https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101555
Bug ID: 101555
Summary: Compile slowdown in tree PRE
Product: gcc
Version: 9.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: wsnyder at wsnyder dot org
Target Milestone: ---
Running
g++ (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
as
g++ -I. -c -Os -ftime-report -o /dev/null Vara___024root__114_0.cpp
takes 297 CPU seconds.
On clang version 10.0.0-4ubuntu1 it takes 37 seconds total.
In GCC, 276 of 297 seconds are in the "tree PRE" stage. Splitting the large
function into roughly three pieces (Vara___024root__114_0.cpp, _1 and _2)
the "tree PRE" stage takes 12.7, 7.8 and 3.9 seconds (totalling 24.4).
Therefore I suspect some O(n^2) issue in tree PRE.
Note removing -Os makes gcc much faster, but still signifcantly slower than
clang.
This issue was originally reported using gcc 8.2.0 by Matheus Cavalcante
at https://github.com/verilator/verilator/issues/3072
