* [PATCH] clang: disable the gcc-compat warnings for read_atomic
@ 2017-04-11 7:54 Roger Pau Monne
2017-04-11 8:35 ` Jan Beulich
` (2 more replies)
0 siblings, 3 replies; 6+ messages in thread
From: Roger Pau Monne @ 2017-04-11 7:54 UTC (permalink / raw)
To: xen-devel; +Cc: Andrew Cooper, Jan Beulich, Roger Pau Monne
clang gcc-compat warnings can wrongly fire when certain constructions are used,
at least the following flow:
switch ( ... )
{
case ...:
while ( ({ int x; switch ( foo ) { case 1: x = 1; break; } x }) )
{
...
Will cause clang to emit the following warning "'break' is bound to loop, GCC
binds it to switch", which is a false positive, and both gcc and clang bound
the break to the inner switch. In order to workaround this issue, disable the
gcc-compat checks for the usage of the read_atomic macro.
This has been reported upstream as http://bugs.llvm.org/show_bug.cgi?id=32595.
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
---
xen/include/asm-x86/atomic.h | 2 ++
xen/include/xen/compiler.h | 27 +++++++++++++++++++++++++++
2 files changed, 29 insertions(+)
diff --git a/xen/include/asm-x86/atomic.h b/xen/include/asm-x86/atomic.h
index 2fbe705518..b997a1726b 100644
--- a/xen/include/asm-x86/atomic.h
+++ b/xen/include/asm-x86/atomic.h
@@ -45,6 +45,7 @@ void __bad_atomic_size(void);
#define read_atomic(p) ({ \
unsigned long x_; \
+ CLANG_DISABLE_WARN_GCC_COMPAT_START \
switch ( sizeof(*(p)) ) { \
case 1: x_ = read_u8_atomic((uint8_t *)(p)); break; \
case 2: x_ = read_u16_atomic((uint16_t *)(p)); break; \
@@ -52,6 +53,7 @@ void __bad_atomic_size(void);
case 8: x_ = read_u64_atomic((uint64_t *)(p)); break; \
default: x_ = 0; __bad_atomic_size(); break; \
} \
+ CLANG_DISABLE_WARN_GCC_COMPAT_END \
(typeof(*(p)))x_; \
})
diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
index 16aeeea7f1..17d1f33a2d 100644
--- a/xen/include/xen/compiler.h
+++ b/xen/include/xen/compiler.h
@@ -100,4 +100,31 @@
# define ASM_FLAG_OUT(yes, no) no
#endif
+/*
+ * NB: we need to disable the gcc-compat warnings for clang in some places or
+ * else it will complain with: "'break' is bound to loop, GCC binds it to
+ * switch" when a switch is used inside of a while expression inside of a
+ * switch statement, ie:
+ *
+ * switch ( ... )
+ * {
+ * case ...:
+ * while ( ({ int x; switch ( foo ) { case 1: x = 1; break; } x }) )
+ * {
+ * ...
+ *
+ * This has already been reported upstream:
+ * http://bugs.llvm.org/show_bug.cgi?id=32595
+ */
+#ifdef __clang__
+# define CLANG_DISABLE_WARN_GCC_COMPAT_START \
+ _Pragma("clang diagnostic push") \
+ _Pragma("clang diagnostic ignored \"-Wgcc-compat\"")
+# define CLANG_DISABLE_WARN_GCC_COMPAT_END \
+ _Pragma("clang diagnostic pop")
+#else
+# define CLANG_DISABLE_WARN_GCC_COMPAT_START
+# define CLANG_DISABLE_WARN_GCC_COMPAT_END
+#endif
+
#endif /* __LINUX_COMPILER_H */
--
2.11.0 (Apple Git-81)
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^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH] clang: disable the gcc-compat warnings for read_atomic
2017-04-11 7:54 [PATCH] clang: disable the gcc-compat warnings for read_atomic Roger Pau Monne
@ 2017-04-11 8:35 ` Jan Beulich
2017-04-11 8:46 ` Roger Pau Monne
2017-04-13 15:11 ` Jan Beulich
2017-08-01 7:04 ` Roger Pau Monne
2 siblings, 1 reply; 6+ messages in thread
From: Jan Beulich @ 2017-04-11 8:35 UTC (permalink / raw)
To: Roger Pau Monne; +Cc: Andrew Cooper, xen-devel
>>> On 11.04.17 at 09:54, <roger.pau@citrix.com> wrote:
> clang gcc-compat warnings can wrongly fire when certain constructions are used,
> at least the following flow:
>
> switch ( ... )
> {
> case ...:
> while ( ({ int x; switch ( foo ) { case 1: x = 1; break; } x }) )
> {
> ...
