[clang] [Clang][CodeGen] Check `isUnderlyingBasePointerConstantNull` in `emitPointerArithmetic` (PR #137849)

[Yingwei Zheng via cfe-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/137849

Do not set inbounds on GEP when the pointer operand is a constant null.

Note: This patch is fragile :( For a more complex case `(int*)0 + offset1 + offset2`, we still set inbounds for the second addition.

Related issue: https://github.com/llvm/llvm-project/issues/137833


>From f1db3a540ec1383451955efab62b64ed8d180349 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Wed, 30 Apr 2025 01:26:41 +0800
Subject: [PATCH] [Clang][CodeGen] Check `isUnderlyingBasePointerConstantNull`
 in `emitPointerArithmetic`

---
 clang/lib/CodeGen/CGExprScalar.cpp                   |  3 ++-
 .../test/CodeGen/catch-nullptr-and-nonzero-offset.c  | 12 ++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 8dbbcdaef25d8..d214d2af52563 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -4238,7 +4238,8 @@ static Value *emitPointerArithmetic(CodeGenFunction &CGF,
   else
     elemTy = CGF.ConvertTypeForMem(elementType);
 
-  if (CGF.getLangOpts().PointerOverflowDefined)
+  if (CGF.getLangOpts().PointerOverflowDefined ||
+      CGF.isUnderlyingBasePointerConstantNull(pointerOperand))
     return CGF.Builder.CreateGEP(elemTy, pointer, index, "add.ptr");
 
   return CGF.EmitCheckedInBoundsGEP(
diff --git a/clang/test/CodeGen/catch-nullptr-and-nonzero-offset.c b/clang/test/CodeGen/catch-nullptr-and-nonzero-offset.c
index 63b6db2c2adeb..c5ae3f8bcc368 100644
--- a/clang/test/CodeGen/catch-nullptr-and-nonzero-offset.c
+++ b/clang/test/CodeGen/catch-nullptr-and-nonzero-offset.c
@@ -431,6 +431,18 @@ char *void_ptr(void *base, unsigned long offset) {
   return base + offset;
 }
 
+int *constant_null_add(long offset) {
+  // CHECK: define{{.*}} ptr @constant_null_add(i64 noundef %[[OFFSET:.*]])
+  // CHECK-NEXT: [[ENTRY:.*]]:
+  // CHECK-NEXT:   %[[OFFSET_ADDR:.*]] = alloca i64, align 8
+  // CHECK-NEXT:   store i64 %[[OFFSET]], ptr %[[OFFSET_ADDR]], align 8
+  // CHECK-NEXT:   %[[OFFSET_RELOADED:.*]] = load i64, ptr %[[OFFSET_ADDR]], align 8
+  // CHECK-NEXT:   %[[ADD_PTR:.*]] = getelementptr i32, ptr null, i64 %[[OFFSET_RELOADED]]
+  // CHECK-NEXT:   ret ptr %[[ADD_PTR]]
+#line 1800
+  return (int *)0 + offset;
+}
+
 #ifdef __cplusplus
 }
 #endif