[Bug tree-optimization/63169] New: ivopts rewrite_uses pessimizes debug info

[Bug tree-optimization/63169] New: ivopts rewrite_uses pessimizes debug info

[amodra at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63169

            Bug ID: 63169
           Summary: ivopts rewrite_uses pessimizes debug info
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amodra at gmail dot com

Extract from 122t.ivopts for gcc.c-torture/compile/pr60655-2.c on powerpc,
showing loop in mp_compare.  The thing to note is the good expression for #
DEBUG r1, a simple dependency on one iv, but the bad choice for # DEBUG r2
(which triggers pr60655 on powerpc).
# DEBUG r2 => (sizetype) (ivtmp.19_20 + 4294967295) is correct also and leads
to much simpler debug info.

  <bb 3>:
  # ivtmp.17_24 = PHI <ivtmp.17_22(2), ivtmp.17_23(6)>
  # ivtmp.19_20 = PHI <ivtmp.19_26(2), ivtmp.19_25(6)>
  # DEBUG precision => (short int) (((unsigned short) precision_6 - (unsigned
short) (unsigned int) (r1_10 + 1)) + (unsigned short) ivtmp.17_24)
  # DEBUG r2 => r2_12 + (((sizetype) ivtmp.17_24 - (sizetype) r1_10) +
4294967295)
  # DEBUG r1 => (const unit *) (ivtmp.17_24 + 4294967295)
  ivtmp.17_23 = ivtmp.17_24 - 1;
  _28 = (void *) ivtmp.17_23;
  _13 = MEM[base: _28, offset: 0B];
  ivtmp.19_25 = ivtmp.19_20 - 1;
  _29 = (void *) ivtmp.19_25;
  _14 = MEM[base: _29, offset: 0B];
  if (_13 < _14)
    goto <bb 7>;
  else
    goto <bb 4>;

  <bb 4>:
  # DEBUG r1 => (const unit *) (ivtmp.17_23 + 4294967295)
  # DEBUG r2 => r2_12 + (((sizetype) ivtmp.17_23 - (sizetype) r1_10) +
4294967295)
  if (_13 > _14)
    goto <bb 7>;
  else
    goto <bb 5>;

  <bb 5>:
  # DEBUG precision => (short int) (((unsigned short) precision_6 - (unsigned
short) (unsigned int) (r1_10 + 1)) + (unsigned short) ivtmp.17_23)
  if (ivtmp.17_23 != _35)
    goto <bb 6>;
  else
    goto <bb 7>;

  <bb 6>:
  goto <bb 3>;

[Bug tree-optimization/63169] ivopts rewrite_uses pessimizes debug info

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63169

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
IVOPTs depends on CSE to remove the effect of building its (sometimes gigantic)
GENERIC expressions figuring out IVs and inserting them multiple times.

Not sure if we even try to optimize the IV chosen to replace debuginfo uses.