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)
{
if (ISA_HAS_BRANCHLIKELY
&& (optimize_size
|| (mips_tune_info->tune_flags & PTF_AVOID_BRANCHLIKELY) == 0))
target_flags |= MASK_BRANCHLIKELY;
else
target_flags &= ~MASK_BRANCHLIKELY;
}
else if (TARGET_BRANCHLIKELY && !ISA_HAS_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?:
if (ISA_HAS_BRANCHLIKELY
&& ((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.
Thanks!
--
Joshua Kinard
Gentoo/MIPS
kumba@xxxxxxxxxx
"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
