On Mon, 2020-01-20 at 13:03 +0000, Richard Sandiford wrote:
> lra_assign has an assert to make sure that no pseudo is allocated
> to a conflicting hard register. It used to be restricted to
> !flag_ipa_ra, but in g:a1e6ee38e708ef2bdef4 I'd enabled it for
> flag_ipa_ra too. It then tripped while building libstdc++
> for mips-mti-linux.
>
> The failure was due to code at the end of process_bb_lives. For an
> abnormal/EH edge, we need to make sure that all pseudos that are live
> on entry to the destination conflict with all hard registers that are
> clobbered by an abnormal call return. The usual way to do this would
> be to simulate a clobber of the hard registers, by making them live and
> them making them dead again. Making the registers live creates the
> conflict; making them dead again restores the correct live set for
> whatever follows.
>
> However, process_bb_lives skips the second step (making the registers
> dead again) at the start of a BB, presumably on the basis that there's
> no further code that needs a correct live set. The problem for the PR
> is that that wasn't quite true in practice. There was code further
> down process_bb_lives that updated the live-in set of the BB for some
> registers, and this live set was "contaminated" by registers that
> weren't live but that created conflicts. This information then got
> propagated to other blocks, so that registers that were made live
> purely to create a conflict at the start of the EH receiver then became
> needlessly live throughout preceding blocks. This in turn created a
> fake conflict with pseudos in those blocks, invalidating the choices
> made by IRA.
>
> The easiest fix seems to be to update the live-in set *before* adding
> the fake live registers. An alternative would be to simulate the full
> clobber, but that seems a bit wasteful.
>
> Tested on aarch64-linux-gnu and x86_64-linux-gnu, and that the
> preprocessed libstdc++ code now compiles for mipsisa64-elf.
> OK to install?
>
> Richard
>
>
> 2020-01-20 Richard Sandiford <richard.sandif...@arm.com>
>
> gcc/
> PR rtl-optimization/92989
> * lra-lives.c (process_bb_lives): Update the live-in set before
> processing additional clobbers.
OK
jeff
>
