gcc9 snapshot 20190414 is 30x slower than gcc 6.3

[Jason Mancini]

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.