[LLVMdev] InstructionCombining.cpp inconsistency in whether it modifies the CFG?

[Duncan Sands]

Hi Michael,

> InstCombine says in its getAnalysisUsage that it preserves the CFG,
> but for the 4th argument in its INITIALIZE_PASS call, it says false,
> which I believe corresponds to whether it preserves the CFG.

that argument should be set to true if the pass only looks at the CFG, i.e.
whatever it computes/does is only a function of the CFG, and doesn't otherwise
depend on what particular instructions make up the function.  Suppose an
analysis pass A has this property, and a transform pass P announces (using
setPreservesCFG()) that it does not change the CFG.  Then A does not need
to be recomputed after P has run.

So I think it is correct for InstCombine to not set this flag.

Ciao, Duncan.

  Is this a
> mistake, or is there deeper meaning here?
>
> InstructionCombining.cpp:73-82
>
> char InstCombiner::ID = 0;
> INITIALIZE_PASS(InstCombiner, "instcombine",
>                  "Combine redundant instructions", false, false)
>
> void InstCombiner::getAnalysisUsage(AnalysisUsage&AU) const {
>    AU.addPreservedID(LCSSAID);
>    AU.setPreservesCFG();
> }
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