[clang] [Clang] [Sema] Reject non-power-of-2 `_BitInt` matrix element types (PR #117487)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: None (Sirraide)

<details>
<summary>Changes</summary>

Essentially, this pr makes this ill-formed:
```c++
using mat4 = _BitInt(12) [[clang::matrix_type(3, 3)]];
```

This matches preexisting behaviour for vector types (e.g. `ext_vector_type`), and given that LLVM IR intrinsics for matrices also take vector types, it seems like a sensible thing to do.

This is currently especially problematic since we sometimes lower matrix types to LLVM array types instead, and while e.g. `[4 x i32]` and `<4 x i32>` *probably* have the same similar memory layout (though I don’t think it’s sound to rely on that either, see #<!-- -->117486), `[4 x i12]` and `<4 x i12>` definitely don’t.

---
Full diff: https://github.com/llvm/llvm-project/pull/117487.diff


4 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+3) 
- (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+2-2) 
- (modified) clang/lib/Sema/SemaType.cpp (+23-18) 
- (modified) clang/test/SemaCXX/matrix-type.cpp (+11-1) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8bd06fadfdc984..1c0e4043bbe276 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -371,6 +371,9 @@ Non-comprehensive list of changes in this release
 - ``__builtin_reduce_and`` function can now be used in constant expressions.
 - ``__builtin_reduce_or`` and ``__builtin_reduce_xor`` functions can now be used in constant expressions.
 
+- Clang now rejects ``_BitInt`` matrix element types if the bit width is less than ``CHAR_WIDTH`` or
+  not a power of two, matching preexisting behaviour for vector types.
+
 New Compiler Flags
 ------------------
 
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index eb05a6a77978af..f049e72a6b8694 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3233,8 +3233,8 @@ def err_attribute_too_few_arguments : Error<
   "%0 attribute takes at least %1 argument%s1">;
 def err_attribute_invalid_vector_type : Error<"invalid vector element type %0">;
 def err_attribute_invalid_bitint_vector_type : Error<
-  "'_BitInt' vector element width must be %select{a power of 2|"
-  "at least as wide as 'CHAR_BIT'}0">;
+  "'_BitInt' %select{vector|matrix}0 element width must be %select{a power of 2|"
+  "at least as wide as 'CHAR_BIT'}1">;
 def err_attribute_invalid_matrix_type : Error<"invalid matrix element type %0">;
 def err_attribute_bad_neon_vector_size : Error<
   "Neon vector size must be 64 or 128 bits">;
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index f32edc5ac06440..06b779f5ef3aa2 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -2312,6 +2312,18 @@ QualType Sema::BuildArrayType(QualType T, ArraySizeModifier ASM,
   return T;
 }
 
+bool CheckBitIntElementType(Sema &S, SourceLocation AttrLoc,
+                            const BitIntType *BIT, bool ForMatrixType = false) {
+  // Only support _BitInt elements with byte-sized power of 2 NumBits.
+  unsigned NumBits = BIT->getNumBits();
+  if (!llvm::isPowerOf2_32(NumBits) || NumBits < 8) {
+    S.Diag(AttrLoc, diag::err_attribute_invalid_bitint_vector_type)
+        << ForMatrixType << (NumBits < 8);
+    return true;
+  }
+  return false;
+}
+
 QualType Sema::BuildVectorType(QualType CurType, Expr *SizeExpr,
                                SourceLocation AttrLoc) {
   // The base type must be integer (not Boolean or enumeration) or float, and
@@ -2324,15 +2336,10 @@ QualType Sema::BuildVectorType(QualType CurType, Expr *SizeExpr,
     Diag(AttrLoc, diag::err_attribute_invalid_vector_type) << CurType;
     return QualType();
   }
-  // Only support _BitInt elements with byte-sized power of 2 NumBits.
-  if (const auto *BIT = CurType->getAs<BitIntType>()) {
-    unsigned NumBits = BIT->getNumBits();
-    if (!llvm::isPowerOf2_32(NumBits) || NumBits < 8) {
-      Diag(AttrLoc, diag::err_attribute_invalid_bitint_vector_type)
-          << (NumBits < 8);
-      return QualType();
-    }
-  }
+
+  if (const auto *BIT = CurType->getAs<BitIntType>();
+      BIT && CheckBitIntElementType(*this, AttrLoc, BIT))
+    return QualType();
 
   if (SizeExpr->isTypeDependent() || SizeExpr->isValueDependent())
     return Context.getDependentVectorType(CurType, SizeExpr, AttrLoc,
@@ -2402,15 +2409,9 @@ QualType Sema::BuildExtVectorType(QualType T, Expr *ArraySize,
     return QualType();
   }
 
