[PATCH, i386]: Emit cld instruction when stringops are used

[Richard Guenther]

On Tue, Apr 29, 2008 at 7:24 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Mon, Apr 28, 2008 at 11:37 PM, Mark Mitchell <mark@codesourcery.com> wrote:
>
>  > I agree with Joseph as to the strategy. If there is no objection to that
>  > strategy, is there a patch to review?
>
>  I have different opinion on this matter, that I would like to share:
>
>  IMO, there are two classes of users, the ones that get their compiler
>  from their distribution and users that get the compiler from FSF
>  website.
>
>  1) The compiler that is provided by the distribution does not need to
>  be patched, since the distribution also provides relevant update to
>  the kernel (it is a two-line patch). Both, the patch and the compiler
>  are/should be patched and updated together.
>
>  2) Users that download the compiler directly do this because they have
>  an application they care about and they want to squeeze every bit of
>  performance out of this application. Even if the kernel is not updated
>  (please note that due to point 1) above, applications and libraries
>  are compiled with older gcc in this case) it is just a matter of
>  fixing the application, so it doesn't mess with cld anymore.
>
>  Thinking a bit about it, I come to the conclusion, that the impact of
>  this bug is negligible, and I propose to simple close this matter
>  without patching the compiler. The rationale for this proposal is,
>  that adding --enable-cld is not a good solution, since (a)
>  distributions won't need it, since thy will patch the kernel and (b)
>  power users won't use it, since emitting cld has non-negligible
>  performance impact and they will rather fix the application. Also,
>  adding -mcld means that we will have to carry this options far in the
>  future to handle 0.001% of users that want to use old kernel with
>  bleeding-edge compiler and don't want to bother by changing source
>  code of their application to comply with published ABI.
>
>  So, I don't agree that patching the compiler to emit CLD in order to
>  fix bugs that are elsewhere is the solution for this problem.

I agree 100% (but I also respect others opinions).

Richard.