[llvm] [llvm] annotate ABIBreakingChecks symobls for DLL export (PR #145575)

[Andrew Rogers via llvm-commits]

https://github.com/andrurogerz created https://github.com/llvm/llvm-project/pull/145575

## Purpose
This patch is one in a series of code-mods that annotate LLVM’s public interface for export. This patch annotates the ABI Breaking Checks interface in llvm/config. Since config cannot include `llvm/Support/Compiler.h` where the `LLVM_ABI` export macros are defined, the logic from that file is replicated inline. The annotations currently have no meaningful impact on the LLVM build; however, they are a prerequisite to support an LLVM Windows DLL (shared library) build.

## Background
The effort to build LLVM as a Windows DLL is tracked in #109483. Additional context is provided in [this discourse](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307), and documentation for `LLVM_ABI` and related annotations is found in the LLVM repo [here](https://github.com/llvm/llvm-project/blob/main/llvm/docs/InterfaceExportAnnotations.rst).  

## Validation
Local builds and tests to validate cross-platform compatibility. This included llvm, clang, and lldb on the following configurations:

- Windows with MSVC
- Windows with Clang
- Linux with GCC
- Linux with Clang
- Darwin with Clang

>From 14e5bedb1aa35b63af19c91f83e4e88afc961c7d Mon Sep 17 00:00:00 2001
From: Andrew Rogers <andrurogerz at gmail.com>
Date: Tue, 24 Jun 2025 12:07:32 -0700
Subject: [PATCH] [llvm] annotate ABIBreakingChecks symobls for DLL export

---
 llvm/include/llvm/Config/abi-breaking.h.cmake | 28 +++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/llvm/include/llvm/Config/abi-breaking.h.cmake b/llvm/include/llvm/Config/abi-breaking.h.cmake
index 2d27e02b1d545..345d2100f09f8 100644
--- a/llvm/include/llvm/Config/abi-breaking.h.cmake
+++ b/llvm/include/llvm/Config/abi-breaking.h.cmake
@@ -12,12 +12,36 @@
 #ifndef LLVM_ABI_BREAKING_CHECKS_H
 #define LLVM_ABI_BREAKING_CHECKS_H
 
+// llvm-config.h is required for LLVM_ENABLE_LLVM_EXPORT_ANNOTATIONS
+#include "llvm/Config/llvm-config.h"
+
 /* Define to enable checks that alter the LLVM C++ ABI */
 #cmakedefine01 LLVM_ENABLE_ABI_BREAKING_CHECKS
 
 /* Define to enable reverse iteration of unordered llvm containers */
 #cmakedefine01 LLVM_ENABLE_REVERSE_ITERATION
 
+// Properly annotate EnableABIBreakingChecks or DisableABIBreakingChecks for
+// export from shared library.
+#if !defined(LLVM_ABI_GENERATING_ANNOTATIONS)
+// TODO(https://github.com/llvm/llvm-project/issues/145406): eliminate need for
+// two preprocessor definitions to gate LLVM_ABI macro definitions.
+#if defined(LLVM_ENABLE_LLVM_EXPORT_ANNOTATIONS) && !defined(LLVM_BUILD_STATIC)
+#if defined(_WIN32) && !defined(__MINGW32__)
+#if defined(LLVM_EXPORTS)
+#define ABI_BREAKING_EXPORT_ABI __declspec(dllexport)
+#else
+#define ABI_BREAKING_EXPORT_ABI __declspec(dllimport)
+#endif
+#elif defined(__has_attribute) && __has_attribute(visibility)
+#define ABI_BREAKING_EXPORT_ABI __attribute__((visibility("default")))
+#endif
+#endif
+#if !defined(ABI_BREAKING_EXPORT_ABI)
+#define ABI_BREAKING_EXPORT_ABI
+#endif
+#endif
+
 /* Allow selectively disabling link-time mismatch checking so that header-only
    ADT content from LLVM can be used without linking libSupport. */
 #if !defined(LLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING) || !LLVM_DISABLE_ABI_BREAKING_CHECKS_ENFORCING
@@ -43,12 +67,12 @@
 #endif
 namespace llvm {
 #if LLVM_ENABLE_ABI_BREAKING_CHECKS
-extern int EnableABIBreakingChecks;
+ABI_BREAKING_EXPORT_ABI extern int EnableABIBreakingChecks;
 LLVM_HIDDEN_VISIBILITY
 __attribute__((weak)) int *VerifyEnableABIBreakingChecks =
     &EnableABIBreakingChecks;
 #else
-extern int DisableABIBreakingChecks;
+ABI_BREAKING_EXPORT_ABI extern int DisableABIBreakingChecks;
 LLVM_HIDDEN_VISIBILITY
 __attribute__((weak)) int *VerifyDisableABIBreakingChecks =
     &DisableABIBreakingChecks;