[llvm-dev] LLVM 3.9 RC2's SCCP pass removing calls to external functions?!

[Sanjoy Das via llvm-dev]

Btw, looks like the bug is in runSCCP in

    for (BasicBlock::iterator BI = BB.begin(), E = BB.end(); BI != E;) {
      Instruction *Inst = &*BI++;
      if (Inst->getType()->isVoidTy() || isa<TerminatorInst>(Inst))
        continue;

      if (tryToReplaceInstWithConstant(Solver, Inst,
                                       true /* shouldEraseFromParent */)) {
        // Hey, we just changed something!
        MadeChanges = true;
        ++NumInstRemoved;
      }

It should not be passing in true for shouldEraseFromParent.  I think
the right fix here is to not have the shouldEraseFromParent parameter
at all, but in tryToReplaceInstWithConstant to do:

  // replace Inst with constant
  llvm::RecursivelyDeleteTriviallyDeadInstructions(Inst, TLI);

Let me know if you want to take a crack at this, otherwise I'll get to
it this week.

-- Sanjoy


On Wed, Aug 24, 2016 at 12:12 AM, Sanjoy Das
<sanjoy at playingwithpointers.com> wrote:
> Hi Félix,
>
> Sanjoy Das wrote:
>> Félix Cloutier via llvm-dev wrote:
>>  > Assuming that this is a bug, what are the next steps?
>>
>> Looks like you already have a very small test case -- have you tried
>> sticking it in a debugger to see why SCCP thinks removing the call is
>> okay?
>>
>> Alternatively, file a bug at llvm.org/bugs and someone will get to it.
>
> The third thing you can do is run a bisection to find the problematic
> commit, since you said this was a regression.
>
> -- Sanjoy



-- 
Sanjoy Das
http://playingwithpointers.com