[PATCH] malloc: remove __builtin_expect

[Jeff Law]

On 01/26/2016 05:37 PM, Steven Munroe wrote:
> On Tue, 2016-01-26 at 14:42 -0700, Jeff Law wrote:
>> On 01/26/2016 02:34 PM, Steven Munroe wrote:
>>> 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
>> Ha, already fixed that one :-)
>
> But there are more, we are still digging them out.
And we'll keep squashing 'em!

I suspect the FSM threader is going to be the source of a lot of these 
issues in the future.

jeff