llvm dev - May 2011 - [LLVMdev] InstructionCombining.cpp inconsistency in whether it modifies the CFG?

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. 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();
}

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();
> }
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Ah, I see, thanks for the clarification. I interpreted "Pass only looks at
the CFG" as didn't modify it, only looked at it, but I see now that it
can
also mean "Pass looks only at the CFG", as in not the instructions.

Thanks!

On Mon, May 16, 2011 at 1:58 PM, Duncan Sands <baldrick at free.fr> wrote:
> 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();
> > }
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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