[Bug target/59239] New: [SH] Improve decrement-and-test insn

["olegendo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59239

            Bug ID: 59239
           Summary: [SH] Improve decrement-and-test insn
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: olegendo at gcc dot gnu.org
            Target: sh*-*-*

Created attachment 31264
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31264&action=edit
Possible cleanup patch

The decrement-and-test insn seems to rely on the define_peephole.  At least
taking it out shows some missed opportunities in the CSiBE set.
I've tried replacing it with a define_peephole2, but it would still show missed
cases such as:
   mov.l  @(...), Rn
   add    #-1,Rn
   mov.l  Rn,@(...)
   tst    Rn,Rn

Towards the very end of compilation the insns often get reordered to something
like
   mov.l  @(...), Rn
   add    #-1,Rn
   tst    Rn,Rn
   mov.l  Rn,@(...)

and the define_peephole will catch it in the final RTL pass when outputting asm
code.

Attached is a patch that replaces the old define_peephole with a manual insn
combine in cmpeqsi_t.  This catches some more of those cases where the
individual decrement and test insns are interleaved with something else.

There is one weird case in CSiBE in linux-2.4.23-pre3-testplatform/fs/iobuf.c
(alloc_kiobuf_bhs) though, where the individual decrement and test insns end up
in different basic blocks and only at the very end are emitted right next to
each other without a label in between.