Re: [PATCH] aarch64: Don't record hazards against paired insns [PR113356]

[Richard Sandiford <richard.sandiford@arm.com>]

Alex Coplan <alex.coplan@arm.com> writes:
> Hi,
>
> For the testcase in the PR, we try to pair insns where the first has
> writeback and the second uses the updated base register.  This causes us
> to record a hazard against the second insn, thus narrowing the move
> range away from the end of the BB.
>
> However, it isn't meaningful to record hazards against the other insn
> in the pair, as this doesn't change which pairs can be formed, and also
> doesn't change where the pair is formed (from the perspective of
> nondebug insns).
>
> To see why this is the case, consider the two cases:
>
>  - Suppoe we are finding hazards for insns[0].  If we record a hazard
>    against insns[1], then range.last becomes
>    insns[1]->prev_nondebug_insn (), but note that this is equivalent to
>    inserting after insns[1] (since insns[1] is being changed).
>  - Now consider finding hazards for insns[1].  Suppose we record
>    insns[0] as a hazard.  Then we set range.first = insns[0], which is a
>    no-op.
>
> As such, it seems better to never record hazards against the other insn
> in the pair, as we check whether the insns themselves are suitable for
> combination separately (e.g. for ldp checking that they use distinct
> transfer registers).  Avoiding unnecessarily narrowing the move range
> avoids unnecessarily re-ordering over debug insns.
>
> This should also mean that we can only narrow the move range away from
> the end of the BB in the case that we record a hazard for insns[0]
> against insns[1]->prev_nondebug_insn () or earlier.  This means that for
> the non-call-exceptions case, either the move range includes insns[1],
> or we reject the pair (thus the assert tripped in the PR should always
> hold).
>
> Bootstrapped/regtested on aarch64-linux-gnu with/without ldp passes
> enabled on top of the PR113070 fixes, OK for trunk?
>
> Thanks,
> Alex
>
> gcc/ChangeLog:
>
> 	PR target/113356
> 	* config/aarch64/aarch64-ldp-fusion.cc (ldp_bb_info::try_fuse_pair):
> 	Don't record hazards against the opposite insn in the pair.
>
> gcc/testsuite/ChangeLog:
>
> 	PR target/113356
> 	* gcc.target/aarch64/pr113356.C: New test.

OK, thanks.

Richard