[Bug rtl-optimization/54369] New: Delayed-branch pass in reorg.c removes too many instructions

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54369

             Bug #: 54369
           Summary: Delayed-branch pass in reorg.c removes too many
                    instructions
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: chaoyingfu@gcc.gnu.org


Created attachment 28079
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28079
A simple test for an issue in reorg.c

Please check this example when using GCC 4.6 for mips-linux-gnu.
(I think the issue is in GCC 4.7 and later as well.  But due to the basic block
ordering, I cannot reproduce this issue.)
Ex:
# /gcc46branch/build-mips/gcc/cc1plus -quiet Bug.i -o Bug.s -mips32r2 -O2
# cat Bug.s
_Z3bugP9ValueRTRK:
        .frame  $sp,0,$31               # vars= 0, regs= 0/0, args= 0, gp= 0
        .mask   0x00000000,0
        .fmask  0x00000000,0
        .set    noreorder
        .set    nomacro
        lw      $2,0($4) <---------
        lw      $2,0($2) <--------- (NO RETURN IN THIS FUNCTION.)
        .set    macro
        .set    reorder
        .end    _Z3bugP9ValueRTRK
        .cfi_endproc
$LFE13:
        .size   _Z3bugP9ValueRTRK, .-_Z3bugP9ValueRTRK
        .ident  "GCC: (GNU) 4.6.4 20120823 (prerelease) [gcc-4_6-branch
revision
 190613]"

  The delayed-branch pass in reorg.c is too aggressive, such that a return
instruction and others are removed.  From debugging code in reorg.c:
delete_jump() -> delete_computation() -> delete_related_insn(), I think we
should not use delete_related_insn() that removes branches and all the
following instructions.  A simple fix may be just using delete_insn() in
delete_computation().
Ex: # svn diff reorg.c
Index: reorg.c
===================================================================
--- reorg.c     (revision 190636)
+++ reorg.c     (working copy)
@@ -3298,7 +3298,7 @@
       delete_prior_computation (note, insn);
     }

-  delete_related_insns (insn);
+  delete_insn (insn);
 }

 /* If all INSN does is set the pc, delete it,

Ex: (Bug.s after this fix.)
# cat Bug.s
_Z3bugP9ValueRTRK:
        .frame  $sp,0,$31               # vars= 0, regs= 0/0, args= 0, gp= 0
        .mask   0x00000000,0
        .fmask  0x00000000,0
        .set    noreorder
        .set    nomacro
        lw      $2,0($4)
        lw      $2,0($2)
$L4:
        j       $31      <------------- OK
        srl     $2,$2,8  <------------- OK

        .set    macro
        .set    reorder
        .end    _Z3bugP9ValueRTRK
        .cfi_endproc
$LFE13:
        .size   _Z3bugP9ValueRTRK, .-_Z3bugP9ValueRTRK
        .ident  "GCC: (GNU) 4.6.4 20120823 (prerelease) [gcc-4_6-branch
revision
 190613]"

Thanks!