Alp Mestanogullari authored Jul 11, 2019 and Marge Bot committed Jul 13, 2019

This patch was motivated by some performance characterization work done
for #16822, where we suspected that GHC was spending a lot of time waiting
on the linker to be done. (That turned out to be true.)

The tracing is taken care of by ErrUtils.withTiming, so this patch just defines
and uses a little wrapper around that function in all the helpers for
calling the various systools (C compiler, linker, unlit, ...).

With this patch, assuming a GHC executable linked against an eventlog-capable
RTS (RTS ways that contain the debug, profiling or eventlog way units), we can
measure how much time is spent in each of the SysTools when building hello.hs
by simply doing:

  ghc hello.hs -ddump-timings +RTS -l

The event names are "systool:{cc, linker, as, unlit, ...}".