Target/X86/README.txt
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@ -350,3 +350,27 @@ much sense (e.g. its an infinite loop). :)
None of the FPStack instructions are handled in
X86RegisterInfo::foldMemoryOperand, which prevents the spiller from
folding spill code into the instructions.
//===---------------------------------------------------------------------===//
In many cases, LLVM generates code like this:
_test:
movl 8(%esp), %eax
cmpl %eax, 4(%esp)
setl %al
movzbl %al, %eax
ret
on some processors (which ones?), it is more efficient to do this:
_test:
movl 8(%esp), %ebx
xor %eax, %eax
cmpl %ebx, 4(%esp)
setl %al
ret
Doing this correctly is tricky though, as the xor clobbers the flags.