[Bug middle-end/79238] New: Combine generates paradoxical subreg of memory

[vogt at linux dot vnet.ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79238

            Bug ID: 79238
           Summary: Combine generates paradoxical subreg of memory
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vogt at linux dot vnet.ibm.com
                CC: krebbel at gcc dot gnu.org
  Target Milestone: ---
              Host: s390x
            Target: s390x

Segher asked me to open a bug report for this:
https://gcc.gnu.org/ml/gcc/2017-01/msg00227.html

On Wed, Jan 25, 2017 at 06:36:04PM +0100, Dominik Vogt wrote:
> What's the matter with paradoxical subregs of memory?  So, as far
> as I understand, Recog refuses to match them unless they are
> explicitly expressed as a pattern.  On the other hand, Combine
> does not know that they are "outlawed" and happily generates
> them.
>
> This happens sometimes with some new patterns for s390x for the
> RISBG instruction, which does complex operations on registers.  As
> it's beneficial to allow memory operands before reload,
> paradoxical subregs of memory are sometimes combined from such
> patterns, for example
>
>   (set (reg:DI 68)
>        (and:DI (subreg:DI (lshiftrt:SI (reg:SI 67 [ *f_5(D) ])
>                           (const_int 8 [0x8])) 0)
>                (const_int 16777215 [0xffffff])))
>
> +
>
>   (set (reg/i:DI 2 %r2)
>         (and:DI (reg:DI 68)
>                 (const_int 1 [0x1])))
>
>
> =
>
> (set (reg/i:DI 2 %r2)
>      (and:DI (subreg:DI (mem:QI (plus:DI (reg:DI 2 %r2 [ f ])
>               ^^^^^^^^^^^^^^^^^
>                         (const_int 2 [0x2])) [1 *f_5(D)+2 S1 A16]) 0)
>              (const_int 1 [0x1])))
>
> Which doesn't match because of the paradoxical subreg of memory (I
> think).

(No example code that generates this available, sorry.  The expressions below
have been generated by patterns that are under development.)

--

On Wed, Jan 25, 2017 at 04:45:23PM -0600, Segher Boessenkool wrote:
> On Wed, Jan 25, 2017 at 06:36:04PM +0100, Dominik Vogt wrote:
> > What's the matter with paradoxical subregs of memory?  So, as far
> > as I understand, Recog refuses to match them unless they are
> > explicitly expressed as a pattern.
>
> If INSN_SCHEDULING is defined recog does not allow paradoxical subregs
> of memory as a general_operand (memory_operand does not allow any
> subregs of memory).
>
> > On the other hand, Combine
> > does not know that they are "outlawed" and happily generates
> > them.
>
> combine should not generate things that can never match.  Of course it
> sometimes does.  This should be improved; please open a PR.