Investigate LLD Build Time Discrepancy

[theduke]

As discussed on Zulip, build times with cg_clif appear to be consistently faster than llvm with ld, but slower with lld.

https://github.com/theduke/rustc-cranelift-bench/blob/main/timings.csv

[bjorn3]

I can reproduce this. While trying to repro I even found a rustc ICE: rust-lang/rust#83138

[bjorn3]

Excepts from -Ztime-passes:

cg_llvm:

time:   0.056; rss:   67MB ->  126MB (  +59MB)  configure_and_expand
time:   0.208; rss:  142MB ->  143MB (   +0MB)  link
time:   0.360; rss:   51MB ->  141MB (  +90MB)  total

cg_clif:

time:   0.201; rss:   51MB ->  276MB ( +224MB)  configure_and_expand
time:   0.149; rss:  154MB ->  150MB (   -4MB)  link
time:   0.455; rss:   45MB ->  101MB (  +56MB)  total

I think this is #927.

[bjorn3]

With #927 implemented in b1d14ca, I get:

time:   0.104; rss:   50MB ->  130MB (  +81MB)  configure_and_expand
time:   0.155; rss:  135MB ->  132MB (   -4MB)  link
time:   0.379; rss:   44MB ->   94MB (  +51MB)  total

According to hyperfine cg_clif is now 8% slower than cg_llvm instead of the ~25% I believe it was before.

[bjorn3]

I think the difference is small enough now.