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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/97077] Missed loop unrolling with range for over initializer list
Date: Thu, 17 Sep 2020 06:32:26 +0000 [thread overview]
Message-ID: <bug-97077-4-CmWrxo8JSV@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-97077-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97077
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Last reconfirmed| |2020-09-17
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
This is because the second loop has a load from {0,1,2,3,4} in its body and
thus
appears larger to unroll (we don't estimate those loads to go away - a missed
optimization).
static const int C.0[5] = {0, 1, 2, 3, 4};
...
<bb 4> [local count: 894749065]:
# __for_begin_19 = PHI <__for_begin_10(5), &C.0(7)>
# prephitmp_3 = PHI <pretmp_15(5), 0(7)>
# ivtmp_14 = PHI <ivtmp_8(5), 5(7)>
foo (prephitmp_3);
__for_begin_10 = __for_begin_19 + 4;
ivtmp_8 = ivtmp_14 - 1;
if (ivtmp_8 == 0)
goto <bb 6>; [20.00%]
else
goto <bb 5>; [80.00%]
<bb 5> [local count: 715756304]:
pretmp_15 = MEM[(const int *)__for_begin_19 + 4B];
goto <bb 4>; [100.00%]
Estimating sizes for loop 2
BB: 4, after_exit: 0
size: 2 foo (prephitmp_3);
size: 1 __for_begin_10 = __for_begin_19 + 4;
size: 1 ivtmp_8 = ivtmp_14 - 1;
Induction variable computation will be folded away.
size: 2 if (ivtmp_8 == 0)
Exit condition will be eliminated in peeled copies.
Exit condition will be eliminated in last copy.
Constant conditional.
BB: 5, after_exit: 1
size: 1 pretmp_15 = MEM[(const int *)__for_begin_19 + 4B];
size: 7-3, last_iteration: 6-3
Loop size: 7
Estimated size after unrolling: 12
Not unrolling loop 2: size would grow.
Not unrolling loop 2: contains call and code would grow.
at some point I had patches to improve this but they had negative ripple-down
effects so I reverted them.
next prev parent reply other threads:[~2020-09-17 6:32 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-16 21:01 [Bug c++/97077] New: " bmburstein at gmail dot com
2020-09-17 6:32 ` rguenth at gcc dot gnu.org [this message]
2021-07-04 14:13 ` [Bug c++/97077] " magiblot at hotmail dot com
2021-12-11 1:31 ` pinskia at gcc dot gnu.org
2021-12-27 4:11 ` [Bug tree-optimization/97077] " pinskia at gcc dot gnu.org
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