>
> Will cause clang to emit the following warning "'break' is bound to loop, GCC
> binds it to switch", which is a false positive, and both gcc and clang bound
bind (I think)
> the break to the inner switch. In order to workaround this issue, disable the
> gcc-compat checks for the usage of the read_atomic macro.
Hmm, so far it wasn't clear to me that this is also needing an outer
switch() - can you confirm switch inside the while control expression
alone does not trigger the warning?
> This has been reported upstream as
> http://bugs.llvm.org/show_bug.cgi?id=32595.
>
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
The patch itself is fine with me, i.e.
Acked-by: Jan Beulich <jbeulich@suse.com>
but besides wanting to wait for above confirmation, I'd also like
to allow some time for others to voice objections to this new
approach.
Jan
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] clang: disable the gcc-compat warnings for read_atomic
2017-04-11 8:35 ` Jan Beulich
@ 2017-04-11 8:46 ` Roger Pau Monne
0 siblings, 0 replies; 6+ messages in thread
From: Roger Pau Monne @ 2017-04-11 8:46 UTC (permalink / raw)
To: Jan Beulich; +Cc: Andrew Cooper, xen-devel
On Tue, Apr 11, 2017 at 02:35:59AM -0600, Jan Beulich wrote:
> >>> On 11.04.17 at 09:54, <roger.pau@citrix.com> wrote:
> > clang gcc-compat warnings can wrongly fire when certain constructions are used,
> > at least the following flow:
> >
> > switch ( ... )
> > {
> > case ...:
> > while ( ({ int x; switch ( foo ) { case 1: x = 1; break; } x }) )
^ missing ;
> > {
> > ...
> >
> > Will cause clang to emit the following warning "'break' is bound to loop, GCC
> > binds it to switch", which is a false positive, and both gcc and clang bound
>
> bind (I think)
Right, I think so.
>
> > the break to the inner switch. In order to workaround this issue, disable the
> > gcc-compat checks for the usage of the read_atomic macro.
>
> Hmm, so far it wasn't clear to me that this is also needing an outer
> switch() - can you confirm switch inside the while control expression
> alone does not trigger the warning?
Yes, I can confirm that the following:
while ( ({ int x; switch ( foo ) { case 1: x = 1; break; } x; }) )
{
...
Does not trigger the warning.
> > This has been reported upstream as
> > http://bugs.llvm.org/show_bug.cgi?id=32595.
> >
> > Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
>
> The patch itself is fine with me, i.e.
> Acked-by: Jan Beulich <jbeulich@suse.com>
>
> but besides wanting to wait for above confirmation, I'd also like
> to allow some time for others to voice objections to this new
> approach.
Thanks, Roger.
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] clang: disable the gcc-compat warnings for read_atomic
2017-04-11 7:54 [PATCH] clang: disable the gcc-compat warnings for read_atomic Roger Pau Monne
2017-04-11 8:35 ` Jan Beulich
@ 2017-04-13 15:11 ` Jan Beulich
2017-04-13 15:23 ` Julien Grall
2017-08-01 7:04 ` Roger Pau Monne
2 siblings, 1 reply; 6+ messages in thread
From: Jan Beulich @ 2017-04-13 15:11 UTC (permalink / raw)
To: Julien Grall; +Cc: Andrew Cooper, xen-devel, Roger Pau Monne
>>> On 11.04.17 at 09:54, <roger.pau@citrix.com> wrote:
> clang gcc-compat warnings can wrongly fire when certain constructions are used,
> at least the following flow:
>
> switch ( ... )
> {
> case ...:
> while ( ({ int x; switch ( foo ) { case 1: x = 1; break; } x }) )
> {
> ...
