[cfe-dev] Instrumenting the run-time sizes of SmallVector et al.

[Hans Wennborg]

I've always wondered how the in-line sizes of SmallVector,
SmallString, etc. that we use in LLVM and Clang hold up against the
actual sizes of the containers at run-time.

To figure this out, I added a destructor to each such class, that
would dump the container's final size into a table that got printed at
exit. I also added a special field, __clang_ObjectLocation, to each
such class and hacked Clang to populate that field with the source
location of the object's declarator each time its constructor was
called.

I used this instrumented compiler to build vanilla Clang and captured
the results (attached, along with the patches I used). It looks like
we mostly get it right, but I have patches for a few size tweaks I'd
like to do. (The measurements were done on Clang r215325 / LLVM
r215315).

Cheers,
Hans
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