atomic asm snippets: Replace "memory" clobber with "Q" constraint

In our atomic asm snippets, using the "Q" constraint,
(https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html tells gcc
which exact memory is modified i.e. the bytes covered by the atomic variable),
is better than using the "memory" clobber (tells gcc that all memory is
modified, effectively a compiler memory barrier which prevents compiler
optimizations).

diff --git a/open-vm-tools/lib/include/vm_atomic.h b/open-vm-tools/lib/include/vm_atomic.h
index 45775938e9a6f3ecd7c24fd752ba77909e22a0fe..e089d0027bc1c97145d3d8cba24c7d1bd94a3344 100644 (file)
--- a/open-vm-tools/lib/include/vm_atomic.h
+++ b/open-vm-tools/lib/include/vm_atomic.h
@@ -307,23 +307,24 @@ Atomic_ReadIfEqualWrite128(Atomic_uint128 *ptr,   // IN/OUT
    uint32 failed;
 
    LDST_LDST_MEM_BARRIER();
-   __asm__ __volatile__ (
-      "1: ldxp    %x0, %x1, [%3]      \n\t"
+   __asm__ __volatile__(
+      "1: ldxp    %x0, %x1, %3        \n\t"
       "   cmp     %x0, %x4            \n\t"
       "   ccmp    %x1, %x5, #0, eq    \n\t"
       "   b.ne    2f                  \n\t"
-      "   stxp    %w2, %x6, %x7, [%3] \n\t"
+      "   stxp    %w2, %x6, %x7, %3   \n\t"
       "   cbnz    %w2, 1b             \n\t"
       "2: clrex                       \n\t"
       : "=&r" (res.lo),
         "=&r" (res.hi),
-        "=&r" (failed)
-      : "r" (&ptr->value),
-        "r" (_old.lo),
+        "=&r" (failed),
+        "+Q" (ptr->value)
+      : "r" (_old.lo),
         "r" (_old.hi),
         "r" (_new.lo),
         "r" (_new.hi)
-      : "cc", "memory");
+      : "cc"
+   );
    LDST_LDST_MEM_BARRIER();
 
    return res.raw;