[llvm-commits] [llvm] r123060 - in /llvm/trunk: lib/Transforms/Scalar/LoopRotation.cpp test/Transforms/LoopRotate/phi-duplicate.ll

[Duncan Sands]

Hi Chris,

> Have loop-rotate simplify instructions (yay instsimplify!) as it clones
> them into the loop preheader, eliminating silly instructions like
> "icmp i32 0, 100" in fixed tripcount loops.  This also better exposes the
> bigger problem with loop rotate that I'd like to fix: once this has been
> folded, the duplicated conditional branch *often* turns into an uncond branch.

...

> @@ -205,9 +206,24 @@
>       // Otherwise, create a duplicate of the instruction.
>       Instruction *C = Inst->clone();
>
> -    C->setName(Inst->getName());
> -    C->insertBefore(LoopEntryBranch);
> -    ValueMap[Inst] = C;
> +    // Eagerly remap the operands of the instruction.
> +    RemapInstruction(C, ValueMap,
> +                     RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
> +
> +    // With the operands remapped, see if the instruction constant folds or is
> +    // otherwise simplifyable.  This commonly occurs because the entry from PHI
> +    // nodes allows icmps and other instructions to fold.
> +    if (Value *V = SimplifyInstruction(C)) {
> +      // If so, then delete the temporary instruction and stick the folded value
> +      // in the map.
> +      delete C;
> +      ValueMap[Inst] = V;
> +    } else {

since instsimplify can look through phi nodes, it doesn't automatically preserve
LCSSA form.  Passes like loop-rotate that need to preserve LCSSA form should do
something like this:

   if (Value *V = SimplifyInstruction(C))
     if (LI->replacementPreservesLCSSAForm(C, V)) {
       .. use the simplification ...
     }

Ciao, Duncan.