[PATCH] support: add __address_diff function
Yury Khrustalev
yury.khrustalev@arm.com
Wed Mar 25 10:59:00 GMT 2026
Some malloc tests compare pointers meaning to compare addresses.
On AArch64, the 64-bit value of the pointer may contain metadata
along with the values of the address.
In order to correctly compare addresses, we add new function for
AArch64 target that will use the AArch64 64 SUBP (subtract pointer)
instruction when it is available. This instruction uses the 56-bit
addresses ignoring top-byte metadata.
Best implementation is selected using ifunc resolver.
On other targets and also on AArch64 when MTE is not available this
function defaults to PTR_DIFF defined in libc-pointer-arith.h.
Three malloc tests are modified accordingly:
- tst-memalign-2.c
- tst-memalign-3.c
- tst-realloc.c
---
base-commit: 4a97c12302
passes regression on aarch64 and x86-64
there is probably a better way to add target-specific code
to the support library?
---
malloc/tst-memalign-2.c | 10 +++---
malloc/tst-memalign-3.c | 10 +++---
malloc/tst-realloc.c | 5 +--
support/Makefile | 1 +
support/pointer-arith.h | 31 +++++++++++++++++++
support/support_pointer_arith.c | 55 +++++++++++++++++++++++++++++++++
6 files changed, 100 insertions(+), 12 deletions(-)
create mode 100644 support/pointer-arith.h
create mode 100644 support/support_pointer_arith.c
diff --git a/malloc/tst-memalign-2.c b/malloc/tst-memalign-2.c
index b3e393c249..c402f2c356 100644
--- a/malloc/tst-memalign-2.c
+++ b/malloc/tst-memalign-2.c
@@ -22,7 +22,7 @@
#include <string.h>
#include <unistd.h>
#include <array_length.h>
-#include <libc-pointer-arith.h>
+#include <support/pointer-arith.h>
#include <support/check.h>
#include "tst-malloc-aux.h"
@@ -82,9 +82,9 @@ do_test (void)
/* This should return the same chunk as was just free'd. */
tcache_allocs[i].ptr2 = memalign (tcache_allocs[i].alignment, sz2);
CHECK (tcache_allocs[i].ptr2, tcache_allocs[i].alignment);
+ TEST_VERIFY (
+ __address_diff (tcache_allocs[i].ptr1, tcache_allocs[i].ptr2) == 0);
free (tcache_allocs[i].ptr2);
-
- TEST_VERIFY (tcache_allocs[i].ptr1 == tcache_allocs[i].ptr2);
}
/* Test for non-head tcache hits. This exercises the memalign
@@ -110,7 +110,7 @@ do_test (void)
count = 0;
for (i = 0; i < 10; ++ i)
- if (ptr[i] == p)
+ if (__address_diff (ptr[i], p) == 0)
++ count;
free (p);
TEST_VERIFY (count > 0);
@@ -146,7 +146,7 @@ do_test (void)
{
int ok = 0;
for (j = 0; j < LN; ++ j)
- if (large_allocs[i].ptr1 == large_allocs[j].ptr2)
+ if (__address_diff (large_allocs[i].ptr1, large_allocs[j].ptr2) == 0)
ok = 1;
if (ok == 1)
count ++;
diff --git a/malloc/tst-memalign-3.c b/malloc/tst-memalign-3.c
index 435e1c94d5..81756d265f 100644
--- a/malloc/tst-memalign-3.c
+++ b/malloc/tst-memalign-3.c
@@ -23,7 +23,7 @@
#include <string.h>
#include <unistd.h>
#include <array_length.h>
-#include <libc-pointer-arith.h>
+#include <support/pointer-arith.h>
#include <support/check.h>
#include <support/xthread.h>
#include "tst-malloc-aux.h"
@@ -85,9 +85,9 @@ mem_test (void *closure)
/* This should return the same chunk as was just free'd. */
tcache_allocs[i].ptr2 = memalign (tcache_allocs[i].alignment, sz2);
CHECK (tcache_allocs[i].ptr2, tcache_allocs[i].alignment);
+ TEST_VERIFY (
+ __address_diff (tcache_allocs[i].ptr1, tcache_allocs[i].ptr2) == 0);
free (tcache_allocs[i].ptr2);
-
- TEST_VERIFY (tcache_allocs[i].ptr1 == tcache_allocs[i].ptr2);
}
/* Test for non-head tcache hits. */
