[clang] Fix a regression with alignas on structure members in C (PR #98642)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman created https://github.com/llvm/llvm-project/pull/98642

This was a 19.x regression and thus has no release note.

Fixes #95032

>From 41b7064dce91fdb1cf73d0e7508ad83a1edbb42a Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aaron at aaronballman.com>
Date: Fri, 12 Jul 2024 10:38:09 -0400
Subject: [PATCH] Fix a regression with alignas on structure members in C

This was a 19.x regression and thus has no release note.

Fixes #95032
---
 clang/include/clang/Sema/DeclSpec.h |  3 ++-
 clang/lib/Parse/ParseDecl.cpp       | 10 ++++++++++
 clang/lib/Sema/ParsedAttr.cpp       |  2 +-
 clang/test/C/C2y/n3254.c            |  4 ++--
 clang/test/Sema/alignas.c           | 19 ++++++++++++++++---
 5 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/clang/include/clang/Sema/DeclSpec.h b/clang/include/clang/Sema/DeclSpec.h
index 9c22c35535ede..1dd9781bba5ea 100644
--- a/clang/include/clang/Sema/DeclSpec.h
+++ b/clang/include/clang/Sema/DeclSpec.h
@@ -2042,7 +2042,8 @@ class Declarator {
     assert(llvm::all_of(DeclarationAttrs,
                         [](const ParsedAttr &AL) {
                           return (AL.isStandardAttributeSyntax() ||
-                                  AL.isRegularKeywordAttribute());
+                                  AL.isRegularKeywordAttribute() ||
+                                  AL.isAlignas());
                         }) &&
            "DeclarationAttrs may only contain [[]] and keyword attributes");
   }
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 7ce9a9cea1c7a..ae1801f8a572a 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -4919,6 +4919,16 @@ void Parser::ParseStructDeclaration(
   // Parse the common specifier-qualifiers-list piece.
   ParseSpecifierQualifierList(DS);
 
+  // FIXME: _Alignas has special handling that really shouldn't be necessary.
+  // Loop over all the attributes on the specifier qualifier, find any that
+  // are alignment attributes, and shift those off the specifier qualifier and
+  // onto the declarator.
+  ParsedAttributes &DSAttrs = DS.getAttributes();
+  for (auto &PAttr : DSAttrs) {
+    if (PAttr.isAlignas())
+      Attrs.takeOneFrom(DSAttrs, &PAttr);
+  }
+
   // If there are no declarators, this is a free-standing declaration
   // specifier. Let the actions module cope with it.
   if (Tok.is(tok::semi)) {
diff --git a/clang/lib/Sema/ParsedAttr.cpp b/clang/lib/Sema/ParsedAttr.cpp
index 6abc90336c994..2109494aa5889 100644
--- a/clang/lib/Sema/ParsedAttr.cpp
+++ b/clang/lib/Sema/ParsedAttr.cpp
@@ -225,7 +225,7 @@ bool ParsedAttr::slidesFromDeclToDeclSpecLegacyBehavior() const {
     // atributes.
     return false;
 
-  assert(isStandardAttributeSyntax());
+  assert(isStandardAttributeSyntax() || isAlignas());
 
   // We have historically allowed some type attributes with standard attribute
   // syntax to slide to the decl-specifier-seq, so we have to keep supporting
diff --git a/clang/test/C/C2y/n3254.c b/clang/test/C/C2y/n3254.c
index e08659cf377aa..347360a87faed 100644
--- a/clang/test/C/C2y/n3254.c
+++ b/clang/test/C/C2y/n3254.c
@@ -80,9 +80,9 @@ struct T {
 // CHECK-LABEL: define dso_local signext i8 @quux(
 // CHECK-SAME: ) #[[ATTR0]] {
 // CHECK-NEXT:  [[ENTRY:.*:]]
-// CHECK-NEXT:    [[T:%.*]] = alloca [[STRUCT_T:%.*]], align 1
+// CHECK-NEXT:    [[T:%.*]] = alloca [[STRUCT_T:%.*]], align 4
 // CHECK-NEXT:    [[S_PTR:%.*]] = alloca ptr, align 8
-// CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 1 [[T]], i8 0, i64 12, i1 false)
+// CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 4 [[T]], i8 0, i64 12, i1 false)
 // CHECK-NEXT:    [[BUFFER:%.*]] = getelementptr inbounds [[STRUCT_T]], ptr [[T]], i32 0, i32 0
 // CHECK-NEXT:    [[ARRAYDECAY:%.*]] = getelementptr inbounds [12 x i8], ptr [[BUFFER]], i64 0, i64 0
 // CHECK-NEXT:    store ptr [[ARRAYDECAY]], ptr [[S_PTR]], align 8
diff --git a/clang/test/Sema/alignas.c b/clang/test/Sema/alignas.c
index 020eff6a141c0..391553bc540ec 100644
--- a/clang/test/Sema/alignas.c
+++ b/clang/test/Sema/alignas.c
@@ -1,5 +1,6 @@
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c11 -Dalignof=__alignof %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c11 -Dalignof=_Alignof -DUSING_C11_SYNTAX %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c23 -DUSING_C11_SYNTAX %s
 
 _Alignas(3) int align_illegal; //expected-error {{requested alignment is not a power of 2}}
 _Alignas(int) char align_big;
@@ -18,12 +19,24 @@ void f(_Alignas(1) char c) { // expected-error {{'_Alignas' attribute cannot be
 }
 
 #ifdef USING_C11_SYNTAX
-// expected-warning at +4{{'_Alignof' applied to an expression is a GNU extension}}
-// expected-warning at +4{{'_Alignof' applied to an expression is a GNU extension}}
-// expected-warning at +4{{'_Alignof' applied to an expression is a GNU extension}}
+// expected-warning-re at +4{{'{{(_A|a)}}lignof' applied to an expression is a GNU extension}}
+// expected-warning-re at +4{{'{{(_A|a)}}lignof' applied to an expression is a GNU extension}}
+// expected-warning-re at +4{{'{{(_A|a)}}lignof' applied to an expression is a GNU extension}}
 #endif
 _Static_assert(alignof(align_big) == alignof(int), "k's alignment is wrong");
 _Static_assert(alignof(align_small) == 1, "j's alignment is wrong");
 _Static_assert(alignof(align_multiple) == 8, "l's alignment is wrong");
 _Static_assert(alignof(struct align_member) == 8, "quuux's alignment is wrong");
 _Static_assert(sizeof(struct align_member) == 8, "quuux's size is wrong");
+
+struct GH95032_1 {
+  _Alignas(16) char bytes[16];
+};
+_Static_assert(_Alignof(struct GH95032_1) == 16, "");
+
+#if __STDC_VERSION__ >= 202311L
+struct GH95032_2 {
+  alignas(16) char bytes[16];
+};
+static_assert(alignof(struct GH95032_2) == 16);
+#endif