[llvm] d9f5d7b - [InstCombine] Extract bitcast -> gep transform

[David Blaikie via llvm-commits]

On Mon, Jun 21, 2021 at 12:25 PM Nikita Popov via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
>
>
> Author: Nikita Popov
> Date: 2021-06-21T21:24:50+02:00
> New Revision: d9f5d7b959de36085944d4a99a73f3053f953796
>
> URL: https://github.com/llvm/llvm-project/commit/d9f5d7b959de36085944d4a99a73f3053f953796
> DIFF: https://github.com/llvm/llvm-project/commit/d9f5d7b959de36085944d4a99a73f3053f953796.diff
>
> LOG: [InstCombine] Extract bitcast -> gep transform
>
> Move this into a separate function, to make sure that early
> returns do not accidentally skip other transforms. There is
> already one isSized() check that could run into this issue,
> thus this change is not strictly NFC.

Can/should this be tested, then?

>
> Added:
>
>
> Modified:
>     llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
>
> Removed:
>
>
>
> ################################################################################
> diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
> index 9b38eb6ab24d..dcfff4552f11 100644
> --- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
> +++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
> @@ -2603,6 +2603,52 @@ Instruction *InstCombinerImpl::optimizeBitCastFromPhi(CastInst &CI,
>    return RetVal;
>  }
>
> +static Instruction *convertBitCastToGEP(BitCastInst &CI, IRBuilderBase &Builder,
> +                                        const DataLayout &DL) {
> +  Value *Src = CI.getOperand(0);
> +  PointerType *SrcPTy = cast<PointerType>(Src->getType());
> +  PointerType *DstPTy = cast<PointerType>(CI.getType());
> +  Type *DstElTy = DstPTy->getElementType();
> +  Type *SrcElTy = SrcPTy->getElementType();
> +
> +  // When the type pointed to is not sized the cast cannot be
> +  // turned into a gep.
> +  if (!SrcElTy->isSized())
> +    return nullptr;
> +
> +  // If the source and destination are pointers, and this cast is equivalent
> +  // to a getelementptr X, 0, 0, 0...  turn it into the appropriate gep.
> +  // This can enhance SROA and other transforms that want type-safe pointers.
> +  unsigned NumZeros = 0;
> +  while (SrcElTy && SrcElTy != DstElTy) {
> +    SrcElTy = GetElementPtrInst::getTypeAtIndex(SrcElTy, (uint64_t)0);
> +    ++NumZeros;
> +  }
> +
> +  // If we found a path from the src to dest, create the getelementptr now.
> +  if (SrcElTy == DstElTy) {
> +    SmallVector<Value *, 8> Idxs(NumZeros + 1, Builder.getInt32(0));
> +    GetElementPtrInst *GEP =
> +        GetElementPtrInst::Create(SrcPTy->getElementType(), Src, Idxs);
> +
> +    // If the source pointer is dereferenceable, then assume it points to an
> +    // allocated object and apply "inbounds" to the GEP.
> +    bool CanBeNull, CanBeFreed;
> +    if (Src->getPointerDereferenceableBytes(DL, CanBeNull, CanBeFreed)) {
> +      // In a non-default address space (not 0), a null pointer can not be
> +      // assumed inbounds, so ignore that case (dereferenceable_or_null).
> +      // The reason is that 'null' is not treated
> diff erently in these address
> +      // spaces, and we consequently ignore the 'gep inbounds' special case
> +      // for 'null' which allows 'inbounds' on 'null' if the indices are
> +      // zeros.
> +      if (SrcPTy->getAddressSpace() == 0 || !CanBeNull)
> +        GEP->setIsInBounds();
> +    }
> +    return GEP;
> +  }
> +  return nullptr;
> +}
> +
>  Instruction *InstCombinerImpl::visitBitCast(BitCastInst &CI) {
>    // If the operands are integer typed then apply the integer transforms,
>    // otherwise just apply the common ones.
> @@ -2616,11 +2662,6 @@ Instruction *InstCombinerImpl::visitBitCast(BitCastInst &CI) {
>      return replaceInstUsesWith(CI, Src);
>
>    if (isa<PointerType>(SrcTy) && isa<PointerType>(DestTy)) {
> -    PointerType *SrcPTy = cast<PointerType>(SrcTy);
> -    PointerType *DstPTy = cast<PointerType>(DestTy);
> -    Type *DstElTy = DstPTy->getElementType();
> -    Type *SrcElTy = SrcPTy->getElementType();
> -
>      // If we are casting a alloca to a pointer to a type of the same
>      // size, rewrite the allocation instruction to allocate the "right" type.
>      // There is no need to modify malloc calls because it is their bitcast that
> @@ -2629,43 +2670,8 @@ Instruction *InstCombinerImpl::visitBitCast(BitCastInst &CI) {
>        if (Instruction *V = PromoteCastOfAllocation(CI, *AI))
>          return V;
>
> -    // When the type pointed to is not sized the cast cannot be
> -    // turned into a gep.
> -    Type *PointeeType =
> -        cast<PointerType>(Src->getType()->getScalarType())->getElementType();
> -    if (!PointeeType->isSized())
> -      return nullptr;
> -
> -    // If the source and destination are pointers, and this cast is equivalent
> -    // to a getelementptr X, 0, 0, 0...  turn it into the appropriate gep.
> -    // This can enhance SROA and other transforms that want type-safe pointers.
> -    unsigned NumZeros = 0;
> -    while (SrcElTy && SrcElTy != DstElTy) {
> -      SrcElTy = GetElementPtrInst::getTypeAtIndex(SrcElTy, (uint64_t)0);
> -      ++NumZeros;
> -    }
> -
> -    // If we found a path from the src to dest, create the getelementptr now.
> -    if (SrcElTy == DstElTy) {
> -      SmallVector<Value *, 8> Idxs(NumZeros + 1, Builder.getInt32(0));
> -      GetElementPtrInst *GEP =
> -          GetElementPtrInst::Create(SrcPTy->getElementType(), Src, Idxs);
> -
> -      // If the source pointer is dereferenceable, then assume it points to an
> -      // allocated object and apply "inbounds" to the GEP.
> -      bool CanBeNull, CanBeFreed;
> -      if (Src->getPointerDereferenceableBytes(DL, CanBeNull, CanBeFreed)) {
> -        // In a non-default address space (not 0), a null pointer can not be
> -        // assumed inbounds, so ignore that case (dereferenceable_or_null).
> -        // The reason is that 'null' is not treated
> diff erently in these address
> -        // spaces, and we consequently ignore the 'gep inbounds' special case
> -        // for 'null' which allows 'inbounds' on 'null' if the indices are
> -        // zeros.
> -        if (SrcPTy->getAddressSpace() == 0 || !CanBeNull)
> -          GEP->setIsInBounds();
> -      }
> -      return GEP;
> -    }
> +    if (Instruction *I = convertBitCastToGEP(CI, Builder, DL))
> +      return I;
>    }
>
>    if (FixedVectorType *DestVTy = dyn_cast<FixedVectorType>(DestTy)) {
>
>
>
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