[TBAA] Don't emit pointer-tbaa for void pointers.

clang/docs/UsersManual.rst

@@ -2489,6 +2489,82 @@ are listed below.
 
     $ clang -fuse-ld=lld -Oz -Wl,--icf=safe -fcodegen-data-use code.cc
 
+.. _strict_aliasing:
+
+Strict Aliasing
+---------------
+
+The C and C++ standards require accesses to objects in memory to use l-values of
+an appropriate type for the object. This is called *strict aliasing* or
+*type-based alias analysis*. Strict aliasing enhances a variety of powerful
+memory optimizations, including reordering, combining, and eliminating memory
+accesses. These optimizations can lead to unexpected behavior in code that
+violates the strict aliasing rules. For example:
+
+.. code-block:: c++
+
+    void advance(size_t *index, double *data) {
+      double value = data[*index];
+      /* Clang may assume that this store does not change the contents of `data`. */
+      *index += 1;
+      /* Clang may assume that this store does not change the contents of `index`. */
+      data[*index] = value;
+      /* Either of these facts may create significant optimization opportunities
+       if Clang is able to inline this function. */
+  }
+
+Strict aliasing can be explicitly enabled with ``-fstrict-aliasing`` and
+disabled with ``-fno-strict-aliasing``. ``clang-cl`` defaults to
+``-fno-strict-aliasing``; see . Otherwise, Clang defaults to ``-fstrict-aliasing``.
+
+C and C++ specify slightly different rules for strict aliasing. To improve
+language interoperability, Clang allows two types to alias if either language
+would permit it. This includes applying the C++ similar types rule to C,
+allowing ``int **`` to alias ``int const * const *``. Clang also relaxes the
+standard aliasing rules in the following ways:
+
+* All integer types of the same size are permitted to alias each other,
+  including signed and unsigned types.
+* ``void*`` is permitted to alias any pointer type, ``void**`` is permitted to
+  alias any pointer to pointer type, and so on.
+
+Code which violates strict aliasing has undefined behavior. A program that
+works in one version of Clang may not work in another because of changes to the
+optimizer. Clang provides a :doc:`TypeSanitizer` to help detect
+violations of the strict aliasing rules, but it is currently still experimental.
+Code that is known to violate strict aliasing should generally be built with
+``-fno-strict-aliasing`` if the violation cannot be fixed.
+
+Clang supports several ways to fix a violation of strict aliasing:
+
+* L-values of the character types ``char`` and ``unsigned char`` (as well as
+  other types, depending on the standard) are permitted to access objects of
+  any type.
+
+* Library functions such as ``memcpy`` and ``memset`` are specified as treating
+  memory as characters and therefore are not limited by strict aliasing. If a
+  value of one type must be reinterpreted as another (e.g. to read the bits of a
+  floating-point number), use ``memcpy`` to copy the representation to an object
+  of the destination type. This has no overhead over a direct l-value access
+  because Clang should reliably optimize calls to these functions to use simple
+  loads and stores when they are used with small constant sizes.
+
+* The attribute ``may_alias`` can be added to a ``typedef`` to give l-values of
+  that type the same aliasing power as the character types.
+
+Clang makes a best effort to avoid obvious miscompilations from strict aliasing
+by only considering type information when it cannot prove that two accesses must
+refer to the same memory. However, it is not recommended that programmers
+intentionally rely on this instead of using one of the solutions above because
+it is too easy for the compiler's analysis to be blocked in surprising ways.
+
+In Clang 20, Clang strengthened its implementation of strict aliasing for
+accesses of pointer type. Previously, all accesses of pointer type were
+permitted to alias each other, but Clang now distinguishes different pointers
+by their pointee type, except as limited by the relaxations around qualifiers
+and ``void*`` described above. The previous behavior of treating all pointers as
+aliasing can be restored using ``-fno-pointer-tbaa``.
+
 Profile Guided Optimization
 ---------------------------
 
@@ -5272,12 +5348,6 @@ The Visual C++ Toolset has a slightly more elaborate mechanism for detection.
 Restrictions and Limitations compared to Clang
 ----------------------------------------------
 
