Seeking advice on creating reduced testcase

[Alexander Monakov]

On Thu, 24 Sep 2015, Johan Alfredsson wrote:
> I'm seeing this already at -O1 (-O0 is fine, no deadlock). Now, I
> tried to bisect whatever options -O1 implies by looking at what g++ -Q
> -O1... produced, but it turns out that when I use the full list of
> options (except for -fleading-underscore and -fgnu-runtime, which
> obviously are not actually used for my -O1 compile, as either the
> compiler complains that I'm not using LTO or the linker complains
> about ___dso_handle being undefined for libstdc++ code) I don't get
> the deadlock anymore.

As GCC manual mentions, without -O optimizations are disabled, even if
individual flags are passed on the command line (how did you find out about
-Q, and yet not about that restriction?).  Thus, you have to keep -O1 and add
-fno-...  flags to disable optimizations, if you want to bisect on that basis.

(the manual also mentions that not all optimizations have a flag, so even if
the above restriction wasn't there, such bisection would still have to work by
removing optimization from a failing level)

Alexander