Rebuilding webrender/wrench takes a long time

[jrmuizel]

$ git clone https://github.com/servo/webrender/
$ cd wrench
$ cargo build

$ [do some small change to wrench/src/rawtest.rs]
$ cargo build
   Compiling wrench v0.3.0 (file:///tmp/webrender/wrench)
    Finished dev [unoptimized + debuginfo] target(s) in 5.15 secs

These timings are from linux. It's quite a bit worse on Mac, but a lot of that time probably comes from dsymutil (#47784)

[ishitatsuyuki]

@kennytm Issues with a mixture of root cause shouldn't be tagged for the WG; a WG issue normally has a concrete target to achieve, and is normally tagged by a WG lead.

[michaelwoerister]

@ishitatsuyuki There's no harm in marking issues like this one with the WG-compiler-performance label. I personally view the label as a tool for discoverability, so it's better to tag too much than too little.

[michaelwoerister]

I can reproduce this. Rebuilding takes 5.6 seconds on my system. Of that, 2.8 seconds are spent in the linker. The compiler also spends 0.780 seconds in LLVM, but it only re-compiles the two object files corresponding to rawtest.rs, so that's pretty much the best case.

Note that using different linkers gives different results:

	rebuild time
ld	9.38s
gold	5.65s
lld	3.41s

[nikomatsakis]

@michaelwoerister can you clarify why that is the best case? =) that is, can we break down a bit more where the time is going?

[michaelwoerister]

It is the best case in the sense that only the two object files get re-compiled that correspond to the changed module while all the others are re-used. Iow, the best case with respect to object file re-use. I'll post some -Ztime-passes and perf focus numbers later, so we can take a closer look.

[michaelwoerister]

By the way, in order to compile with LLD, I had to use clang as the linker:

RUSTFLAGS="-Clinker=clang -Clink-arg=-fuse-ld=lld" cargo build

[nnethercote]

On my Linux box, using Nightly rustc (from 2018-05-22), linking takes about 4.8s out of a total time of 6.3s. That's with whatever the default linker is.

[michaelwoerister]

That's with whatever the default linker is.

Really? That's very surprising. Maybe it's bottlenecked on I/O on that system?

[nnethercote]

@michaelwoerister: how can I tell which linker I am using? And how can I change the linker?