[InstCombine] Print a debug message when we constant fold an operand during worklist creation

[craig.topper]

Summary

InstCombine tries to constant fold instruction operands during worklist building, but we don't print that we're doing this.

We also set a change flag here that causes us to rebuild and rerun the worklist one more time even if processing the worklist itself created no additional changes. So in the log I saw two inst combine runs that visited all instructions without printing that anything was changed. I may be submitting another patch to remove the change flag unless I can find some reason why we should be doing that.

[spatel]

LGTM.

[davide]

Sure, why not. Feel free to commit debug cleanups without pre-commit review.

llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp

for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E; ) {
         auto *C = cast<Constant>(i);
         Constant *&FoldRes = FoldedConstants[C];
         if (!FoldRes)
           FoldRes = ConstantFoldConstant(C, DL, TLI);
         if (!FoldRes)
           FoldRes = C;
 
         if (FoldRes != C) {
+          DEBUG(dbgs() << "IC: ConstFold operand of: " << *Inst << "\n    Old = " << *C
+                       << "\n    New = " << *FoldRes << '\n');
           *i = FoldRes;
           MadeIRChange = true;
         }
       }
 
       InstrsForInstCombineWorklist.push_back(Inst);
     }