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Re: [PATCH] malloc: remove __builtin_expect
- From: Steven Munroe <munroesj at linux dot vnet dot ibm dot com>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: Yury Gribov <y dot gribov at samsung dot com>, Joern Engel <joern at purestorage dot com>, "GNU C. Library" <libc-alpha at sourceware dot org>, Siddhesh Poyarekar <siddhesh dot poyarekar at gmail dot com>, Joern Engel <joern at purestorage dot org>
- Date: Tue, 26 Jan 2016 15:34:52 -0600
- Subject: Re: [PATCH] malloc: remove __builtin_expect
- Authentication-results: sourceware.org; auth=none
- References: <1453767942-19369-1-git-send-email-joern at purestorage dot com> <1453767942-19369-21-git-send-email-joern at purestorage dot com> <56A726C1 dot 6070206 at samsung dot com> <1453840993 dot 18407 dot 2 dot camel at oc7878010663> <56A7E028 dot 6050204 at redhat dot com>
- Reply-to: munroesj at linux dot vnet dot ibm dot com
On Tue, 2016-01-26 at 22:07 +0100, Florian Weimer wrote:
> On 01/26/2016 09:43 PM, Steven Munroe wrote:
> > On Tue, 2016-01-26 at 10:56 +0300, Yury Gribov wrote:
> >> On 01/26/2016 03:24 AM, Joern Engel wrote:
> >>> From: Joern Engel <joern@purestorage.org>
> >>>
> >>> It was disabled anyway and only served as obfuscation. No change
> >>> post-compilation.
> >>
> >> FYI I've witnessed significant improvements from (real) __builtin_expect
> >> in other projects.
> >>
> > It depends on the platform and if the programmer correctly understands
> > the behavior of the program as written.
> >
> > Net, except for error cases that "should not happen, ever", a bad idea.
>
> Based on what I saw, glibc uses __builtin_expect and the macros derived
> from it in two conflicting ways: to express that one alternative is more
> likely that the other, and to state that some alternative is impossible
> in practice (for a well-behaved program in particular).
>
> GCC's current interpretation leans towards the latter, at least on
> x86_64. I think GCC even puts unlikely code into separate text sections
> in some cases. Most of our __builtin_expect uses seem to be of the
> former nature: things that can and do happen during normal operation,
> like an unusual character in a character set conversion, or a
> locale-related environment variable which is set.
>
I am also concerned the GCC has some serious bugs in its block frequency
handling. For example:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67755
> I found it also problematic that GCC does the hot/cold partitioning (not
> sure if it's actually called this way) on the error paths themselves,
> where we already know that the impossible has happened, and all paths
> eventually head towards an abort. It doesn't make sense for GCC to
> split those that hit a known no-return function from those that hit a
> function which does not return, but GCC cannot infer that from available
> declarations.
>
> Florian
>