Re: [PATCH][ARM] PR 65489: Accept VSTRUCT constants in arm_legitimate_constant_p

[Ramana Radhakrishnan <ramana dot gcc at googlemail dot com>]

On Mon, Mar 23, 2015 at 4:15 PM, Kyrill Tkachov <kyrylo.tkachov@arm.com> wrote:
> Hi all,
>
> The ICE in the PR happens on arm during the hoist pass when the code
> generates a SET rtx of the form:
> (set (reg:OI) (const_int 0)). It checks whether const_int 0 is a
> general_operand for OImode which involves asking the backend whether it's a
> legitimate constant.
>
> arm_legitimate_constant_p_1 explicitly rejects OImode constants as a result
> of the fix for PR 46329
> https://gcc.gnu.org/ml/gcc-patches/2011-04/msg00200.html
>
> This results in gcse using an emit_insn of the SET rtx rather than trying to
> do an emit_move_insn
> which would have worked in this case.
>
> From reading the thread I get the impression that the ICE there was caused
> by reload and coproc_secondary_reload_class behaving weirdly. We're now
> using LRA and
> SECONDARY_INPUT_RELOAD_CLASS and SECONDARY_OUTPUT_RELOAD_CLASS which are the
> only users of coproc_secondary_reload_class are no longer used.
>
> I removed the restriction on constants for these large modes in
> arm_legitimate_constant_p_1 and the testcases in PR 46329 all work fine.
> Bootstrap, testing and building various vector workloads also didn't show
> any problems.
>
> Is this the right way to go for this?
> If so, ok for trunk?
>
> The ICE in this PR happens on a recently added testcase to
> gcc.c-torture/execute/ and this patch fixes that, so no new testcase is
> added. Jakub mentioned that maybe the testcase can be modified to manually
> perform the lowering done in his patch to potentially trigger this on older
> gcc versions, but I'm not sure how that would be done.
> The problem is that something is generating OImode moves that I think can
> only be done in the vector form of the testcase, so I'm not entirely
> convinced that it's reproducible on 4.9 and 4.8.


This is OK for trunk now that we are in wide-int mode on trunk for the ARM port.

Thanks,
Ramana

>
> Thanks,
> Kyrill
>
> Thanks,
> Kyrill
>
> 2015-03-23  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
>     PR target/65489
>     * config/arm/arm.c (arm_legitimate_constant_p_1): Remove restriction
>     on constants for NEON VSTRUCT modes.