[RFC libdrm] Use __sync_val_compare_and_swap to implement DRM_CAS

Matt Turner mattst88 at gmail.com
Thu Oct 28 16:59:13 PDT 2010

Can we use gcc's __sync_val_compare_and_swap to implement DRM_CAS?
(If so, do we actually need the __sync_val version, or can we use

I just threw the patch together in two minutes, so I've got no idea
if it's right, just looking for feedback. The purpose of this is to
remove a lot of inline assembly that hasn't been touched since the
file was added in 2004, including some awesome already-assembled
SPARC instructions.

Apparently someone had this idea before me. Way before me, as this
code already exists in the Itanium section, though commented out for
some reason long forgotten.

I don't have any idea when gcc added this. I just said 4.0, but this
must be wrong, given the previous paragraph.

 xf86drm.h |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/xf86drm.h b/xf86drm.h
index 9b89f56..1459b5a 100644
--- a/xf86drm.h
+++ b/xf86drm.h
@@ -333,7 +333,12 @@ typedef struct _drmSetVersion {
 #define DRM_LOCK_HELD  0x80000000U /**< Hardware lock is held */
 #define DRM_LOCK_CONT  0x40000000U /**< Hardware lock is contended */
-#if defined(__GNUC__) && (__GNUC__ >= 2)
+#if defined(__GNUC__) && (__GNUC__ >= 4)
+#define DRM_CAS(lock, old, new, __ret)	\
+	__ret = __sync_val_compare_and_swap(i&__drm_dummy_lock(lock), (old), (new)) != (old)
+#elif defined(__GNUC__) && (__GNUC__ >= 2)
 # if defined(__i386) || defined(__AMD64__) || defined(__x86_64__) || defined(__amd64__)
 				/* Reflect changes here to drmP.h */
 #define DRM_CAS(lock,old,new,__ret)                                    \

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