[PATCH v2] malloc: Improve performance of __libc_malloc

[Wilco Dijkstra]

v2: rebased to latest trunk

Improve performance of __libc_malloc by splitting it into 2 parts: first handle
the tcache fastpath, then do the rest in a separate tailcalled function.
This results in significant performance gains since __libc_malloc doesn't need
to setup a frame and we delay tcache initialization and setting of errno until
later.

On Neoverse V2, bench-malloc-simple improves by 6.7% overall (up to 8.5% for
ST case) and bench-malloc-thread improves by 20.3% for 1 thread and 14.4% for
32 threads.

Regress passed, OK for commit?

---

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 7e4c1399385051b1989cbc0ac14266d2138695af..8ad8d5da55c2086587e06d824a73344a3a76e7c4 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1325,6 +1325,9 @@ nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 static __always_inline size_t
 checked_request2size (size_t req) __nonnull (1)
 {
+  _Static_assert (PTRDIFF_MAX <= SIZE_MAX / 2,
+                  "PTRDIFF_MAX is not more than half of SIZE_MAX");
+
   if (__glibc_unlikely (req > PTRDIFF_MAX))
     return 0;
 
@@ -3380,26 +3383,17 @@ tcache_thread_shutdown (void)
 #endif /* !USE_TCACHE  */
 
 #if IS_IN (libc)
-void *
-__libc_malloc (size_t bytes)
+
+static void * __attribute_noinline__
+__libc_malloc2 (size_t bytes)
 {
   mstate ar_ptr;
   void *victim;
 
-  _Static_assert (PTRDIFF_MAX <= SIZE_MAX / 2,
-                  "PTRDIFF_MAX is not more than half of SIZE_MAX");
-
   if (!__malloc_initialized)
     ptmalloc_init ();
-#if USE_TCACHE
-  bool err = tcache_try_malloc (bytes, &victim);
-
-  if (err)
-      return NULL;
 
-  if (victim)
-      return tag_new_usable (victim);
-#endif
+  MAYBE_INIT_TCACHE ();
 
   if (SINGLE_THREAD_P)
     {
@@ -3430,6 +3424,19 @@ __libc_malloc (size_t bytes)
           ar_ptr == arena_for_chunk (mem2chunk (victim)));
   return victim;
 }
+
+void *
+__libc_malloc (size_t bytes)
+{
+#if USE_TCACHE
+  size_t tc_idx = csize2tidx (checked_request2size (bytes));
+
+  if (tcache_available (tc_idx))
+    return tcache_get (tc_idx);
+#endif
+
+  return __libc_malloc2 (bytes);
+}
 libc_hidden_def (__libc_malloc)
 
 void