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[Bug tree-optimization/66002] paq8p benchmark 50% slower than clang on sandybridge
- From: "rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Mon, 04 May 2015 14:06:49 +0000
- Subject: [Bug tree-optimization/66002] paq8p benchmark 50% slower than clang on sandybridge
- Auto-submitted: auto-generated
- References: <bug-66002-4 at http dot gcc dot gnu dot org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66002
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Status|NEW |ASSIGNED
Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot gnu.org
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
VRP performs jump-threading to else-if style but phiopt doesn't handle the
min-max case with split PHIs
if (wt_25 < -32768)
goto <bb 5>;
else
goto <bb 4>;
<bb 4>:
if (wt_25 > 32767)
goto <bb 6>;
else
goto <bb 5>;
<bb 5>:
# wt_31 = PHI <wt_25(4), -32768(3)>
<bb 6>:
# wt_3 = PHI <wt_31(5), 32767(4)>
_26 = (short int) wt_3;
vs.
if (wt_24 < -32768)
goto <bb 6>;
else
goto <bb 4>;
<bb 4>:
if (wt_24 > 32767)
goto <bb 6>;
else
goto <bb 5>;
<bb 5>:
<bb 6>:
# wt_2 = PHI <-32768(3), wt_24(5), 32767(4)>
_25 = (short int) wt_2;
so it looks like phiopt "depends" on mergephi (I always wondered what pass
that is useful for...). Currently that pass runs right before VRP which
definitely does _not_ depend on it. I'd move it right before ifcombine
which is the first pass that might care.