[clang] [clang][Fuchsia] Add __Fuchsia_Compiler_ABI__ predefine (PR #211744)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Roland McGrath (frobtech)

<details>
<summary>Changes</summary>

This establishes a new canonical predefine for *-fuchsia targets:
`__Fuchsia_Compiler_ABI__` is set to an integer for the version
number of the Fuchsia Compiler ABI the compiler supports.  The
value will change as appropriate with changes to *-fuchsia target
ABI expectations.  It is not anticipated that the compiler will
support selecting target ABI versions, only the latest as the
canonical target-specific behavior.  Fuchsia Compiler ABI
versions are unrelated to the Fuchsia API levels supported by
Fuchsia systems; newly-built programs may target older API levels
with `-ffuchsia-api-level`, but everything relying on the Fuchsia
Compiler ABI is built and deployed together as an isolated unit.


---
Full diff: https://github.com/llvm/llvm-project/pull/211744.diff


2 Files Affected:

- (modified) clang/lib/Basic/Targets/OSTargets.cpp (+19) 
- (modified) clang/test/Preprocessor/init-fuchsia.c (+1) 


``````````diff
diff --git a/clang/lib/Basic/Targets/OSTargets.cpp b/clang/lib/Basic/Targets/OSTargets.cpp
index 915cd566ed6e4..04d9e13d3c9a6 100644
--- a/clang/lib/Basic/Targets/OSTargets.cpp
+++ b/clang/lib/Basic/Targets/OSTargets.cpp
@@ -306,6 +306,25 @@ void getFuchsiaDefines(MacroBuilder &Builder, const LangOptions &Opts,
   if (Opts.CPlusPlus)
     Builder.defineMacro("_GNU_SOURCE");
   Builder.defineMacro("__Fuchsia_API_level__", Twine(Opts.FuchsiaAPILevel));
+
+  // __Fuchsia_Compiler_ABI__ gives the version of the Fuchsia Compiler ABI
+  // supported by the compiler.  This is a single small integer that increases
+  // monotonically for any kinds of ABI change, across any of the supported
+  // machines, and including language-specific ABI aspects such as the Fuchsia
+  // variant of the Itanium C++ ABI.  Changes affecting just one machine and/or
+  // just one language will cause this single number to advance regardless.
+  //
+  // * Version 0 is the original Fuchsia Compiler ABI.
+  //   - Custom SafeStack ABI on every machine
+  //   - ShadowCallStack ABI on aarch64, riscv64
+  //   - Itanium C++ ABI as modified: ctor/dtor return value; relative vtables
+  //
+  // * Version 1 is first iteration formalizing the version number scheme.
+  //   - SafeStack ABI no longer supported on aarch64, riscv64 (x86 only)
+  //
+  // The predefine can be checked by code relying on the Fuchsia Compiler ABI
+  // to ensure that a compiler new enough for the required version is in use.
+  Builder.defineMacro("__Fuchsia_Compiler_ABI__", Twine(1));
 }
 
 } // namespace targets
diff --git a/clang/test/Preprocessor/init-fuchsia.c b/clang/test/Preprocessor/init-fuchsia.c
index 057218711b8b2..04b5ef573cc07 100644
--- a/clang/test/Preprocessor/init-fuchsia.c
+++ b/clang/test/Preprocessor/init-fuchsia.c
@@ -9,3 +9,4 @@
 
 // FUCHSIA-DAG: #define __Fuchsia__ 1
 // FUCHSIA-DAG: #define __Fuchsia_API_level__ {{[0-9]+}}
+// FUCHSIA-DAG: #define __Fuchsia_Compiler_ABI__ 1

``````````

</details>


https://github.com/llvm/llvm-project/pull/211744