Re: [PATCH]: R10000 Needs LL/SC Workaround in Gcc

[Kumba <kumba@gentoo.org>]

Richard Sandiford wrote:
> 
> To be clear, the first option above was to check -- in mips_override_options --
> that -mfix-r10000 is only used in cases where -mbranch-likely is in effect.
> If we pick that option, it would be an error to use -mfix-r10000 in
> other cases, and any code protected by TARGET_FIX_R10000 would be free
> to use branch-likely instructions.  (Actually, we should use sorry()
> instead of error() to report something like this.)

[snip]

> That's the second option above, yes.  In other words, -mfix-r10000
> would support both -mbranch-likely and -mno-branch-likely, and act
> accordingly.

So do I need to worry about modifying the asm templates at all?  Or is that only 
needed if pursuing option #2?

The branch-likely stuff is only going to work for MIPS-II or higher targets.  In 
the odd (but possible) cases where MIPS-I might be used with -mfix-r10000, I 
assume we'll still have to emit 28 nops prior to a beq/beqz instruction.  Is 
this already taken care of someplace?


> ...that's a good question.  My take is "no".  I don't think we want
> -mfix-r10000 to enable branch-likely instructions in cases where
> it isn't necessary for R10000 errata.  If we take the first option,
> we can simply raise an error if:

Hmm, okay.  Might this work to enable -mbranch-likely if -mfix-r10000?  (Kind of 
guessing by looking at other segments of code).

   if ((target_flags_explicit & MASK_BRANCHLIKELY) == 0)
     {
       if (ISA_HAS_BRANCHLIKELY
           && (optimize_size
               || (!(target_flags_explicit & MASK_FIX_R10000) == 0)
               || (mips_tune_info->tune_flags & PTF_AVOID_BRANCHLIKELY) == 0))
         target_flags |= MASK_BRANCHLIKELY;
       else
         target_flags &= ~MASK_BRANCHLIKELY;
     }



My understanding so far for -mfix-r10000:
- Gets enabled if -march=r10000 is passed (done)
- Enable -mbranch-likely if not already enabled on >= MIPS-II (working on)
- Emits beqzl in the asm templates if enabled and >= MIPS-II (unsure)
- Emits 28 nops prior to beq/beqz if enabled and == MIPS-I (unsure)
- Ditto for asm templates (unsure)
- Documentation (not done)

Missing anything?


> Yeah, I was wondering that too.  I did a search, but couldn't
> find anything.

It seems we just need to use nop only and not worry about ssnop.


-- 
Joshua Kinard
Gentoo/MIPS
kumba@gentoo.org

"The past tempts us, the present confuses us, the future frightens us.  And our 
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