-Strict Aliasing
-^^^^^^^^^^^^^^^
-
-Strict aliasing (TBAA) is always off by default in clang-cl. Whereas in clang,
-strict aliasing is turned on by default for all optimization levels.
-
-To enable LLVM optimizations based on strict aliasing rules (e.g., optimizations
-based on type of expressions in C/C++), user will need to explicitly pass
-`-fstrict-aliasing` to clang-cl.
+Strict aliasing (TBAA) is always off by default in clang-cl whereas in clang,
+strict aliasing is turned on by default for all optimization levels. For more
+details, see :ref:`Strict aliasing <strict_aliasing>`.

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}

clang/test/CodeGenOpenCL/amdgpu-enqueue-kernel.cl

@@ -651,7 +651,7 @@ kernel void test_target_features_kernel(global int *i) {
 //
 // GFX900: Function Attrs: convergent nounwind
 // GFX900-LABEL: define {{[^@]+}}@__test_block_invoke_3_kernel
-// GFX900-SAME: (<{ i32, i32, ptr, ptr addrspace(1), ptr addrspace(1), i64, i8 }> [[TMP0:%.*]], ptr addrspace(3) [[TMP1:%.*]]) #[[ATTR6]] !kernel_arg_addr_space [[META28:![0-9]+]] !kernel_arg_access_qual [[META29:![0-9]+]] !kernel_arg_type [[META30:![0-9]+]] !kernel_arg_base_type [[META30]] !kernel_arg_type_qual [[META31:![0-9]+]] {
+// GFX900-SAME: (<{ i32, i32, ptr, ptr addrspace(1), ptr addrspace(1), i64, i8 }> [[TMP0:%.*]], ptr addrspace(3) [[TMP1:%.*]]) #[[ATTR6]] !kernel_arg_addr_space [[META27:![0-9]+]] !kernel_arg_access_qual [[META28:![0-9]+]] !kernel_arg_type [[META29:![0-9]+]] !kernel_arg_base_type [[META29]] !kernel_arg_type_qual [[META30:![0-9]+]] {
 // GFX900-NEXT:  entry:
 // GFX900-NEXT:    [[TMP2:%.*]] = alloca <{ i32, i32, ptr, ptr addrspace(1), ptr addrspace(1), i64, i8 }>, align 8, addrspace(5)
 // GFX900-NEXT:    store <{ i32, i32, ptr, ptr addrspace(1), ptr addrspace(1), i64, i8 }> [[TMP0]], ptr addrspace(5) [[TMP2]], align 8
@@ -688,7 +688,7 @@ kernel void test_target_features_kernel(global int *i) {
 //
 // GFX900: Function Attrs: convergent norecurse nounwind
 // GFX900-LABEL: define {{[^@]+}}@test_target_features_kernel
-// GFX900-SAME: (ptr addrspace(1) noundef align 4 [[I:%.*]]) #[[ATTR2]] !kernel_arg_addr_space [[META32:![0-9]+]] !kernel_arg_access_qual [[META23]] !kernel_arg_type [[META33:![0-9]+]] !kernel_arg_base_type [[META33]] !kernel_arg_type_qual [[META25]] {
+// GFX900-SAME: (ptr addrspace(1) noundef align 4 [[I:%.*]]) #[[ATTR2]] !kernel_arg_addr_space [[META31:![0-9]+]] !kernel_arg_access_qual [[META23]] !kernel_arg_type [[META32:![0-9]+]] !kernel_arg_base_type [[META32]] !kernel_arg_type_qual [[META25]] {
 // GFX900-NEXT:  entry:
 // GFX900-NEXT:    [[I_ADDR:%.*]] = alloca ptr addrspace(1), align 8, addrspace(5)
 // GFX900-NEXT:    [[DEFAULT_QUEUE:%.*]] = alloca ptr addrspace(1), align 8, addrspace(5)
