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

Richard Sandiford wrote:

As Maciej said, this should really be controlled by an -mfix-r10000 command-line option, not by the _MIPS_ARCH_* macro. (In this context, _MIPS_ARCH_* is a property of the compiler that you're using to build gcc itself.)

There are two ways we could handle this:

  - Make -mfix-r10000 require -mbranch-likely.  (It mustn't _imply_
    -mbranch-likely.  It should simply check that -mbranch-likely is
    already in effect.)

- Make -mfix-r10000 insert nops when -mbranch-likely is not in effect.

Does using -mbranch-likely change the output of those specific asm commands that my original patch was altering? Or will -mfix-r10000 need to not only check the status of -mbranch-likely and set it if not set, but also need to modify the referenced beq/beqzl sets in mips.h?

If so, I assume a test for both TARGET_FIX_R10000 and TARGET_BRANCHLIKELY would be needed, and then if TARGET_BRANCHLIKELY doesn't exist, but TARGET_FIX_R10000 is, insert 28 nops before beq. Sound correct?

On setting -mbranch-likely, I found what I think is the appropriate section in mips.c around Line 13810:

  /* If neither -mbranch-likely nor -mno-branch-likely was given
     on the command line, set MASK_BRANCHLIKELY based on the target
     architecture and tuning flags.  Annulled delay slots are a
     size win, so we only consider the processor-specific tuning
     for !optimize_size.  */
  if ((target_flags_explicit & MASK_BRANCHLIKELY) == 0)
          && (optimize_size
              || (mips_tune_info->tune_flags & PTF_AVOID_BRANCHLIKELY) == 0))
        target_flags |= MASK_BRANCHLIKELY;
        target_flags &= ~MASK_BRANCHLIKELY;
    warning (0, "the %qs architecture does not support branch-likely"
             " instructions", mips_arch_info->name);

I'm kind of thinking that the -mfix-r10000 setting to include -mbranch-likely would fit here (Assuming this is what can enable/disable that option via MASK_BRANCHLIKELY), but if I'm reading it right, optimizing for size disables brach-likely instructions. Shouldn't -mfix-r10000 override that?

Would an equivalent conditional like this be close?:

          && ((optimize_size || TARGET_FIX_R10000)
              || (mips_tune_info->tune_flags & PTF_AVOID_BRANCHLIKELY) == 0))

Also, does anyone have a copy of the R10000 Silicon Errata documentation kicking around? Thiemo brought up a point that we may need ssnop instead of nop, but I'd need to check the errata for that, and that doesn't seem to exist anywhere anymore. I found an old link to it on MIPS' site, but nothing else. I've only got Vr10000 manuals from SGI and NEC, and they don't seem to cover revision-specific errata any.


Joshua Kinard

"The past tempts us, the present confuses us, the future frightens us. And our lives slip away, moment by moment, lost in that vast, terrible in-between."

--Emperor Turhan, Centauri Republic

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