@@ -112,7 +112,7 @@ mem_test (void *closure)
count = 0;
for (i = 0; i < 10; ++ i)
- if (ptr[i] == p)
+ if (__address_diff (ptr[i], p) == 0)
++ count;
free (p);
TEST_VERIFY (count > 0);
@@ -146,7 +146,7 @@ mem_test (void *closure)
{
int ok = 0;
for (j = 0; j < LN; ++ j)
- if (large_allocs[i].ptr1 == large_allocs[j].ptr2)
+ if (__address_diff (large_allocs[i].ptr1, large_allocs[j].ptr2) == 0)
ok = 1;
if (ok == 1)
count ++;
diff --git a/malloc/tst-realloc.c b/malloc/tst-realloc.c
index e02b28d982..b5a0017b21 100644
--- a/malloc/tst-realloc.c
+++ b/malloc/tst-realloc.c
@@ -22,6 +22,7 @@
#include <string.h>
#include <libc-diag.h>
#include <support/check.h>
+#include <support/pointer-arith.h>
#include "tst-malloc-aux.h"
@@ -155,9 +156,9 @@ do_test (void)
size_t newsz = malloc_usable_size (p);
printf ("size: %zu, usable size: %zu, extra: %zu\n",
sz, newsz, newsz - sz);
- uintptr_t oldp = (uintptr_t) p;
+ void *oldp = p;
void *new_p = realloc (p, newsz);
- if ((uintptr_t) new_p != oldp)
+ if (__address_diff (new_p, oldp) != 0)
FAIL_EXIT1 ("Expanding (%zu bytes) to usable size (%zu) moved block",
sz, newsz);
free (new_p);
diff --git a/support/Makefile b/support/Makefile
index 1dae2802cf..67990fe075 100644
--- a/support/Makefile
+++ b/support/Makefile
@@ -94,6 +94,7 @@ libsupport-routines = \
support_openpty \
support_path_support_time64 \
support_paths \
+ support_pointer_arith \
support_process_state \
support_ptrace \
support_quote_blob \
diff --git a/support/pointer-arith.h b/support/pointer-arith.h
new file mode 100644
index 0000000000..fc9cf593ac
--- /dev/null
+++ b/support/pointer-arith.h
@@ -0,0 +1,31 @@
+/* Support functions for pointer arithmetic.
+ Copyright (C) 2026 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <https://www.gnu.org/licenses/>. */
+
+#ifndef _POINTER_ARITH_H
+#define _POINTER_ARITH_H 1
+
+#include <stddef.h>
+#include <libc-pointer-arith.h>
+
+#ifdef __ARM_ARCH_ISA_A64
+ptrdiff_t __address_diff (const void *lhs, const void *rhs);
+#else
+#define __address_diff PTR_DIFF
+#endif /* __ARM_ARCH_ISA_A64 */
+
+#endif
diff --git a/support/support_pointer_arith.c b/support/support_pointer_arith.c
new file mode 100644
index 0000000000..56217aa821
--- /dev/null
+++ b/support/support_pointer_arith.c
@@ -0,0 +1,55 @@
+/* Support functions for pointer arithmetic.
+ Copyright (C) 2026 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <https://www.gnu.org/licenses/>. */
+
+#ifdef __ARM_ARCH_ISA_A64
+
+#include "pointer-arith.h"
+
+#include <sys/ifunc.h>
+#include <sys/auxv.h>
+
+static ptrdiff_t
+__address_diff_mte (const void *lhs, const void *rhs)
+{
+ register const void *x0 asm ("x0") = lhs;
+ register const void *x1 asm ("x1") = rhs;
+ asm (".inst 0x9ac10000 /* subp x0, x0, x1 */" : "+r" (x0) : "r" (x1));
+ return (ptrdiff_t)x0;
+}
+
+static ptrdiff_t
+__address_diff_generic (const void *lhs, const void *rhs)
+{
+ return PTR_DIFF (lhs, rhs);
+}
+
+static void * __attribute__ ((unused))
+__address_diff_resolver (unsigned long a0, const unsigned long *a1)
+{
+ unsigned long hwcap2 = __ifunc_hwcap (_IFUNC_ARG_AT_HWCAP2, a0, a1);
+ if (hwcap2 & HWCAP2_MTE)
+ return (void *)__address_diff_mte;
+ return (void *)__address_diff_generic;
+}
+
+
+ptrdiff_t
+__address_diff (const void *lhs, const void *rhs)
+__attribute__ ((ifunc ("__address_diff_resolver")));
+
+#endif /* __ARM_ARCH_ISA_A64 */
--
2.47.3
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