Re: gcc9 snapshot 20190414 is 30x slower than gcc 6.3

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On Wed, 17 Apr 2019 at 03:20, Xi Ruoyao wrote:
>
> On 2019-04-17 02:09 +0000, Jason Mancini wrote:
> > > Using gcc 6.3, my C++ source file compiles in 1m2s with -O0.  With snapshot
> > > 20190414 (compiled with --disable-checking
> > > and -O2 and make install-strip), it takes 31 minutes to compile the same
> > > file with -O0.  Have I overlooked disabling some
> > > snapshot self-checking code?  Are there known configuration mistakes that
> > > could result in this sort of performance
> > > degradation?  Thanks!  It will take a while to go back and try other gcc 6,
> > > 7, 8, and 9 snapshots to collect points of reference.
> > > Both are pretty heavy on memory, gcc6 uses 3.7G and gcc9 uses 5.4G for this
> > > file.  There's a lot of templatized headers.
> >
> > Latest data points:
> > gcc-6.3/6.4 take about 43 seconds
> > gcc-7.2 takes 30 minutes
> > gcc-8.2 takes 27 minutes
> > gcc-9.0 takes 31 minutes (snapshot 20190414)
> > clang 6.0.1/7.01 take about 31 seconds
> >
> > This is frustrating, as I'm going to have to capitulate to using clang here
> > for a very large user base.  We've been a gcc
> > shop for decades.
>
> We'll never know why unless you can give a testcase to reproduce this issue.

Even better would be a bug report.

The chances of it ever getting fixed are much higher if we know
there's a problem. If you just complain that you have to switch to
clang then nothing will change. And if you'd told us two years ago
that your program started compiling 40 times slower, maybe it would
have been fixed already.