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Re: [PATCH] malloc: remove __builtin_expect

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 <>

It was disabled anyway and only served as obfuscation.  No change

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:
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.


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