cmd/compile: teach rulegen to remove unused decls

First, add cpu and memory profiling flags, as these are useful to see
where rulegen is spending its time. It now takes many seconds to run on
a recent laptop, so we have to keep an eye on what it's doing.

Second, stop writing '_ = var' lines to keep imports and variables used
at all times. Now that rulegen removes all such unused names, they're
unnecessary.

To perform the removal, lean on go/types to first detect what names are
unused. We can configure it to give us all the type-checking errors in a
file, so we can collect all "declared but not used" errors in a single
pass.

We then use astutil.Apply to remove the relevant nodes based on the line
information from each unused error. This allows us to apply the changes
without having to do extra parser+printer roundtrips to plaintext, which
are far too expensive.

We need to do multiple such passes, as removing an unused variable
declaration might then make another declaration unused. Two passes are
enough to clean every file at the moment, so add a limit of three passes
for now to avoid eating cpu uncontrollably by accident.

The resulting performance of the changes above is a ~30% loss across the
table, since go/types is fairly expensive. The numbers were obtained
with 'benchcmd Rulegen go run *.go', which involves compiling rulegen
itself, but that seems reflective of how the program is used.

	name     old time/op         new time/op         delta
	Rulegen          5.61s ± 0%          7.36s ± 0%  +31.17%  (p=0.016 n=5+4)

	name     old user-time/op    new user-time/op    delta
	Rulegen          7.20s ± 1%          9.92s ± 1%  +37.76%  (p=0.016 n=5+4)

	name     old sys-time/op     new sys-time/op     delta
	Rulegen          135ms ±19%          169ms ±17%  +25.66%  (p=0.032 n=5+5)

	name     old peak-RSS-bytes  new peak-RSS-bytes  delta
	Rulegen         71.0MB ± 2%         85.6MB ± 2%  +20.56%  (p=0.008 n=5+5)

We can live with a bit more resource usage, but the time/op getting
close to 10s isn't good. To win that back, introduce concurrency in
main.go. This further increases resource usage a bit, but the real time
on this quad-core laptop is greatly reduced. The final benchstat is as
follows:

	name     old time/op         new time/op         delta
	Rulegen          5.61s ± 0%          3.97s ± 1%   -29.26%  (p=0.008 n=5+5)

	name     old user-time/op    new user-time/op    delta
	Rulegen          7.20s ± 1%         13.91s ± 1%   +93.09%  (p=0.008 n=5+5)

	name     old sys-time/op     new sys-time/op     delta
	Rulegen          135ms ±19%          269ms ± 9%   +99.17%  (p=0.008 n=5+5)

	name     old peak-RSS-bytes  new peak-RSS-bytes  delta
	Rulegen         71.0MB ± 2%        226.3MB ± 1%  +218.72%  (p=0.008 n=5+5)

It might be possible to reduce the cpu or memory usage in the future,
such as configuring go/types to do less work, or taking shortcuts to
avoid having to run it many times. For now, ~2x cpu and ~4x memory usage
seems like a fair trade for a faster and better rulegen.

Finally, we can remove the old code that tried to remove some unused
variables in a hacky and unmaintainable way.

Change-Id: Iff9e83e3f253babf5a1bd48cc993033b8550cee6
Reviewed-on: https://go-review.googlesource.com/c/go/+/189798
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 0 insertions, 15 deletions

diff --git a/src/cmd/compile/internal/ssa/rewritedec64.go b/src/cmd/compile/internal/ssa/rewritedec64.go
index a67ae1ed52..3c15ddefdd 100644
--- a/src/cmd/compile/internal/ssa/rewritedec64.go
+++ b/src/cmd/compile/internal/ssa/rewritedec64.go
@@ -3,18 +3,8 @@
 
 package ssa
 
-import "fmt"
-import "math"
-import "cmd/internal/obj"
-import "cmd/internal/objabi"
 import "cmd/compile/internal/types"
 
-var _ = fmt.Println   // in case not otherwise used
-var _ = math.MinInt8  // in case not otherwise used
-var _ = obj.ANOP      // in case not otherwise used
-var _ = objabi.GOROOT // in case not otherwise used
-var _ = types.TypeMem // in case not otherwise used
-
 func rewriteValuedec64(v *Value) bool {
 	switch v.Op {
 	case OpAdd64:
@@ -2726,11 +2716,6 @@ func rewriteValuedec64_OpZeroExt8to64_0(v *Value) bool {
 	}
 }
 func rewriteBlockdec64(b *Block) bool {
-	config := b.Func.Config
-	typ := &config.Types
-	_ = typ
-	v := b.Control
-	_ = v
 	switch b.Kind {
 	}
 	return false