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Add a note from llvmdev, this time with more info.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122966 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer 2011-01-06 17:35:50 +00:00
parent 4bb46f43eb
commit fa36680076
1 changed files with 26 additions and 0 deletions
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26
lib/Target/README.txt
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@ -2047,3 +2047,29 @@ entry:
}
//===---------------------------------------------------------------------===//
clang -O3 currently compiles this code
int g(unsigned int a) {
unsigned int c[100];
c[10] = a;
c[11] = a;
unsigned int b = c[10] + c[11];
if(b > a*2) a = 4;
else a = 8;
return a + 7;
}
into
define i32 @g(i32 a) nounwind readnone {
%add = shl i32 %a, 1
%mul = shl i32 %a, 1
%cmp = icmp ugt i32 %add, %mul
%a.addr.0 = select i1 %cmp, i32 11, i32 15
ret i32 %a.addr.0
}
The icmp should fold to false. This CSE opportunity is only available
after GVN and InstCombine have run.
//===---------------------------------------------------------------------===//