[llvm] Fix compiler errors in GVN.h for clang versions older than 15 (PR #107636)

[Thomas Fransham via llvm-commits]

https://github.com/fsfod updated https://github.com/llvm/llvm-project/pull/107636

>From 1ee44f3d3ab976b35b1e81e2fe91da86684d8534 Mon Sep 17 00:00:00 2001
From: Thomas Fransham <tfransham at gmail.com>
Date: Fri, 6 Sep 2024 20:05:40 +0100
Subject: [PATCH] Fix compiler errors from clang versions older than 15 not
 supporting gnu style attributes on namespaces

Fixes errors in GVN.h after changes in #96630 to how LLVM_LIBRARY_VISIBILITY is defined
---
 llvm/include/llvm/Support/Compiler.h      | 9 +++++++++
 llvm/include/llvm/Transforms/Scalar/GVN.h | 2 +-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/llvm/include/llvm/Support/Compiler.h b/llvm/include/llvm/Support/Compiler.h
index e1d54af7dfcce1..140ff611af79fb 100644
--- a/llvm/include/llvm/Support/Compiler.h
+++ b/llvm/include/llvm/Support/Compiler.h
@@ -131,13 +131,22 @@
 #if (!(defined(_WIN32) || defined(__CYGWIN__)) ||                              \
      (defined(__MINGW32__) && defined(__clang__)))
 #define LLVM_LIBRARY_VISIBILITY LLVM_ATTRIBUTE_VISIBILITY_HIDDEN
+// Clang compilers older then 15 do not support gnu style attributes on
+// namespaces.
+#if defined(__clang__) && __clang_major__ < 15
+#define LLVM_LIBRARY_VISIBILITY_NAMESPACE [[gnu::visibility("hidden")]]
+#else
+#define LLVM_LIBRARY_VISIBILITY_NAMESPACE LLVM_ATTRIBUTE_VISIBILITY_HIDDEN
+#endif
 #define LLVM_ALWAYS_EXPORT LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #elif defined(_WIN32)
 #define LLVM_ALWAYS_EXPORT __declspec(dllexport)
 #define LLVM_LIBRARY_VISIBILITY
+#define LLVM_LIBRARY_VISIBILITY_NAMESPACE
 #else
 #define LLVM_LIBRARY_VISIBILITY
 #define LLVM_ALWAYS_EXPORT
+#define LLVM_LIBRARY_VISIBILITY_NAMESPACE
 #endif
 
 /// LLVM_ABI is the main export/visibility macro to mark something as explicitly
diff --git a/llvm/include/llvm/Transforms/Scalar/GVN.h b/llvm/include/llvm/Transforms/Scalar/GVN.h
index debe2ee7991728..be6c0ec5edab07 100644
--- a/llvm/include/llvm/Transforms/Scalar/GVN.h
+++ b/llvm/include/llvm/Transforms/Scalar/GVN.h
@@ -56,7 +56,7 @@ class TargetLibraryInfo;
 class Value;
 /// A private "module" namespace for types and utilities used by GVN. These
 /// are implementation details and should not be used by clients.
-namespace LLVM_LIBRARY_VISIBILITY gvn {
+namespace LLVM_LIBRARY_VISIBILITY_NAMESPACE gvn {
 
 struct AvailableValue;
 struct AvailableValueInBlock;