[clang] [Clang][CodeGen] Do not use the GEP result to infer offset and result type (PR #134221)

[Yingwei Zheng via cfe-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/134221

If `CreateConstInBoundsGEP2_32` returns a constant null/gep, the cast to GetElementPtrInst will fail.
This patch uses two static helpers `GEPOperator::accumulateConstantOffset/GetElementPtrInst::getIndexedType` to infer offset and result type instead of depending on the GEP result.

This patch is extracted from https://github.com/llvm/llvm-project/pull/130734. It may be useful to fix https://github.com/llvm/llvm-project/issues/132449.


>From e07237777e3a2215f69ced633fd281ac026eda61 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Thu, 3 Apr 2025 16:32:56 +0800
Subject: [PATCH] [Clang][CodeGen] Do not use the GEP result to infer offset

---
 clang/lib/CodeGen/CGBuilder.h | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/clang/lib/CodeGen/CGBuilder.h b/clang/lib/CodeGen/CGBuilder.h
index b8036cf6e6a30..090f75d3b5d3c 100644
--- a/clang/lib/CodeGen/CGBuilder.h
+++ b/clang/lib/CodeGen/CGBuilder.h
@@ -64,21 +64,25 @@ class CGBuilderTy : public CGBuilderBaseTy {
   Address createConstGEP2_32(Address Addr, unsigned Idx0, unsigned Idx1,
                              const llvm::Twine &Name) {
     const llvm::DataLayout &DL = BB->getDataLayout();
-    llvm::GetElementPtrInst *GEP;
+    llvm::Value *V;
     if (IsInBounds)
-      GEP = cast<llvm::GetElementPtrInst>(CreateConstInBoundsGEP2_32(
-          Addr.getElementType(), emitRawPointerFromAddress(Addr), Idx0, Idx1,
-          Name));
+      V = CreateConstInBoundsGEP2_32(Addr.getElementType(),
+                                     emitRawPointerFromAddress(Addr), Idx0,
+                                     Idx1, Name);
     else
-      GEP = cast<llvm::GetElementPtrInst>(CreateConstGEP2_32(
-          Addr.getElementType(), emitRawPointerFromAddress(Addr), Idx0, Idx1,
-          Name));
+      V = CreateConstGEP2_32(Addr.getElementType(),
+                             emitRawPointerFromAddress(Addr), Idx0, Idx1, Name);
     llvm::APInt Offset(
         DL.getIndexSizeInBits(Addr.getType()->getPointerAddressSpace()), 0,
         /*isSigned=*/true);
-    if (!GEP->accumulateConstantOffset(DL, Offset))
-      llvm_unreachable("offset of GEP with constants is always computable");
-    return Address(GEP, GEP->getResultElementType(),
+    if (!llvm::GEPOperator::accumulateConstantOffset(
+            Addr.getElementType(), {getInt32(Idx0), getInt32(Idx1)}, DL,
+            Offset))
+      llvm_unreachable(
+          "accumulateConstantOffset with constant indices should not fail.");
+    llvm::Type *ElementTy = llvm::GetElementPtrInst::getIndexedType(
+        Addr.getElementType(), {Idx0, Idx1});
+    return Address(V, ElementTy,
                    Addr.getAlignment().alignmentAtOffset(
                        CharUnits::fromQuantity(Offset.getSExtValue())),
                    IsInBounds ? Addr.isKnownNonNull() : NotKnownNonNull);