-  // Only support _BitInt elements with byte-sized power of 2 NumBits.
-  if (T->isBitIntType()) {
-    unsigned NumBits = T->castAs<BitIntType>()->getNumBits();
-    if (!llvm::isPowerOf2_32(NumBits) || NumBits < 8) {
-      Diag(AttrLoc, diag::err_attribute_invalid_bitint_vector_type)
-          << (NumBits < 8);
-      return QualType();
-    }
-  }
+  if (const auto *BIT = T->getAs<BitIntType>();
+      BIT && CheckBitIntElementType(*this, AttrLoc, BIT))
+    return QualType();
 
   if (!ArraySize->isTypeDependent() && !ArraySize->isValueDependent()) {
     std::optional<llvm::APSInt> vecSize =
@@ -2455,6 +2456,10 @@ QualType Sema::BuildMatrixType(QualType ElementTy, Expr *NumRows, Expr *NumCols,
     return QualType();
   }
 
+  if (const auto *BIT = ElementTy->getAs<BitIntType>();
+      BIT && CheckBitIntElementType(*this, AttrLoc, BIT, true))
+    return QualType();
+
   if (NumRows->isTypeDependent() || NumCols->isTypeDependent() ||
       NumRows->isValueDependent() || NumCols->isValueDependent())
     return Context.getDependentSizedMatrixType(ElementTy, NumRows, NumCols,
diff --git a/clang/test/SemaCXX/matrix-type.cpp b/clang/test/SemaCXX/matrix-type.cpp
index af31e267fdcae8..bb7a8421ca9e37 100644
--- a/clang/test/SemaCXX/matrix-type.cpp
+++ b/clang/test/SemaCXX/matrix-type.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -pedantic -fenable-matrix -std=c++11 -verify -triple x86_64-apple-darwin %s
+// RUN: %clang_cc1 -fsyntax-only -fenable-matrix -std=c++11 -verify -triple x86_64-apple-darwin %s
 
 using matrix_double_t = double __attribute__((matrix_type(6, 6)));
 using matrix_float_t = float __attribute__((matrix_type(6, 6)));
@@ -29,3 +29,13 @@ void matrix_unsupported_element_type() {
   using matrix3_t = bool __attribute__((matrix_type(1, 1)));     // expected-error{{invalid matrix element type 'bool'}}
   using matrix4_t = TestEnum __attribute__((matrix_type(1, 1))); // expected-error{{invalid matrix element type 'TestEnum'}}
 }
+
+void matrix_unsupported_bit_int() {
+  using m1 = _BitInt(2) __attribute__((matrix_type(4, 4))); // expected-error{{'_BitInt' matrix element width must be at least as wide as 'CHAR_BIT'}}
+  using m2 = _BitInt(7) __attribute__((matrix_type(4, 4))); // expected-error{{'_BitInt' matrix element width must be at least as wide as 'CHAR_BIT'}}
+  using m3 = _BitInt(9) __attribute__((matrix_type(4, 4))); // expected-error{{'_BitInt' matrix element width must be a power of 2}}
+  using m4 = _BitInt(12) __attribute__((matrix_type(4, 4))); // expected-error{{'_BitInt' matrix element width must be a power of 2}}
+  using m5 = _BitInt(8) __attribute__((matrix_type(4, 4)));
+  using m6 = _BitInt(64) __attribute__((matrix_type(4, 4)));
+  using m7 = _BitInt(256) __attribute__((matrix_type(4, 4)));
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/117487