
Improving C++ Builds with Split DWARF - mrich
http://www.productive-cpp.com/improving-cpp-builds-with-split-dwarf/
======
mewse
In my moderate-sized real-world project (about 700 C++ source files, about
130k lines of code), adding -gsplit-dwarf reduced the time of a single-file
recompile+relink from 7.2 seconds to 5.8 seconds, which isn't bad at all for a
single extra compiler flag that you never have to think about again (assuming
that it works with all your other debugging tools).

But much better than that was switching to using `gold` instead of the regular
`ld` (mentioned in the "remarks" section near the bottom of the article);
doing this brought my time down substantially further, from 5.8 seconds down
to 1.6 seconds. Even without using -gsplit-dwarf, it brought the
recompile+relink time down from the original 7.2 seconds to 2.6 seconds.

I guess my codebase might be hitting a particularly bad case for `ld`. I'm
kind of startled to see such a large difference for what seemed like a
throwaway extra suggestion at the end of the article!

~~~
glandium
Try lld, it's even faster than gold.

Also note that not so long ago, ccache didn't support split dwarf. If you're
using ccache, check that your version supports it.

~~~
mewse
With lld, that single-file rebuild+relink drops from 1.6 seconds (with gold)
to 1.2 seconds.

lld spits out a bunch of warnings that neither ld nor gold cared about, about
finding local symbols in the global symbol table of some of the third-party
.so libraries I link against. Not sure to what extent I should care about
that; the final build product does run the same as always.

And just so that nobody else needs to go searching; looks like ccache added
support for split dwarf debugging data in its version 3.2.3, about three years
ago. (although I haven't actually tried it to confirm, yet)

------
claudius
Relatedly, I found it useful to pass -g1 and not just -g to GCC. The resulting
debug information is a fair bit smaller and GCC itself also takes less RAM and
less CPU time to compile template-heavy code (going from 10s of GB to 1-2 GB
on one particularly nasty object file).

~~~
saagarjha
Have you noticed any noticeable decrease in the quality of debug information
available with -g1?

~~~
glandium
-g1 omits information about local variables and line numbers. It's fine if you want stack traces, not so good when you actually want to debug.

------
LHxB
13 minutes -> 11 minutes: "Elapsed time goes down by 15%."

3:16 minutes -> 1:42 minutes: "We get a roughly 90% speedup in elapsed time."

What's the formula for the "speedup"? I think it would be more expressive if
one would compare it in the same terms.

~~~
EdSchouten
I think the math was done inversely.

1:42 -> 3:16: 90% slowdown?

~~~
leni536
We could use a logarithmic scale instead of percentages. If we defined a
decibel scale for time then this would be a 2.83 dB change both ways (if we
defined dB-sec as 10*log10(time/(1 sec)) ).

------
cryptonector
It might be useful to have things like version-script mapfiles as defining
interfaces separate from the actual libraries (ELF objects). Then for shared
linking the build system (make, whatever) could notice that though an object
changed, its interfaces haven't, and then not re-link dependents.

------
roel_v
Is this a new thing? Isn't this like the pdb files that msvc had since the
late 1990's?

~~~
hohenheim
Not new, for one thing I did not use it. Thinking about all the time lost
waiting for linker to finish makes me sad, but I'll change my work flow now!

------
grandinj
Just be aware that various downstream dev tools like profilers don't support
this very well yet

