FoldGEP only for Constant

dms · July 13, 2023, 7:10pm

Hi,

Why FoldGEP is performed only for Constants ?

llvm/llvm-project/blob/289ae6525dffd9eb9a23753acd407235e125c690/llvm/include/llvm/IR/ConstantFolder.h#L111


      
            if (LC && RC)
              return ConstantExpr::getCompare(P, LC, RC);
            return nullptr;
          }
          
          Value *FoldGEP(Type *Ty, Value *Ptr, ArrayRef<Value *> IdxList,
                         bool IsInBounds = false) const override {
            if (!ConstantExpr::isSupportedGetElementPtr(Ty))
              return nullptr;
          
            if (auto *PC = dyn_cast<Constant>(Ptr)) {
              // Every index must be constant.
              if (any_of(IdxList, [](Value *V) { return !isa<Constant>(V); }))
                return nullptr;
          
              if (IsInBounds)
                return ConstantExpr::getInBoundsGetElementPtr(Ty, PC, IdxList);
              else
                return ConstantExpr::getGetElementPtr(Ty, PC, IdxList);
            }
            return nullptr;

Could we perform this also for alloca ?

Thanks

jdoerfert · July 13, 2023, 7:14pm

What exactly do you want to transform to what?

dms · July 13, 2023, 7:35pm

Right now llvm will generate:

%0 = alloca [4 x float]
%1 = getelementptr [4 x float], ptr addrspace(0) %0, i32 0
%2 = load float, ptr addrspace(0) %1, align 4

I would expect to see something like this:

%0 = alloca [4 x float]
%1 = load float, ptr addrspace(0) %0, align 4

Is it something illegal ?

nikic · July 13, 2023, 7:38pm

IRBuilder supports multiple different “folders” that allow simplifying operations before instructions get generated for them. By default it uses ConstantFolder, which performs data-layout independent folding. The other standard folders are NoFolder (does the obvious), TargetFolder (performs data-layout dependent folding) and InstSimplifyFolder.

If you specify the InstSimplifyFolder, then the simplification you suggest should get applied.

Most places will stick to just ConstantFolder/TargetFolder and leave everything else to InstCombine though.

dms · July 13, 2023, 7:47pm

If you specify the InstSimplifyFolder, then the simplification you suggest should get applied.

This is what I was looking for. Thanks for the help !