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add a note about overflow idiom recognition.

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llvm-svn: 121853
This commit is contained in:
Chris Lattner 2010-12-15 07:25:55 +00:00
parent 102a0b7b74
commit ec899c30d9
1 changed files with 19 additions and 1 deletions
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20
lib/Target/README.txt
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@ -73,7 +73,25 @@ This has a number of uses:
//===---------------------------------------------------------------------===//
Make the PPC branch selector target independant
We should recognized various "overflow detection" idioms and translate them into
llvm.uadd.with.overflow and similar intrinsics. For example, we compile this:
size_t add(size_t a,size_t b) {
if (a+b<a)
exit(0);
return a+b;
}
into:
addq %rdi, %rbx
cmpq %rdi, %rbx
jae LBB0_2
when it would be better to generate:
addq %rdi, %rbx
jno LBB0_2
//===---------------------------------------------------------------------===//