malloc: Ensure mtag code path in checked_request2size is cold nsz/mtag-2

This is a workaround (hack) for a gcc optimization issue (PR 99551).
Without this the generated code may evaluate the expression in the
cold path which causes performance regression for small allocations
in the memory tagging disabled (common) case.

1 files changed, 7 insertions, 2 deletions

diff --git a/malloc/malloc.c b/malloc/malloc.c
index cb1837d0d7..ef1f5b1621 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1357,8 +1357,13 @@ checked_request2size (size_t req, size_t *sz) __nonnull (1)
      must be a macro that produces a compile time constant if passed
      a constant literal.  */
   if (__glibc_unlikely (mtag_enabled))
-    req = (req + (__MTAG_GRANULE_SIZE - 1)) &
-	  ~(size_t)(__MTAG_GRANULE_SIZE - 1);
+    {
+      /* Ensure this is not evaluated if !mtag_enabled, see gcc PR 99551.  */
+      asm ("");
+
+      req = (req + (__MTAG_GRANULE_SIZE - 1)) &
+	    ~(size_t)(__MTAG_GRANULE_SIZE - 1);
+    }
 
   *sz = request2size (req);
   return true;