RFC: save lots of memory

[Joel Brobecker]

> I'm looking forward to measuring the impact on memory and performance
> with one of the bigger apps we have in house :-).

I finally took the time to make some measurements.  The app I used has
about 600kSLOCs (with comments, it is a little over a million lines)
of Ada, builds and link to about 145MB of code and 70MB of debug info
making a grand total of 215MB.

    GDB startup time: 3.125s -> 2.369s => delta: -0.756s / -24.1 %
    Peak memory usage: 158.3MB -> 70.9MB => delta: -87.4MB / -55.0 %

    The same, but with -readnow:
    GDB startup time: 7.339s -> 6.461s => delta: 0.878s / - 11.9%
    Peak memory usage: 568.4MB -> 429.5MB => delta: -138.9MB / -24.4 %

Unsurprisingly, symbol lookup time did not appear to be affected either way.

I also quickly tried a patch suggested by Tom (attached), and got worse
results on both startup time and memory usage. No visible effect on lookup
performance. Not sure why, yet. I will keep the idea in mind, though.
For now, I must switch to something else again.

Impressive numbers nonetheless. This is a really nice catch, Tom!
-- 
Joel