From: Carlos O'Donell <carlos@redhat.com>
To: Florian Weimer <fweimer@redhat.com>,
GNU C Library <libc-alpha@sourceware.org>
Subject: Re: [PATCH] Fix -Os related -Werror failures.
Date: Fri, 28 Oct 2016 13:07:00 -0000 [thread overview]
Message-ID: <96d1926b-4b53-1453-88e3-9bf5e050439e@redhat.com> (raw)
In-Reply-To: <f2153190-d62b-e04f-feae-39ed52c93589@redhat.com>
On 10/28/2016 08:08 AM, Carlos O'Donell wrote:
> On 10/28/2016 02:32 AM, Florian Weimer wrote:
>> On 10/28/2016 06:46 AM, Carlos O'Donell wrote:
>>> +/* With GCC 5.3 when compiling with -Os the compiler emits a warning
>>> + that buf[0] and buf[1] may be used uninitialized. This can only
>>> + happen in the case where tmpbuf[3] is used, and in that case the
>>> + write to the tmpbuf[1] and tmpbuf[2] was assured because
>>> + ucs4_to_cns11643 would have filled in those entries. The difficulty
>>> + is in getting the compiler to see this logic because tmpbuf[0] is
>>> + involved in determining the code page and is the indicator that
>>> + tmpbuf[2] is initialized. */
>>> +DIAG_PUSH_NEEDS_COMMENT;
>>> +DIAG_IGNORE_NEEDS_COMMENT (5.3, "-Wmaybe-uninitialized");
>>
>> This hides the warning for -O2 builds as well, so I don't think this is a good idea.
>>
>> Those who want to build with -Os or other special compiler flags
>> should just configure with --disable-werror. We can't account for
>> every optimization someone might want to disable in their build.
>
> I agree that we can't account for _all_ optimizations someone might want
> to disable in their build, but I think it is a reasonable goal to target
> a few key _default_ optimization including -O3, -O2, and -Os.
>
> In the case above we only limit the emitted warnings for the narrow
> code involved in iso-2022-cn-ext conversions. I'd be more worried if it
> required a widely used function with broadly disabled warnings.
>
> I agree with Arnd that this code is _overly_ complex and could be
> rewritten such that it's a little clearer and makes sense to the compiler
> at -Os.
>
> Should I try to cleanup the BODY code a bit to remove this particular
> diagnostic disabling?
>
> I know we've had several real uninitialized variable problems in the
> conversion code recently, so I'm also interested in having the compiler
> help us find more of these problems.
For example, initializing the tmpbuf in this fallback case is enough to
silence the compiler warning:
diff --git a/iconvdata/iso-2022-cn-ext.c b/iconvdata/iso-2022-cn-ext.c
index df5b5df..d0b32df 100644
--- a/iconvdata/iso-2022-cn-ext.c
+++ b/iconvdata/iso-2022-cn-ext.c
@@ -456,7 +456,7 @@ enum
used = CNS11643_2_set; \
else \
{ \
- unsigned char tmpbuf[3]; \
+ unsigned char tmpbuf[3] = { 0, 0, 0 }; \
\
switch (0) \
{ \
---
We already initialize buf similarly e.g.
429 unsigned char buf[2] = { 0, 0 }; \
At -Os the compiler is unable to determine if tmpbuf can or can't be used
in one of the failure cases e.g. return __UNKNOWN_10646_CHAR;.
This particular case we are into the 3rd conversion attempt of an unknown
character, so it can't possibly be a performance case to zero tmpbuf and
simplify the analysis for all kinds of static analysis tooling.
Thoughts?
--
Cheers,
Carlos.
next prev parent reply other threads:[~2016-10-28 13:07 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-28 4:48 Carlos O'Donell
2016-10-28 6:25 ` Andreas Schwab
2016-10-28 6:32 ` Florian Weimer
2016-10-28 6:44 ` Jeff Law
2016-10-28 8:12 ` Arnd Bergmann
2016-10-28 8:17 ` Andrew Pinski
2016-10-28 13:28 ` Jeff Law
2016-10-28 20:10 ` Paul Eggert
2016-10-29 3:03 ` Jeff Law
2016-10-30 4:25 ` Paul Eggert
2016-10-28 12:09 ` Carlos O'Donell
2016-10-28 12:43 ` Florian Weimer
2016-10-28 13:04 ` Joseph Myers
2016-10-28 13:07 ` Carlos O'Donell [this message]
2016-10-28 12:49 ` Joseph Myers
2016-10-28 12:55 ` Florian Weimer
2016-10-28 13:18 ` Carlos O'Donell
2016-10-28 13:58 ` [PATCH v2] Fix -Os related build and test failures Carlos O'Donell
2016-10-28 14:17 ` Joseph Myers
2016-10-29 2:59 ` [PATCH v3] " Carlos O'Donell
2016-10-29 3:26 ` Carlos O'Donell
2016-10-29 17:35 ` Joseph Myers
2016-10-30 3:51 ` [PATCH v4] " Carlos O'Donell
2016-10-31 8:33 ` Andreas Schwab
2016-10-31 9:16 ` Carlos O'Donell
2016-10-31 9:22 ` Florian Weimer
2016-10-31 12:56 ` David Miller
2016-10-31 19:56 ` Carlos O'Donell
2016-11-01 22:59 ` Joseph Myers
2016-11-02 12:52 ` Carlos O'Donell
2016-11-01 9:17 ` Andreas Schwab
2016-11-01 11:13 ` Joseph Myers
2016-11-01 15:58 ` Tamar Christina
2016-11-01 16:06 ` David Miller
2016-11-01 16:15 ` Tamar Christina
2016-11-02 11:53 ` Carlos O'Donell
2016-11-02 17:03 ` Carlos O'Donell
2016-11-02 13:22 ` Carlos O'Donell
2016-10-31 18:38 ` [PATCH v3] " Steve Ellcey
2016-10-31 19:50 ` Carlos O'Donell
2016-10-31 19:57 ` Steve Ellcey
2016-10-31 20:50 ` Carlos O'Donell
2016-10-31 21:00 ` Steve Ellcey
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