[Loads] Use more powerful constant folding API

[nikic]

Summary

This follows up on D111023 by exporting the generic "load value from constant at given offset as given type" and using it in the store to load forwarding code. We now need to make sure that the load size is smaller than the store size, previously this was implicitly ensured by ConstantFoldLoadThroughBitcast().

[aeubanks]

llvm/test/Transforms/InstCombine/load-store-forward.ll

4–5

comment needs updating

llvm/include/llvm/Analysis/ConstantFolding.h

 
 /// Attempt to constant fold a shufflevector instruction with the
 /// specified operands and mask.  See class ShuffleVectorInst for a description
 /// of the mask representation. The constant result is returned if successful;
 /// if not, null is returned.
 Constant *ConstantFoldShuffleVectorInstruction(Constant *V1, Constant *V2,
                                                ArrayRef<int> Mask);
 
+/// Extract value of C at the given Offset reinterpreted as Ty. If bits past
+/// the end of C are accessed, they are assumed to be poison and may take any
+/// value.
+Constant *ConstantFoldLoadFromConst(Constant *C, Type *Ty, const APInt &Offset,
+                                    const DataLayout &DL);
+
+/// Extract value of C reinterpreted as Ty. Same as previous API with zero
+/// offset.
+Constant *ConstantFoldLoadFromConst(Constant *C, Type *Ty,
+                                    const DataLayout &DL);
+
 /// ConstantFoldLoadFromConstPtr - Return the value that a load from C would
 /// produce if it is constant and determinable.  If this is not determinable,
 /// return null.
 Constant *ConstantFoldLoadFromConstPtr(Constant *C, Type *Ty, const DataLayout &DL);
 
 /// ConstantFoldLoadThroughGEPConstantExpr - Given a constant and a
 /// getelementptr constantexpr, return the constant value being addressed by the
 /// constant expression, or null if something is funny and we can't decide.

llvm/lib/Analysis/ConstantFolding.cpp

for (const APInt &Index : drop_begin(Indices)) {
     C = C->getAggregateElement(Index.getZExtValue());
     if (!C)
       return nullptr;
   }
 
   return C;
 }
 
-Constant *ConstantFoldLoadFromConst(Constant *C, Type *Ty, const APInt &Offset,
+} // end anonymous namespace
+
+Constant *llvm::ConstantFoldLoadFromConst(Constant *C, Type *Ty,
+                                          const APInt &Offset,
                                           const DataLayout &DL) {
   if (Constant *AtOffset = getConstantAtOffset(C, Offset, DL))
     if (Constant *Result = ConstantFoldLoadThroughBitcast(AtOffset, Ty, DL))
       return Result;
 
   // Try hard to fold loads from bitcasted strange and non-type-safe things.
   if (Offset.getMinSignedBits() <= 64)
     return FoldReinterpretLoadFromConst(C, Ty, Offset.getSExtValue(), DL);
 
   return nullptr;
 }
 
-} // end anonymous namespace
+Constant *llvm::ConstantFoldLoadFromConst(Constant *C, Type *Ty,
+                                          const DataLayout &DL) {
+  return ConstantFoldLoadFromConst(C, Ty, APInt(64, 0), DL);
+}
 
 Constant *llvm::ConstantFoldLoadFromConstPtr(Constant *C, Type *Ty,
                                              const DataLayout &DL) {
   APInt Offset(DL.getIndexTypeSizeInBits(C->getType()), 0);
   C = cast<Constant>(C->stripAndAccumulateConstantOffsets(
           DL, Offset, /* AllowNonInbounds */ true));
 
   if (auto *GV = dyn_cast<GlobalVariable>(C))

llvm/lib/Analysis/Loads.cpp

if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
 
     if (IsLoadCSE)
       *IsLoadCSE = false;
 
     Value *Val = SI->getValueOperand();
     if (CastInst::isBitOrNoopPointerCastable(Val->getType(), AccessTy, DL))
       return Val;
 
+    TypeSize StoreSize = DL.getTypeStoreSize(Val->getType());
+    TypeSize LoadSize = DL.getTypeStoreSize(AccessTy);
+    if (TypeSize::isKnownLE(LoadSize, StoreSize))
       if (auto *C = dyn_cast<Constant>(Val))
-      return ConstantFoldLoadThroughBitcast(C, AccessTy, DL);
+        return ConstantFoldLoadFromConst(C, AccessTy, DL);
   }
 
   return nullptr;
 }
 
 Value *llvm::findAvailablePtrLoadStore(
     const MemoryLocation &Loc, Type *AccessTy, bool AtLeastAtomic,
     BasicBlock *ScanBB, BasicBlock::iterator &ScanFrom, unsigned MaxInstsToScan,

llvm/test/Transforms/InstCombine/load-store-forward.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -S -instcombine < %s | FileCheck %s --check-prefixes=CHECK,LITTLE
 ; RUN: opt -S -instcombine -data-layout="E" < %s | FileCheck %s --check-prefixes=CHECK,BIG
 
-; Some cases where store to load forwarding is principally possible,
-; but is non-trivial.
-
 define i8 @load_smaller_int(i16* %p) {
-; CHECK-LABEL: @load_smaller_int(
-; CHECK-NEXT:    store i16 258, i16* [[P:%.*]], align 2
-; CHECK-NEXT:    [[P2:%.*]] = bitcast i16* [[P]] to i8*
-; CHECK-NEXT:    [[LOAD:%.*]] = load i8, i8* [[P2]], align 1
-; CHECK-NEXT:    ret i8 [[LOAD]]
+; LITTLE-LABEL: @load_smaller_int(
+; LITTLE-NEXT:    store i16 258, i16* [[P:%.*]], align 2
+; LITTLE-NEXT:    ret i8 2
+;
+; BIG-LABEL: @load_smaller_int(
+; BIG-NEXT:    store i16 258, i16* [[P:%.*]], align 2
+; BIG-NEXT:    ret i8 1
 ;
   store i16 258, i16* %p
   %p2 = bitcast i16* %p to i8*
   %load = load i8, i8* %p2
   ret i8 %load
 }
 
 ; This case can *not* be forwarded, as we only see part of the stored value.