malloc: Only support zeroing and not arbitrary memset with mtag

The memset api is suboptimal and does not provide much benefit. Memory
tagging only needs a zeroing memset (and only for memory that's sized
and aligned to multiples of the tag granule), so change the internal
api and the target hooks accordingly.  This is to simplify the
implementation of the target hook.

Reviewed-by: DJ Delorie <dj@redhat.com>

malloc/malloc.c

@@ -413,12 +413,11 @@ void *(*__morecore)(ptrdiff_t) = __default_morecore;
    operations can continue to be used.  Support macros are used to do
    this:
 
-   void *tag_new_memset (void *ptr, int, val, size_t size)
+   void *tag_new_zero_region (void *ptr, size_t size)
 
-   Has the same interface as memset(), but additionally allocates a
-   new tag, colors the memory with that tag and returns a pointer that
-   is correctly colored for that location.  The non-tagging version
-   will simply call memset.
+   Allocates a new tag, colors the memory with that tag, zeros the
+   memory and returns a pointer that is correctly colored for that
+   location.  The non-tagging version will simply call memset with 0.
 
    void *tag_region (void *ptr, size_t size)
 
@@ -458,11 +457,11 @@ tag_region (void *ptr, size_t size)
 }
 
 static __always_inline void *
-tag_new_memset (void *ptr, int val, size_t size)
+tag_new_zero_region (void *ptr, size_t size)
 {
   if (__glibc_unlikely (mtag_enabled))
-    return __libc_mtag_memset_with_tag (__libc_mtag_new_tag (ptr), val, size);
-  return memset (ptr, val, size);
+    return __libc_mtag_tag_zero_region (__libc_mtag_new_tag (ptr), size);
+  return memset (ptr, 0, size);
 }
 
 /* Defined later.  */
@@ -3679,7 +3678,7 @@ __libc_calloc (size_t n, size_t elem_size)
      regardless of MORECORE_CLEARS, so we zero the whole block while
      doing so.  */
 #ifdef USE_MTAG
-  return tag_new_memset (mem, 0, CHUNK_AVAILABLE_SIZE (p) - CHUNK_HDR_SZ);
+  return tag_new_zero_region (mem, CHUNK_AVAILABLE_SIZE (p) - CHUNK_HDR_SZ);
 #else
   INTERNAL_SIZE_T csz = chunksize (p);
 

sysdeps/aarch64/Makefile

@@ -41,7 +41,7 @@ endif
 ifeq ($(subdir),misc)
 sysdep_headers += sys/ifunc.h
 sysdep_routines += __mtag_address_get_tag \
-		   __mtag_memset_tag \
+		   __mtag_tag_zero_region \
 		   __mtag_new_tag \
 		   __mtag_tag_region
 

sysdeps/aarch64/__mtag_tag_zero_region.S

@@ -20,9 +20,6 @@
 
 #ifdef USE_MTAG
 
-/* Use the same register names and assignments as memset.  */
-#include "memset-reg.h"
-
 	.arch armv8.5-a
 	.arch_extension memtag
 
@@ -31,16 +28,15 @@
 /* FIXME: This is a minimal implementation.  We could do much better than
    this for large values of COUNT.  */
 
-ENTRY(__libc_mtag_memset_with_tag)
+#define dstin x0
+#define count x1
+#define dst   x2
+
+ENTRY(__libc_mtag_tag_zero_region)
 
-	and	valw, valw, 255
-	orr	valw, valw, valw, lsl 8
-	orr	valw, valw, valw, lsl 16
-	orr	val, val, val, lsl 32
 	mov	dst, dstin
-
 L(loop):
-	stgp	val, val, [dst], #16
+	stzg	dst, [dst], #16
 	subs	count, count, 16
 	bne	L(loop)
 #if 0
@@ -49,5 +45,5 @@ L(loop):
 	ldg	dstin, [dstin] // Recover the tag created (might be untagged).
 #endif
 	ret
-END (__libc_mtag_memset_with_tag)
+END (__libc_mtag_tag_zero_region)
 #endif /* USE_MTAG */

sysdeps/aarch64/libc-mtag.h

@@ -39,8 +39,8 @@
    void *__libc_mtag_tag_region (const void *, size_t)  */
 void *__libc_mtag_tag_region (void *, size_t);
 
-/* Optimized equivalent to __libc_mtag_tag_region followed by memset.  */
-void *__libc_mtag_memset_with_tag (void *, int, size_t);
+/* Optimized equivalent to __libc_mtag_tag_region followed by memset to 0.  */
+void *__libc_mtag_tag_zero_region (void *, size_t);
 
 /* Convert address P to a pointer that is tagged correctly for that
    location.

sysdeps/generic/libc-mtag.h

@@ -44,12 +44,12 @@ __libc_mtag_tag_region (void *p, size_t n)
   return p;
 }
 
-/* Optimized equivalent to __libc_mtag_tag_region followed by memset.  */
+/* Optimized equivalent to __libc_mtag_tag_region followed by memset to 0.  */
 static inline void *
-__libc_mtag_memset_with_tag (void *p, int c, size_t n)
+__libc_mtag_tag_zero_region (void *p, size_t n)
 {
   __libc_mtag_link_error ();
-  return memset (p, c, n);
+  return memset (p, 0, n);
 }
 
 /* Convert address P to a pointer that is tagged correctly for that