[Bug target/112999] New: riscv: Infinite loop with mask extraction

[Bug target/112999] New: riscv: Infinite loop with mask extraction

[rdapp at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112999

            Bug ID: 112999
           Summary: riscv: Infinite loop with mask extraction
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rdapp at gcc dot gnu.org
                CC: juzhe.zhong at rivai dot ai, pan2.li at intel dot com
  Target Milestone: ---
            Target: riscv

Pan Li found the following problematic case in his "full-coverage" testing and
I'm just documenting it here for reference.

/* { dg-do compile } */
/* { dg-options "-march=rv64gcv_zvl512b -mabi=lp64d
--param=riscv-autovec-lmul=m8 --param=riscv-autovec-preference=fixed-vlmax -O3
-fno-vect-cost-model -fno-tree-loop-distribute-patterns" } */

int a[1024];
int b[1024];

_Bool
fn1 ()
{
  _Bool tem;
  for (int i = 0; i < 1024; ++i)
    {
      tem = !a[i];
      b[i] = tem;
    }
  return tem;
}

We try to extract the last bit from a 128-bit value of a mask vector.  In order
to do so we first subreg by a tieable vector mode (here RVVMF4QI) then, because
we do not have a RVVMF4QI -> BI vector extraction, try type punning with a
TImode subreg.
As we do not natively support TImode, the result needs to be subreg'd again to
DImode.  In the course of doing so we get lost in subreg moves and hit an
infinite loop.  I have not tracked down the real root cause but the problem is
fixed by providing a movti pattern and special casing subreg:TI extraction from
vectors (just like we do in legitimize_move for other scalar subregs of vectors
- and wich I don't particularly like either :) ).

[Bug target/112999] riscv: Infinite loop with mask extraction

[rdapp at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112999

--- Comment #1 from Robin Dapp <rdapp at gcc dot gnu.org> ---
What actually gets in the way of vec_extract here is changing to a "better"
vector mode (which is RVVMF4QI here).  If we tried to extract from the mask
directly everything would work directly.

I have a patch locally that does this by refactoring extract_bit_field_1
slightly.  Going to post it soon but not sure if people agree with that idea.

[Bug target/112999] riscv: Infinite loop with mask extraction

[juzhe.zhong at rivai dot ai]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112999

--- Comment #2 from JuzheZhong <juzhe.zhong at rivai dot ai> ---
Ok.It's mask bit field again. It's an annoying issue.

I think explict movti pattern is controversial since it may have
risks that affect scalar part.

[Bug target/112999] riscv: Infinite loop with mask extraction

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112999

--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Robin Dapp <rdapp@gcc.gnu.org>:

https://gcc.gnu.org/g:e5e1999aa664333f766f3e6cc6996f769d50ae7a

commit r14-6556-ge5e1999aa664333f766f3e6cc6996f769d50ae7a
Author: Robin Dapp <rdapp@ventanamicro.com>
Date:   Wed Dec 13 16:42:28 2023 +0100

    expmed: Compare unit_precision for better mode.

    In extract_bit_field_1 we try to get a better vector mode before
    extracting from it.  Better refers to the case when the requested target
    mode does not equal the inner mode of the vector to extract from and we
    have an equivalent tieable vector mode with a fitting inner mode.

    On riscv this triggered an ICE (PR112999) because we would take the
    detour of extracting from a mask-mode vector via a vector integer mode.
    One element of that mode could be subreg-punned with TImode which, in
    turn, would need to be operated on in DImode chunks.

    This patch adds

        && known_eq (bitsize, GET_MODE_UNIT_PRECISION (new_mode))
        && multiple_p (bitnum, GET_MODE_UNIT_PRECISION (new_mode))

    to the list of criteria for a better mode.

    gcc/ChangeLog:

            PR target/112999

            * expmed.cc (extract_bit_field_1):  Ensure better mode
            has fitting unit_precision.

    gcc/testsuite/ChangeLog:

            * gcc.target/riscv/rvv/autovec/pr112999.c: New test.

[Bug target/112999] riscv: Infinite loop with mask extraction

[rdapp at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112999

Robin Dapp <rdapp at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #4 from Robin Dapp <rdapp at gcc dot gnu.org> ---
Should be fixed on trunk.