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Add a comment describing why transforming (shl x, 1) to (add x, x) is to be

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considered safe enough in this context.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133159 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2011-06-16 15:55:48 +00:00
parent ec3141b27f
commit a0697a7ef5
1 changed files with 5 additions and 0 deletions
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5
lib/Target/X86/X86InstrCompiler.td
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@ -1368,6 +1368,11 @@ def : Pat<(store (i8 (trunc_su (srl_su GR16:$src, (i8 8)))), addr:$dst),
// (shl x, 1) ==> (add x, x)
// Note that if x is undef (immediate or otherwise), we could theoretically
// end up with the two uses of x getting different values, producing a result
// where the least significant bit is not 0. However, the probability of this
// happening is considered low enough that this is officially not a
// "real problem".
def : Pat<(shl GR8 :$src1, (i8 1)), (ADD8rr GR8 :$src1, GR8 :$src1)>;
def : Pat<(shl GR16:$src1, (i8 1)), (ADD16rr GR16:$src1, GR16:$src1)>;
def : Pat<(shl GR32:$src1, (i8 1)), (ADD32rr GR32:$src1, GR32:$src1)>;