[PATCH, i386]: Do not emit "cld" instructions

["Uros Bizjak" <ubizjak at gmail dot com>]

On 12/4/06, Jan Hubicka <hubicka@ucw.cz> wrote:


> As author of the current cld emitting code, I can just say that I kept
> it around for single reason, that the old GCC always emitted it and I
> didn't find any specification in the 32bit ABI (in 64bit ABI we specify
> CLD as clear and I did have patch to disable CLD codegen there that
> probably got lost somewhere on it's way to mainline).   Other minor
> reason for me was that we never specified if the asm inline blocks are
> required to cld after std, but as Michael correctly observed, there is
> very little need for this.
>
> I was also originally thinking about memmove inline expander, but it
> don't seem that important to me (since we don't really want to copy
> backwards anyway, we probably might want to inline memmove internally in
> a way checking that blocks don't overlap in negative direction and are
> small and use our memcpy inliners, emitting library call otherwise)
>
> I would not be oposed to not emitting it at all - it just never seemed
> important enought for me as it is short and rather fast on most modern
> CPUs.  It seems better alternative to me than introducing the mode
> switching pass that has some compilation time implications.

Based on all comments and suggestions, I'd propose that we keep all
cld functionality, but emit empty asm instead of "cld". If gcc ever
emits std instruction, we could re-enable cld and perhaps emit it via
optimize mode switching.

A comment in front of cld insn pattern hopefully clarifies this situiation.

> Also note that CSE/GCSE is currently pretty capable on eliminating many
> redundant CLDs.

Unfortunatelly, for some reasons clds were not eliminated from loops...

> > > Finally, according to pentium optimization guide by Agner Fog, std and
> > > cld have astonishing latency of 48 and 52 clks (I still hope for the
> > > possibility that there is some kind of error).
>
> This is prety weird.  Does this apply to original Pentium?

According to the guide, it applies to pentium4.

2006-12-06  Uros Bizjak  <ubizjak@gmail.com>

	* config/i386/i386.md (cld): Emit empty asm string.

Patch was bootstrapped on i686-pc-linux-gnu, regression tested for c,
c++ and fortran.

OK for mainline?

Uros.

Attachment: i386-nocld.diff
Description: Binary data