https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98265
Bug ID: 98265
Summary: gcc-10 has significantly worse code generated with -O2
compared to -O1 (or gcc-9 -O2) when using the Eigen
C++ library
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: ipa
Assignee: unassigned at gcc dot gnu.org
Reporter: kartikmohta at gmail dot com
CC: marxin at gcc dot gnu.org
Target Milestone: ---
I was checking the generated assembly for the following simple code using the
Eigen C++ library:
#include <Eigen/Core>
Eigen::Matrix<float, 3, 1> f(float x, float y, float z, float scale)
{
return Eigen::Matrix<float, 3, 1>(x, y, z) * scale;
}
The Eigen::Matrix structure can be thought of as just a wrapper around an array
in this case. The preprocessed file is attached.
When compiled with -O1 optimization, this generates the expected reasonable
looking code, but when using -O2 it looks like the generated code is extremely
bad. The code generated by gcc-9.3 with -O2 also looks good. A quick comparison
of the code generated by gcc 10.2 vs gcc 9.3 can be seen at
https://godbolt.org/z/186c19.
Upon bisecting this, it seems like the offending changes are from the pair of
commits which changed some of the inliner params:
commit 1e83bd7003e03160b7d71fb959111e89b53446ab
Author: Jan Hubicka <hubi...@gcc.gnu.org>
Date: Sat Nov 23 05:13:23 2019
Convert inliner to new param infrastructure
commit 9340d34599e6d5e7a6f3614de44b2c578b180c1b
Author: Jan Hubicka <hubi...@ucw.cz>
Date: Sat Nov 23 05:11:25 2019
Convert inliner to function specific param infrastructure
By playing with the optimization parameters, I saw that using either
-fno-partial-inlining or --param early-inlining-insns=14 fixes the generated
code with -O2.
Upon further investigation, it looks like earlier there were two separate
params for early-inlining-insns for -O3 and -O2 but in the consider_split
function (ipa-split.c) only the value for -O3 was used irrespective of the
optimization level. This code was not changed when the inliner params changes
were made leading to a much smaller value of param_early_inlining_insns being
used in consider_split for -O2 now, which may be causing problems with inlining
in this case.
