poison and select

Project Infrastructure LLVM Dev List Archives
1

While the documentation may not match the facts on the ground,?InstructionSimplify does (in my opinion) the
right thing; an undef operand doesn't mean the entire SelectInst folds away to undef:
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?revision=217342&view=m
arkup#l2880

It seems pretty clear that this is the right semantics for select.

If we believe the documentation, then select is only useful when we can
prove that it won't introduce a spurious poison value. This will not often
be the case.

I'll go file a bug against the LLVM instruction reference to remind
someone to tweak the doc. My proposed fix would be:

* Values other than phi nodes and select instructions depend on their
operands.

* Phi nodes depend on the operand corresponding to their dynamic
predecessor basic block.

* Select instructions depend on their selected operand.

Doesn't a select instruction also depend on the predicate (boolean) value?

John

--Vikram Adve
Professor, Department of Computer Science
University of Illinois at Urbana-Champaign
vadve@illinois.edu
http://llvm.org

2

* Select instructions depend on their selected operand.

Doesn't a select instruction also depend on the predicate (boolean) value?

Oops, yes, of course. So maybe:

* Select instructions depend on their condition and their selected
   operand.

John