cmd/compile: lay out exit post-dominated blocks at the end
Complete a long-standing TODO in the code.
Exit blocks are cold code, so we lay them out at the end of the function.
Blocks that are post-dominated by exit blocks are also ipso facto exit blocks.
Treat them as such.
Implement using a simple loop, because there are generally very few exit blocks.
In addition to improved instruction cache, this empirically yields
better register allocation.
Binary size impact:
file before after Δ %
cgo
4812872 4808776 -4096 -0.085%
fix
3370072 3365976 -4096 -0.122%
vet
8252280 8248184 -4096 -0.050%
total
115052984 115040696 -12288 -0.011%
This also appears to improve compiler performance
(-0.15% geomean time/op, -1.20% geomean user time/op),
but that could just be alignment effects.
Compiler benchmarking hasn't been super reliably recently,
and there's no particular reason to think this should
speed up the compiler that much.
Change-Id: I3d262c4f5cb80626a67a5c17285e2fa09f423c00
Reviewed-on: https://go-review.googlesource.com/c/go/+/227217
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>