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| author | Dan Gohman <gohman@apple.com> | 2011-06-16 15:55:48 +0000 |
|---|---|---|
| committer | Dan Gohman <gohman@apple.com> | 2011-06-16 15:55:48 +0000 |
| commit | a0697a7ef5a980727d8361acd1fc429a5b1b8e2b (patch) | |
| tree | 627ceda3603cc6f949599b26e6f1f54fec9a2288 /lib/Target/X86/X86InstrCompiler.td | |
| parent | ec3141b27f695585957cb045601e715c367a0cde (diff) | |
| download | external_llvm-a0697a7ef5a980727d8361acd1fc429a5b1b8e2b.zip external_llvm-a0697a7ef5a980727d8361acd1fc429a5b1b8e2b.tar.gz external_llvm-a0697a7ef5a980727d8361acd1fc429a5b1b8e2b.tar.bz2 | |
Add a comment describing why transforming (shl x, 1) to (add x, x) is to be
considered safe enough in this context.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133159 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib/Target/X86/X86InstrCompiler.td')
| -rw-r--r-- | lib/Target/X86/X86InstrCompiler.td | 5 |
1 files changed, 5 insertions, 0 deletions
diff --git a/lib/Target/X86/X86InstrCompiler.td b/lib/Target/X86/X86InstrCompiler.td index 33534cd..adcc747 100644 --- a/lib/Target/X86/X86InstrCompiler.td +++ b/lib/Target/X86/X86InstrCompiler.td @@ -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)>; |