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| -rw-r--r-- | lib/Target/X86/README.txt | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/lib/Target/X86/README.txt b/lib/Target/X86/README.txt index 36f287bcf9..c8e359fced 100644 --- a/lib/Target/X86/README.txt +++ b/lib/Target/X86/README.txt @@ -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. + + |