[llvm] [GISel] Set more MIFlags when translating GEPs (PR #151708)

[Nikita Popov via llvm-commits]

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@@ -1592,9 +1592,19 @@ bool IRTranslator::translateGetElementPtr(const User &U,
   Type *OffsetIRTy = DL->getIndexType(PtrIRTy);
   LLT OffsetTy = getLLTForType(*OffsetIRTy, *DL);
 
-  uint32_t Flags = 0;
+  uint32_t PtrAddFlags = 0;
+  // Each PtrAdd generated to implement the GEP inherits its nuw, nusw, inbounds
+  // flags.
   if (const Instruction *I = dyn_cast<Instruction>(&U))
-    Flags = MachineInstr::copyFlagsFromInstruction(*I);
+    PtrAddFlags = MachineInstr::copyFlagsFromInstruction(*I);
+
+  auto PtrAddFlagsWithConst = [&](int64_t Offset) {
+    // For nusw/inbounds GEP with an offset that is nonnegative when interpreted
+    // as signed, assume there is no unsigned overflow.
+    if (Offset >= 0 && (PtrAddFlags & MachineInstr::MIFlag::NoUSWrap))
+      return PtrAddFlags | MachineInstr::MIFlag::NoUWrap;
+    return PtrAddFlags;
+  };
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nikic wrote:

Okay, I guess we can keep this for now.

https://github.com/llvm/llvm-project/pull/151708