Re: [PATCH 2/2] validate_subreg before call gen_lowpart to avoid ICE.

[Richard Biener <richard.guenther@gmail.com>]

On September 10, 2021 3:27:09 PM GMT+02:00, Hongtao Liu <crazylht@gmail.com> wrote:
>On Fri, Sep 10, 2021 at 9:16 PM Richard Biener via Gcc-patches
><gcc-patches@gcc.gnu.org> wrote:
>>
>> On Fri, Sep 10, 2021 at 2:58 PM liuhongt <hongtao.liu@intel.com> wrote:
>> >
>> > gcc/ChangeLog:
>> >
>> >         * expmed.c (extract_bit_field_using_extv): validate_subreg
>> >         before call gen_lowpart.
>> > ---
>> >  gcc/expmed.c | 6 +++++-
>> >  1 file changed, 5 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/gcc/expmed.c b/gcc/expmed.c
>> > index 3143f38e057..10d62d857a8 100644
>> > --- a/gcc/expmed.c
>> > +++ b/gcc/expmed.c
>> > @@ -1571,12 +1571,16 @@ extract_bit_field_using_extv (const extraction_insn *extv, rtx op0,
>> >
>> >    if (GET_MODE (target) != ext_mode)
>> >      {
>> > +      machine_mode tmode = GET_MODE (target);
>> >        /* Don't use LHS paradoxical subreg if explicit truncation is needed
>> >          between the mode of the extraction (word_mode) and the target
>> >          mode.  Instead, create a temporary and use convert_move to set
>> >          the target.  */
>> >        if (REG_P (target)
>> > -         && TRULY_NOOP_TRUNCATION_MODES_P (GET_MODE (target), ext_mode))
>>
>> ^^^
>>
>> I wonder if herein lies the problem in that the HFmode "truncation" from SImode
>> is considered noop?  Note the underlying target hook only looks at the mode
>> precision and thus receives 16 and 32, and thus maybe that
>> TRULY_NOOP_TRUNCATION_MODES_P query only makes sense for
>> integer modes?  Though the documentation of the hook only talks about
>> "conversion" of "values" ...
>>
>> So maybe a targetm.modes_tieable_p (GET_MODE (target), extmode) check
>> is missing?
>
>According to document, it should be true for
>targetm.modes_tieable_p(HFmode, SImode) since HFmode can be allocated
>to gpr.
>
>----------------
>This hook returns true if a value of mode mode1 is accessible in mode
>mode2 without
>copying
>-------------------
>
>and also here gen_lowpart (SImode, HFmode, target) is called and hit
>gcc_assert, not (subreg:HF (reg:SI) 0)

I see. Of course that leads to a suggestion to allow the subreg based on modes_tieable_p, but then others will know why that's the wrong thing to do? 

Richard. 

>>
>> > +         && TRULY_NOOP_TRUNCATION_MODES_P (tmode, ext_mode)
>> > +         && validate_subreg (ext_mode, tmode,
>> > +                             target,
>> > +                             subreg_lowpart_offset (ext_mode, tmode)))
>> >         {
>> >           target = gen_lowpart (ext_mode, target);
>> >           if (partial_subreg_p (GET_MODE (spec_target), ext_mode))
>> > --
>> > 2.27.0
>> >
>
>
>