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[6/6] malloc: Ensure mtag code path in checked_request2size is cold

Message ID 57b96253346fe679c82b7f88e2398ec21553b52c.1616155129.git.szabolcs.nagy@arm.com
State New
Headers show
Series malloc: more memory tagging optimizations | expand

Commit Message

Szabolcs Nagy March 19, 2021, 1:27 p.m. UTC 
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.
---
 malloc/malloc.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)
diff mbox series

Patch

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;