Hi Jeff,
On 24/07/15 19:43, Jeff Law wrote:
On 07/24/2015 03:31 AM, Kyrill Tkachov wrote:
Wouldn't it be better to walk BB_A, gathering the set of all the
registers modified, then do a single walk through BB testing for uses of
those registers?
I think so, yes. I'll try that.
You might look at resource.c -- I haven't looked at it in a long time,
but it might have many of the interfaces you need to do this kind of
book keeping.
I experimented with resource.c and the roadblock I hit is that it seems
to have an assumption that it operates on hard regs (in fact the struct it uses
to
describe the resources has a HARD_REG_SET for the regs) and so it triggers
various
HARD_REGISTER_P asserts when I try to use the functions there.
if-conversion runs before register allocation, so we're dealing with pseudos
here.
My other attempt was to go over BB_A and mark the set registers in a bitmap then
go over BB_B and do a FOR_EACH_SUBRTX of the SET_SRC of each insn. If a sub-rtx
is
a reg that is set in the bitmap from BB_A we return false. This seemed to do
the job
and testing worked out ok. That would require one walk over BB_A, one walk over
BB_B
but I don't know how expensive FOR_EACH_SUBRTX walks are...
Would that be an acceptable solution?
<snip>
It fails when the last insn is not recognised, because
noce_try_cmove_arith can modify the last insn, but I have
not seen it cause any trouble. If it fails then back in
noce_try_cmove_arith we goto end_seq_and_fail which ends
the sequence and throws it away (and cancels if-conversion
down that path), so it should be safe.
OK, I was working for the assumption that memoization ought not fail,
but it seems that was a bad assumption on my part. So given
noce_try_cmove_arith can change the last insn and make it
no-recognizable this code seems reasoanble.
So I think the only outstanding issues are:
1. Investigate moving rather than re-emitting insns.
I'll look into that, but what is the machinery by which one moves insns?
Thanks,
Kyrill
2. Investigate a more efficient means to find set/use conflicts
between the two blocks, possibly using resource.[ch]
jeff
