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GCC missing -flto optimizations? SPEC lbm benchmark
- From: Steve Ellcey <sellcey at marvell dot com>
- To: "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Thu, 14 Feb 2019 19:29:51 +0000
- Subject: GCC missing -flto optimizations? SPEC lbm benchmark
I have a question about SPEC CPU 2017 and what GCC can and cannot do
with -flto. As part of some SPEC analysis I am doing I found that with
-Ofast, ICC and GCC were not that far apart (especially spec int rate,
spec fp rate was a slightly larger difference).
But when I added -ipo to the ICC command and -flto to the GCC command,
the difference got larger. In particular the 519.lbm_r was more than
twice as fast with ICC and -ipo, but -flto did not help GCC at all.
There are other tests that also show this type of improvement with -ipo
like 538.imagick_r, 544.nab_r, 525.x264_r, 531.deepsjeng_r, and
548.exchange2_r, but none are as dramatic as 519.lbm_r. Anyone have
any idea on what ICC is doing that GCC is missing? Is GCC just not
agressive enough with its inlining?