Compile DUB binaries with LDC

[wilzbach]

(from a recent discussion #1369 (comment) in which the resulting size of a DUB's binary was discussed)

Compiler	Size
DMD	17M
LDC	13M
GDC	19M

DUB is currently compiled with DMD, because LDC doesn't support showing the stack for exceptions, but imho the increase in speed and reduced side justifies overlooking this. After all, this is a production release - not a debug build.

LDC has LTO easily available which results usually in ~10% performance improvement and also a noticeable binary size reduction.
I haven't done big tests, but a build of vibe.d without needing to rebuild goes reproducible down from 0.22s to 0.15s

So my question: how big of a performance gain would justify the missing stack trace for exceptions?
(reminder: here's what DMD generates https://run.dlang.io/is/vzByQy vs. LDC: https://run.dlang.io/is/6iG8ai)

This issue is just about the separately built binaries.
(there's https://issues.dlang.org/show_bug.cgi?id=18656 for the bundled dub with the official release archives)

[kinke]

LDC doesn't enforce the ld option --export-dynamic as DMD does, that's why the stacktrace is missing the symbol names (or rather the stack trace generator in druntime not reading from dwarf debuginfo but from the dynamic symbols table IIRC). So adding -L--export-dynamic makes them available, but may also lead to a big size increase: https://run.dlang.io/is/NXqZH5

[wilzbach]

So adding -L--export-dynamic makes them available, but may also lead to a big size increase:

Nice! I just tested this (without LTO) and with -L--export-dynamic the generated binary takes up 17M as well (that's the same as DMD's build). As the dub binary take 3.5M in a tar.gz archive currently, I doubt that this is a big problem.

I haven't gotten around creating a real benchmark for dub, so I just use a naive "dub describe".
On my system a dub describe of the dub-registry takes 0.32s - whereas the LDC binary takes in at 0.19s. This needs further testing of course before making a statement, but it looks like dub can be ~one third faster just by switching to LDC :)

Anyhow, I made a PR for this: #1420