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Re: [PATCH]: R10000 Needs LL/SC Workaround in Gcc
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
lives slip away, moment by moment, lost in that vast, terrible in-between."
--Emperor Turhan, Centauri Republic