[clang] [TBAA] Don't emit pointer-tbaa for void pointers. (PR #122116)

[Florian Hahn via cfe-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/122116

While there are no special rules in the standards regarding void pointers and strict aliasing, emitting distinct tags for void pointers break some common idioms and there is no good alternative to re-write the code without strict-aliasing violations. An example is to count the entries in an array of pointers:

    int count_elements(void * values) {
      void **seq = values;
      int count;
      for (count = 0; seq && seq[count]; count++);
      return count;
    }

https://clang.godbolt.org/z/8dTv51v8W

An example in the wild is from
https://github.com/llvm/llvm-project/issues/119099

This patch avoids emitting distinct tags for void pointers, to avoid those idioms causing mis-compiles for now.

>From 0144b06ee8cb50d914b667f08c45f719446a8b3a Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Wed, 8 Jan 2025 13:07:01 +0000
Subject: [PATCH] [TBAA] Don't emit pointer-tbaa for void pointers.

While there are no special rules in the standards regarding
void pointers and strict aliasing, emitting distinct tags for void
pointers break some common idioms and there is no good alternative to
re-write the code without strict-aliasing violations. An example is
to count the entries in an array of pointers:

    int count_elements(void * values) {
      void **seq = values;
      int count;
      for (count = 0; seq && seq[count]; count++);
      return count;
    }

https://clang.godbolt.org/z/8dTv51v8W

An example in the wild is from
https://github.com/llvm/llvm-project/issues/119099

This patch avoids emitting distinct tags for void pointers, to avoid
those idioms causing mis-compiles for now.
---
 clang/lib/CodeGen/CodeGenTBAA.cpp  |  8 ++++++++
 clang/test/CodeGen/tbaa-pointers.c | 13 +++----------
 2 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/clang/lib/CodeGen/CodeGenTBAA.cpp b/clang/lib/CodeGen/CodeGenTBAA.cpp
index 75e66bae79afdc..3f1a24791ddd81 100644
--- a/clang/lib/CodeGen/CodeGenTBAA.cpp
+++ b/clang/lib/CodeGen/CodeGenTBAA.cpp
@@ -226,6 +226,14 @@ llvm::MDNode *CodeGenTBAA::getTypeInfoHelper(const Type *Ty) {
       PtrDepth++;
       Ty = Ty->getPointeeType()->getBaseElementTypeUnsafe();
     } while (Ty->isPointerType());
+
+    // While there are no special rules in the standards regarding void pointers
+    // and strict aliasing, emitting distinct tags for void pointers break some
+    // common idioms and there is no good alternative to re-write the code
+    // without strict-aliasing violations.
+    if (Ty->isVoidType())
+      return AnyPtr;
+
     assert(!isa<VariableArrayType>(Ty));
     // When the underlying type is a builtin type, we compute the pointee type
     // string recursively, which is implicitly more forgiving than the standards
diff --git a/clang/test/CodeGen/tbaa-pointers.c b/clang/test/CodeGen/tbaa-pointers.c
index 4aae2552f107a3..48adac503357f0 100644
--- a/clang/test/CodeGen/tbaa-pointers.c
+++ b/clang/test/CodeGen/tbaa-pointers.c
@@ -208,12 +208,9 @@ int void_ptrs(void **ptr) {
 // COMMON-LABEL: define i32 @void_ptrs(
 // COMMON-SAME: ptr noundef [[PTRA:%.+]])
 // COMMON:        [[PTR_ADDR:%.+]]  = alloca ptr, align 8
-// DISABLE-NEXT:  store ptr [[PTRA]], ptr [[PTR_ADDR]], align 8, !tbaa [[ANYPTR]]
-// DISABLE-NEXT:  [[L0:%.+]] = load ptr, ptr  [[PTR_ADDR]], align 8, !tbaa  [[ANYPTR]]
-// DISABLE-NEXT:  [[L1:%.+]] = load ptr, ptr [[L0]], align 8, !tbaa  [[ANYPTR]]
-// DEFAULT-NEXT:  store ptr [[PTRA]], ptr [[PTR_ADDR]], align 8, !tbaa [[P2VOID:!.+]]
-// DEFAULT-NEXT:  [[L0:%.+]] = load ptr, ptr  [[PTR_ADDR]], align 8, !tbaa  [[P2VOID]]
-// DEFAULT-NEXT:  [[L1:%.+]] = load ptr, ptr [[L0]], align 8, !tbaa  [[P1VOID:!.+]]
+// COMMON-NEXT:   store ptr [[PTRA]], ptr [[PTR_ADDR]], align 8, !tbaa [[ANYPTR]]
+// COMMON-NEXT:   [[L0:%.+]] = load ptr, ptr  [[PTR_ADDR]], align 8, !tbaa  [[ANYPTR]]
+// COMMON-NEXT:   [[L1:%.+]] = load ptr, ptr [[L0]], align 8, !tbaa  [[ANYPTR]]
 // COMMON-NEXT:   [[BOOL:%.+]] = icmp ne ptr [[L1]], null
 // COMMON-NEXT:   [[BOOL_EXT:%.+]] = zext i1 [[BOOL]] to i64
 // COMMON-NEXT:   [[COND:%.+]] = select i1 [[BOOL]], i32 0, i32 1
@@ -254,7 +251,3 @@ int void_ptrs(void **ptr) {
 // COMMON:  [[INT_TAG]] = !{[[INT_TY:!.+]], [[INT_TY]], i64 0}
 // COMMON:  [[INT_TY]] = !{!"int", [[CHAR]], i64 0}
 // DEFAULT: [[ANYPTR]] = !{[[ANY_POINTER]],  [[ANY_POINTER]], i64 0}
-// DEFAULT: [[P2VOID]] = !{[[P2VOID_TY:!.+]], [[P2VOID_TY]], i64 0}
-// DEFAULT: [[P2VOID_TY]] = !{!"p2 void", [[ANY_POINTER]], i64 0}
-// DEFAULT: [[P1VOID]] = !{[[P1VOID_TY:!.+]], [[P1VOID_TY]], i64 0}
-// DEFAULT: [[P1VOID_TY]] = !{!"p1 void", [[ANY_POINTER]], i64 0}