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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang-codegen

@llvm/pr-subscribers-clang

Author: Florian Hahn (fhahn)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/122116.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CodeGenTBAA.cpp (+8) 
- (modified) clang/test/CodeGen/tbaa-pointers.c (+3-10) 


``````````diff
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}

``````````

</details>


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