[Clang] Fix build with GCC 14 on ARM

[nikic]

GCC 14 defines __arm_streaming as a macro expanding to [[arm::streaming]]. Due to the nested macro use, this gets expanded prior to concatenation.

It doesn't look like C++ has a really clean way to prevent macro expansion. The best I have found is to use EMPTY ## X where EMPTY is an empty macro argument, so this is the hack I'm implementing here.

Fixes #78691.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Nikita Popov (nikic)

Changes

GCC 14 defines __arm_streaming as a macro expanding to [[arm::streaming]]. Due to the nested macro use, this gets expanded prior to concatenation.

It doesn't look like C++ has a really clean way to prevent macro expansion. The best I have found is to use EMPTY ## X where EMPTY is an empty macro argument, so this is the hack I'm implementing here.

Fixes #78691.

---

Full diff: https://github.com/llvm/llvm-project/pull/78704.diff

3 Files Affected:

• (modified) clang/include/clang/Basic/AttributeCommonInfo.h (+1-1)
• (modified) clang/include/clang/Basic/TokenKinds.def (+2-1)
• (modified) clang/utils/TableGen/ClangAttrEmitter.cpp (+1-1)

diff --git a/clang/include/clang/Basic/AttributeCommonInfo.h b/clang/include/clang/Basic/AttributeCommonInfo.h
index d787e4959bfee3..ef2ddf525c9814 100644
--- a/clang/include/clang/Basic/AttributeCommonInfo.h
+++ b/clang/include/clang/Basic/AttributeCommonInfo.h
@@ -260,7 +260,7 @@ inline bool doesKeywordAttributeTakeArgs(tok::TokenKind Kind) {
   switch (Kind) {
   default:
     return false;
-#define KEYWORD_ATTRIBUTE(NAME, HASARG)                                        \
+#define KEYWORD_ATTRIBUTE(NAME, HASARG, ...)                                   \
   case tok::kw_##NAME:                                                         \
     return HASARG;
 #include "clang/Basic/RegularKeywordAttrInfo.inc"
diff --git a/clang/include/clang/Basic/TokenKinds.def b/clang/include/clang/Basic/TokenKinds.def
index d15e4970b7d8f1..c10e2adfbe6e96 100644
--- a/clang/include/clang/Basic/TokenKinds.def
+++ b/clang/include/clang/Basic/TokenKinds.def
@@ -760,8 +760,9 @@ KEYWORD(__builtin_available              , KEYALL)
 KEYWORD(__builtin_sycl_unique_stable_name, KEYSYCL)
 
 // Keywords defined by Attr.td.
+// The "EMPTY ## X" is used to prevent early macro-expansion of the keyword.
 #ifndef KEYWORD_ATTRIBUTE
-#define KEYWORD_ATTRIBUTE(X, ...) KEYWORD(X, KEYALL)
+#define KEYWORD_ATTRIBUTE(X, HASARG, EMPTY) KEYWORD(EMPTY ## X, KEYALL)
 #endif
 #include "clang/Basic/RegularKeywordAttrInfo.inc"
 
diff --git a/clang/utils/TableGen/ClangAttrEmitter.cpp b/clang/utils/TableGen/ClangAttrEmitter.cpp
index a1827f8ce0ead8..3888e6c08ab0f4 100644
--- a/clang/utils/TableGen/ClangAttrEmitter.cpp
+++ b/clang/utils/TableGen/ClangAttrEmitter.cpp
@@ -3589,7 +3589,7 @@ void EmitClangRegularKeywordAttributeInfo(RecordKeeper &Records,
 
       OS << "KEYWORD_ATTRIBUTE("
          << S.getSpellingRecord().getValueAsString("Name") << ", "
-         << (HasArgs ? "true" : "false") << ")\n";
+         << (HasArgs ? "true" : "false") << ", )\n";
     }
   OS << "#undef KEYWORD_ATTRIBUTE\n";
 }

[sdesmalen-arm]

Would the following suffice?

#define EMPTY
#define KEYWORD_ATTRIBUTE(X, HASARG) KEYWORD(EMPTY ## X, KEYALL)

Or is the crux of this fix that EMPTY must be an argument to the same macro in order for it not to get expanded?

[nikic]

Macros don't get expanded in ## so this will produce something like EMPTY__arm_streaming. I believe the only way to actually get an empty concatenation is via an empty macro argument.

Here's a godbolt to experiment: https://cpp.godbolt.org/z/d9zc7hs3a

[sdesmalen-arm]

I see, thanks for sharing the example.