Re: [PATCH] x86: Align jump targets to 1 byte boundaries

Markus Trippelsdorf
2015-04-11T13:49:03Z

On 2015.04.10 at 14:50 +0200, Denys Vlasenko wrote:
> New-ish versions of gcc allow people to specify optimization
> options per function:
>
> https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html#Function-Attributes
>
> optimize
> The optimize attribute is used to specify that a function is to be compiled
> with different optimization options than specified on the command line.
> Arguments can either be numbers or strings. Numbers are assumed to be an
> optimization level. Strings that begin with O are assumed to be an
> optimization option, while other options are assumed to be used with
> a -f prefix.
>
> How about not aligning code by default, and using
>
> #define hot_func __attribute__((optimize("O2","align-functions=16","align-jumps=16")))
> ...
>
> void hot_func super_often_called_func(...) {...}
>
> in hot code paths?

__attribute__((optimize)) is meant for compiler debugging only (to help
folks to reduce their testcases to a single function).
It should _not_ be used in production code, because the implementation
is very buggy.

--
Markus