https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98775
Bug ID: 98775
Summary: missing optimization opportunity on nbody
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: vanyacpp at gmail dot com
Target Milestone: ---
Created attachment 50015
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50015&action=edit
nbody.cpp
On the attached sample (208 LOC), clang 11.0 generates the code that is almost
twice as fast as the one generated by GCC 10.2 (-O3 -ffast-math -flto).
$ ./nbody 50000000
4.0s for clang vs 7.5s for GCC.
A quick look at the generated code shows that clang aggressively unrolled all
inner loops. If I unroll all inner loops manually I get:
$ ./nbody-unrolled 50000000
3.7s for clang vs 6.3s for GCC.
17.6B instructions for clang vs 29.6B instructions for GCC.
While the first sample is a subject to unrolling heuristic, the second is about
optimizing the completely linear chunk of code with many floating point
multiplications and additions.
I tried reducing the sample further, but I only came up with PR98774.
