[clang] [llvm] [ConstantFolding] Canonicalize constexpr GEPs to i8 (PR #89872)

[Nikita Popov via llvm-commits]

================
@@ -944,43 +943,18 @@ Constant *SymbolicallyEvaluateGEP(const GEPOperator *GEP,
     return ConstantExpr::getIntToPtr(C, ResTy);
   }
 
-  // Otherwise form a regular getelementptr. Recompute the indices so that
-  // we eliminate over-indexing of the notional static type array bounds.
-  // This makes it easy to determine if the getelementptr is "inbounds".
-
-  // For GEPs of GlobalValues, use the value type, otherwise use an i8 GEP.
-  if (auto *GV = dyn_cast<GlobalValue>(Ptr))
-    SrcElemTy = GV->getValueType();
-  else
-    SrcElemTy = Type::getInt8Ty(Ptr->getContext());
-
-  if (!SrcElemTy->isSized())
-    return nullptr;
-
-  Type *ElemTy = SrcElemTy;
-  SmallVector<APInt> Indices = DL.getGEPIndicesForOffset(ElemTy, Offset);
-  if (Offset != 0)
-    return nullptr;
-
-  // Try to add additional zero indices to reach the desired result element
-  // type.
-  // TODO: Should we avoid extra zero indices if ResElemTy can't be reached and
-  // we'll have to insert a bitcast anyway?
-  while (ElemTy != ResElemTy) {
-    Type *NextTy = GetElementPtrInst::getTypeAtIndex(ElemTy, (uint64_t)0);
-    if (!NextTy)
-      break;
-
-    Indices.push_back(APInt::getZero(isa<StructType>(ElemTy) ? 32 : BitWidth));
-    ElemTy = NextTy;
+  // Try to infer inbounds for GEPs of globals.
+  if (!InBounds && Offset.isNonNegative()) {
+    bool CanBeNull, CanBeFreed;
+    uint64_t DerefBytes =
+        Ptr->getPointerDereferenceableBytes(DL, CanBeNull, CanBeFreed);
+    InBounds = DerefBytes != 0 && !CanBeNull && Offset.sle(DerefBytes);
----------------
nikic wrote:

I've added this inference in this patch, because we lose out on this inference in the DataLayout-independent constant folding: https://github.com/llvm/llvm-project/blob/88b6186af3908c55b357858eb348b5143f21c289/llvm/lib/IR/ConstantFold.cpp#L1717-L1724 As such, we need a (more powerful version of) this inference in the DataLayout-aware constant folding code.

https://github.com/llvm/llvm-project/pull/89872