On 11/17/20 1:44 PM, Richard Earnshaw (lists) wrote:
> On 03/11/2020 15:08, Bernd Edlinger wrote:
>> Hi,
>>
>> this fixes a problem with a missing symbol __sync_synchronize
>> which happens when newlib is used together with libstdc++ for
>> the non-threaded simulator target arm-none-eabi.
>>
>> There are several questions on stackoverflow about this issue.
>>
>> I would like to add a weak symbol for this target, since this
>> is only a default implementation and not meant to override a
>> possibly more sophisticated synchronization function from the
>> c-runtime.
>>
>>
>> Regression tested successfully on arm-none-eabi with newlib-3.3.0.
>>
>> Is it OK for trunk?
>>
>>
>> Thanks
>> Bernd.
>>
>
> I seem to recall that this was a deliberate decision - you can't guess
> this correctly, at least when trying to build portable code - you just
> have to know which runtime you will be using.
>
Therefore I suggest to use the weak attribute. It is on purpose not
implementing all of the atomics.
The use case, is a C++ program which initializes a local static variable.
$ cat test.cc
#include <string>
main(int argc, char **argv)
{
static std::string x = "test";
return 0;
}
compiles to this:
sub sp, sp, #20
str r0, [fp, #-24]
str r1, [fp, #-28]
ldr r3, .L14
ldrb r4, [r3]
bl __sync_synchronize
and r3, r4, #255
and r3, r3, #1
cmp r3, #0
moveq r3, #1
movne r3, #0
and r3, r3, #255
cmp r3, #0
beq .L8
ldr r0, .L14
bl __cxa_guard_acquire
mov r3, r0
so __sync_synchronize is not defined in newlib since the target (arm-sim)
is known to be not multi-threaded,
but __cxa_guard_acquire is also not a thread safe function,
because __GTHREADS is not defined by libgcc, since it is known
at configure time, that the target does not support threads.
So libstdc++ does not try to use a mutex or any atomics either,
because it is not compiled with __GTHREADS.
I can further narrow down the patch by only defining this function when
__GTHREADS is not defined, to make it more clear.
> I think Ramana had some changes in the works at one point to address
> (some) of this, but I'm not sure what happened to them. Ramana?
>
>
> +#if defined (__ARM_ARCH_6__) || defined (__ARM_ARCH_6J__) \
> + || defined (__ARM_ARCH_6K__) || defined (__ARM_ARCH_6T2__) \
> + || defined (__ARM_ARCH_6Z__) || defined (__ARM_ARCH_6ZK__) \
> + || defined (__ARM_ARCH_7__) || defined (__ARM_ARCH_7A__)
> +#if defined (__ARM_ARCH_7__) || defined (__ARM_ARCH_7A__)
>
> Ug, no! Use the ACLE macros to avoid this sort of mess.
>
Ah, thanks, copy-paste from freebsd-atomic.c :)
I've attached the updated patch.
Is it OK?
Thanks
Bernd.
From ca44e1fcb4b991306cbcde6293d20c77ce74ad68 Mon Sep 17 00:00:00 2001
From: Bernd Edlinger <bernd.edlin...@hotmail.de>
Date: Mon, 2 Nov 2020 11:43:44 +0100
Subject: [PATCH] libgcc: Add a weak stub for __sync_synchronize
This patch adds a default implementation for __sync_synchronize,
which prevents many unresolved symbol errors on arm-none-eabi.
This happens often in C++ programs even without any threading.
libgcc:
2020-11-17 Bernd Edlinger <bernd.edlin...@hotmail.de>
* config.host: Use t-eabi for arm-none-eabi.
* config/arm/t-eabi: New.
* config/arm/eabi-sync.c: New.
---
libgcc/config.host | 2 +-
libgcc/config/arm/eabi-sync.c | 38 ++++++++++++++++++++++++++++++++++++++
libgcc/config/arm/t-eabi | 1 +
3 files changed, 40 insertions(+), 1 deletion(-)
create mode 100644 libgcc/config/arm/eabi-sync.c
create mode 100644 libgcc/config/arm/t-eabi
diff --git a/libgcc/config.host b/libgcc/config.host
index 66af834..eaf74f1 100644
--- a/libgcc/config.host
+++ b/libgcc/config.host
@@ -496,7 +496,7 @@ arm*-*-eabi* | arm*-*-symbianelf* | arm*-*-rtems*)
tm_file="$tm_file arm/bpabi-lib.h"
case ${host} in
arm*-*-eabi* | arm*-*-rtems*)
- tmake_file="${tmake_file} arm/t-bpabi t-crtfm"
+ tmake_file="${tmake_file} arm/t-bpabi t-crtfm arm/t-eabi"
extra_parts="crtbegin.o crtend.o crti.o crtn.o"
;;
arm*-*-symbianelf*)
diff --git a/libgcc/config/arm/eabi-sync.c b/libgcc/config/arm/eabi-sync.c
new file mode 100644
index 0000000..c37bacf
--- /dev/null
+++ b/libgcc/config/arm/eabi-sync.c
@@ -0,0 +1,38 @@
+/* Copyright (C) 2020 Free Software Foundation, Inc.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+GCC 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 General Public License
+for more details.
+
+Under Section 7 of GPL version 3, you are granted additional
+permissions described in the GCC Runtime Library Exception, version
+3.1, as published by the Free Software Foundation.
+
+You should have received a copy of the GNU General Public License and
+a copy of the GCC Runtime Library Exception along with this program;
+see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
+<http://www.gnu.org/licenses/>. */
+
+#include "gthr.h"
+
+#ifndef __GTHREADS
+void __attribute__ ((weak))
+__sync_synchronize (void)
+{
+#if __ARM_ARCH >= 7
+ __asm __volatile ("dmb" : : : "memory");
+#elif ARM_ARCH == 6
+ __asm __volatile ("mcr p15, 0, r0, c7, c10, 5" : : : "memory");
+#else
+ __asm __volatile ("nop" : : : "memory");
+#endif
+}
+#endif
diff --git a/libgcc/config/arm/t-eabi b/libgcc/config/arm/t-eabi
new file mode 100644
index 0000000..556032f
--- /dev/null
+++ b/libgcc/config/arm/t-eabi
@@ -0,0 +1 @@
+LIB2ADD_ST += $(srcdir)/config/arm/eabi-sync.c
--
1.9.1
