[llvm] [CMake] Fix __builtin_thread_pointer check with LTO (PR #70968)

[Sam James via llvm-commits]

https://github.com/thesamesam updated https://github.com/llvm/llvm-project/pull/70968

>From a54545ba6514802178cf7cf1c1dd9f7efbf3cde7 Mon Sep 17 00:00:00 2001
From: Sam James <sam at gentoo.org>
Date: Wed, 1 Nov 2023 18:12:16 +0000
Subject: [PATCH] [CMake] Fix __builtin_thread_pointer check with LTO
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With LTO, gcc's IPA passes might drop the foo() function and then the test
will pass even on platforms where __builtin_thread_pointer is unavailable.

On PPC64, we get this as a result:
```
llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp:361:61: error: ‘__builtin_thread_pointer’ is not supported on this targ
```

Just mark the function in the CMake configure test with the 'used' attribute to
avoid it being optimised out. The test then behaves correctly with -flto.

Tested with e.g. 'powerpc64le-linux-gnu-gcc -O2 -flto a.c'.

Reported-by: matoro
Reviewed-by: maskray
Closes: https://github.com/llvm/llvm-project/pull/70968
Signed-off-by: Sam James <sam at gentoo.org>
---
 llvm/cmake/config-ix.cmake | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/llvm/cmake/config-ix.cmake b/llvm/cmake/config-ix.cmake
index 64c63e199ffa4e4..7bb3e98333effb5 100644
--- a/llvm/cmake/config-ix.cmake
+++ b/llvm/cmake/config-ix.cmake
@@ -65,7 +65,12 @@ check_include_file(fenv.h HAVE_FENV_H)
 check_symbol_exists(FE_ALL_EXCEPT "fenv.h" HAVE_DECL_FE_ALL_EXCEPT)
 check_symbol_exists(FE_INEXACT "fenv.h" HAVE_DECL_FE_INEXACT)
 check_c_source_compiles("
-        void *foo() {
+        #if __has_attribute(used)
+        #define LLVM_ATTRIBUTE_USED __attribute__((__used__))
+        #else
+        #define LLVM_ATTRIBUTE_USED
+        #endif
+        LLVM_ATTRIBUTE_USED void *foo() {
           return __builtin_thread_pointer();
         }
         int main(void) { return 0; }"