@@ -700,7 +700,7 @@ kernel void test_target_features_kernel(global int *i) {
 // GFX900-NEXT:    [[FLAGS_ASCAST:%.*]] = addrspacecast ptr addrspace(5) [[FLAGS]] to ptr
 // GFX900-NEXT:    [[NDRANGE_ASCAST:%.*]] = addrspacecast ptr addrspace(5) [[NDRANGE]] to ptr
 // GFX900-NEXT:    [[TMP_ASCAST:%.*]] = addrspacecast ptr addrspace(5) [[TMP]] to ptr
-// GFX900-NEXT:    store ptr addrspace(1) [[I]], ptr [[I_ADDR_ASCAST]], align 8, !tbaa [[TBAA34:![0-9]+]]
+// GFX900-NEXT:    store ptr addrspace(1) [[I]], ptr [[I_ADDR_ASCAST]], align 8, !tbaa [[TBAA33:![0-9]+]]
 // GFX900-NEXT:    call void @llvm.lifetime.start.p5(i64 8, ptr addrspace(5) [[DEFAULT_QUEUE]]) #[[ATTR8]]
 // GFX900-NEXT:    call void @llvm.lifetime.start.p5(i64 4, ptr addrspace(5) [[FLAGS]]) #[[ATTR8]]
 // GFX900-NEXT:    store i32 0, ptr [[FLAGS_ASCAST]], align 4, !tbaa [[TBAA17]]
@@ -803,16 +803,15 @@ kernel void test_target_features_kernel(global int *i) {
 // GFX900: [[META23]] = !{!"none"}
 // GFX900: [[META24]] = !{!"__block_literal"}
 // GFX900: [[META25]] = !{!""}
-// GFX900: [[TBAA26]] = !{[[META27:![0-9]+]], [[META27]], i64 0}
-// GFX900: [[META27]] = !{!"p1 void", [[META9]], i64 0}
-// GFX900: [[META28]] = !{i32 0, i32 3}
-// GFX900: [[META29]] = !{!"none", !"none"}
-// GFX900: [[META30]] = !{!"__block_literal", !"void*"}
-// GFX900: [[META31]] = !{!"", !""}
-// GFX900: [[META32]] = !{i32 1}
-// GFX900: [[META33]] = !{!"int*"}
-// GFX900: [[TBAA34]] = !{[[META35:![0-9]+]], [[META35]], i64 0}
-// GFX900: [[META35]] = !{!"p1 int", [[META9]], i64 0}
+// GFX900: [[TBAA26]] = !{[[META9]], [[META9]], i64 0}
+// GFX900: [[META27]] = !{i32 0, i32 3}
+// GFX900: [[META28]] = !{!"none", !"none"}
+// GFX900: [[META29]] = !{!"__block_literal", !"void*"}
+// GFX900: [[META30]] = !{!"", !""}
+// GFX900: [[META31]] = !{i32 1}
+// GFX900: [[META32]] = !{!"int*"}
+// GFX900: [[TBAA33]] = !{[[META34:![0-9]+]], [[META34]], i64 0}
+// GFX900: [[META34]] = !{!"p1 int", [[META9]], i64 0}
 //.
 //// NOTE: These prefixes are unused and the list is autogenerated. Do not add tests below this line:
 // CHECK: {{.*}}

clang/unittests/CodeGen/TBAAMetadataTest.cpp

@@ -117,15 +117,9 @@ TEST(TBAAMetadataTest, BasicTypes) {
   ASSERT_TRUE(I);
 
   I = matchNext(I,
-      MInstruction(Instruction::Store,
-        MValType(PointerType::getUnqual(Compiler.Context)),
-        MMTuple(
-          MMTuple(
-            MMString("p1 void"),
-            AnyPtr,
-            MConstInt(0)),
-          MSameAs(0),
-          MConstInt(0))));
+                MInstruction(Instruction::Store,
+                             MValType(PointerType::getUnqual(Compiler.Context)),
+                             MMTuple(AnyPtr, MSameAs(0), MConstInt(0))));
   ASSERT_TRUE(I);
 
   I = matchNext(I,