>
> Will cause clang to emit the following warning "'break' is bound to loop, GCC
> binds it to switch", which is a false positive, and both gcc and clang bound
> the break to the inner switch. In order to workaround this issue, disable the
> gcc-compat checks for the usage of the read_atomic macro.
>
> This has been reported upstream as
> http://bugs.llvm.org/show_bug.cgi?id=32595.
>
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Julien,
Roger forgot to Cc you on this - thoughts wrt 4.9 (without it Xen
won't build with clang aiui)?
Jan
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] clang: disable the gcc-compat warnings for read_atomic
2017-04-13 15:11 ` Jan Beulich
@ 2017-04-13 15:23 ` Julien Grall
0 siblings, 0 replies; 6+ messages in thread
From: Julien Grall @ 2017-04-13 15:23 UTC (permalink / raw)
To: Jan Beulich; +Cc: Andrew Cooper, xen-devel, Roger Pau Monne
Hi Jan,
On 13/04/17 16:11, Jan Beulich wrote:
>>>> On 11.04.17 at 09:54, <roger.pau@citrix.com> wrote:
>> clang gcc-compat warnings can wrongly fire when certain constructions are used,
>> at least the following flow:
>>
>> switch ( ... )
>> {
>> case ...:
>> while ( ({ int x; switch ( foo ) { case 1: x = 1; break; } x }) )
>> {
>> ...
>>
>> Will cause clang to emit the following warning "'break' is bound to loop, GCC
>> binds it to switch", which is a false positive, and both gcc and clang bound
>> the break to the inner switch. In order to workaround this issue, disable the
>> gcc-compat checks for the usage of the read_atomic macro.
>>
>> This has been reported upstream as
>> http://bugs.llvm.org/show_bug.cgi?id=32595.
>>
>> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
>
> Julien,
>
> Roger forgot to Cc you on this - thoughts wrt 4.9 (without it Xen
> won't build with clang aiui)?
On the basis that it will break Xen build with clang:
Release-acked-by: Julien Grall <julien.grall@arm.com>
Cheers,
--
Julien Grall
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^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] clang: disable the gcc-compat warnings for read_atomic
2017-04-11 7:54 [PATCH] clang: disable the gcc-compat warnings for read_atomic Roger Pau Monne
2017-04-11 8:35 ` Jan Beulich
2017-04-13 15:11 ` Jan Beulich
@ 2017-08-01 7:04 ` Roger Pau Monne
2 siblings, 0 replies; 6+ messages in thread
From: Roger Pau Monne @ 2017-08-01 7:04 UTC (permalink / raw)
To: xen-devel, Jan Beulich, Andrew Cooper
On Tue, Apr 11, 2017 at 08:54:20AM +0100, Roger Pau Monne wrote:
> clang gcc-compat warnings can wrongly fire when certain constructions are used,
> at least the following flow:
>
> switch ( ... )
> {
> case ...:
> while ( ({ int x; switch ( foo ) { case 1: x = 1; break; } x }) )
> {
> ...
>
> Will cause clang to emit the following warning "'break' is bound to loop, GCC
> binds it to switch", which is a false positive, and both gcc and clang bound
> the break to the inner switch. In order to workaround this issue, disable the
> gcc-compat checks for the usage of the read_atomic macro.
>
> This has been reported upstream as http://bugs.llvm.org/show_bug.cgi?id=32595.
FWIW, this has now been fixed upstream:
https://reviews.llvm.org/rL307051
Roger.
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^ permalink raw reply [flat|nested] 6+ messages in thread
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-- links below jump to the message on this page --
2017-04-11 7:54 [PATCH] clang: disable the gcc-compat warnings for read_atomic Roger Pau Monne
2017-04-11 8:35 ` Jan Beulich
2017-04-11 8:46 ` Roger Pau Monne
2017-04-13 15:11 ` Jan Beulich
2017-04-13 15:23 ` Julien Grall
2017-08-01 7:04 ` Roger Pau Monne
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