[llvm-branch-commits] [llvm] [InstCombine] Improve inbounds preservation for ADD+GEP -> GEP+GEP (PR #135155)

[Nikita Popov via llvm-branch-commits]

================
@@ -3086,13 +3086,47 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
   if (GEPType->isVectorTy())
     return nullptr;
 
+  if (!GEP.isInBounds()) {
+    unsigned IdxWidth =
+        DL.getIndexSizeInBits(PtrOp->getType()->getPointerAddressSpace());
+    APInt BasePtrOffset(IdxWidth, 0);
+    Value *UnderlyingPtrOp =
+        PtrOp->stripAndAccumulateInBoundsConstantOffsets(DL, BasePtrOffset);
+    bool CanBeNull, CanBeFreed;
+    uint64_t DerefBytes = UnderlyingPtrOp->getPointerDereferenceableBytes(
+        DL, CanBeNull, CanBeFreed);
+    if (!CanBeNull && !CanBeFreed && DerefBytes != 0) {
+      if (GEP.accumulateConstantOffset(DL, BasePtrOffset) &&
+          BasePtrOffset.isNonNegative()) {
+        APInt AllocSize(IdxWidth, DerefBytes);
+        if (BasePtrOffset.ule(AllocSize)) {
+          return GetElementPtrInst::CreateInBounds(
+              GEP.getSourceElementType(), PtrOp, Indices, GEP.getName());
+        }
+      }
+    }
+  }
+
+  // nusw + nneg -> nuw
+  if (GEP.hasNoUnsignedSignedWrap() && !GEP.hasNoUnsignedWrap() &&
+      all_of(GEP.indices(), [&](Value *Idx) {
+        return isKnownNonNegative(Idx, SQ.getWithInstruction(&GEP));
+      })) {
+    GEP.setNoWrapFlags(GEP.getNoWrapFlags() | GEPNoWrapFlags::noUnsignedWrap());
+    return &GEP;
+  }
+
+  // These rewrites is trying to preserve inbounds/nuw attributes. So we want to
----------------
nikic wrote:

```suggestion
  // These rewrites are trying to preserve inbounds/nuw attributes. So we want to
```

https://github.com/llvm/llvm-project/pull/135155