[Mesa-dev] [PATCH] configure.ac: Fix -latomic test

[Nicolas Boichat]

From: Nicolas Boichat <drinkcat at chromium.org>

When compiling with LLVM 6.0, the test fails to detect that
-latomic is actually required, as the atomic call is inlined.

In the code itself (src/util/disk_cache.c), we see this pattern:
p_atomic_add(cache->size, - (uint64_t)size);
where cache->size is an uint64_t *, and results in the following
link time error without -latomic:
src/util/disk_cache.c:628: error: undefined reference to '__atomic_fetch_add_8'

Fix the configure test to replicate this pattern, which then
correctly realizes the need for -latomic.

Signed-off-by: Nicolas Boichat <drinkcat at chromium.org>
---
 configure.ac | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index e874f8ebfb2..eff9a0ef88f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -445,9 +445,11 @@ if test "x$GCC_ATOMIC_BUILTINS_SUPPORTED" = x1; then
     AC_MSG_CHECKING(whether -latomic is needed)
     AC_LINK_IFELSE([AC_LANG_SOURCE([[
     #include <stdint.h>
-    uint64_t v;
+    struct {
+        uint64_t* v;
+    } x;
     int main() {
-        return (int)__atomic_load_n(&v, __ATOMIC_ACQUIRE);
+        return (int)__atomic_load_n(x.v, __ATOMIC_ACQUIRE);
     }]])], GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC=no, GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC=yes)
     AC_MSG_RESULT($GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC)
     if test "x$GCC_ATOMIC_BUILTINS_NEED_LIBATOMIC" = xyes; then
-- 
2.17.0.rc1.321.gba9d0f2565-goog