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From: "steven at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/64928] [4.8/4.9/5 Regression] Inordinate cpu time and memory usage in "phase opt and generate" with -ftest-coverage -fprofile-arcs
Date: Thu, 05 Mar 2015 23:07:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64928

Steven Bosscher <steven at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |steven at gcc dot gnu.org

--- Comment #12 from Steven Bosscher <steven at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #9)
> It seems that loop invariant motion is responsible for most of the abnormals,
> thus -fno-tree-loop-im restores performance.
> 
> The loop LIM detects is of style
> 
>   <bb 6>: (header)
>   # ___fp_3(ab) = PHI <___fp_41(4), ___fp_5(21)>
>   # ___r1_7(ab) = PHI <___r1_42(4), ___r1_9(21)>
>   # ___r2_11(ab) = PHI <___r2_43(4), ___r3_17(21)>
>   # ___r3_19(ab) = PHI <___r3_44(4), ___r3_23(21)>
>   # ___r4_25 = PHI <___r4_45(4), ___r4_26(21)>
>   # gotovar.17_29 = PHI <_51(4), _69(21)>
>   goto gotovar.17_29;

Perhaps disable LIM (and maybe PRE) if the CFG has a large edge/bb ratio (i.e.
dense CFG)? There's probably no benefit in such cases anyway.