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From: "ian at airs dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug debug/101064] long compile time in var_tracking for Go package
Date: Tue, 29 Jun 2021 19:00:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101064

--- Comment #5 from Ian Lance Taylor <ian at airs dot com> ---
I have attached a C test case that demonstrates the problem.  The C case ma=
y be
somewhat worse because I had to use ordinary variables, whereas the Go test
case is used compiler-generated temporary variables.

When I compiled the C test case with mainline GCC with "-g -O2" on my laptop
the compilation takes 20 minutes, and according to -ftime-report 91% of that
time is spent on var-tracking:

 var-tracking dataflow              : 541.98 ( 44%)   0.02 (  2%) 542.26 ( =
44%)
 1288k (  0%)
 var-tracking emit                  : 575.86 ( 47%)   0.04 (  4%) 576.31 ( =
47%)
 3125k (  0%)

This test case is, of course, generated code, as is the original Go test ca=
se
(the generated Go code is
https://go.googlesource.com/go/+/refs/heads/master/src/cmd/internal/obj/x86=
/avx_optabs.go).

The var-tracking pass already has a fast exit for functions with lots of ba=
sic
blocks.  Perhaps it also needs a fast exit for functions that are very long
even if they don't have many basic blocks.=