[llvm-dev] funnel shift, select, and poison (original) (raw)
John Regehr via llvm-dev llvm-dev at lists.llvm.org
Mon Feb 25 11:24:22 PST 2019
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Consider this:
%v0 = call i32 @foo(poison) nounwind/readnone store i32 %v0, i32* %validaddress If we assume that poison propagates through calls, we could then optimize this to %v0 = poison store poison, i32* %validaddress
This is a sound transformation only if foo() returns poison when it is called with poison as an argument.
If this is what foo() looks like:
define i32 @foo(i32) { ret i32 0 }
then of course the transformation above is wrong.
John
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