[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
#119099

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

Author: fhahn

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

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}