cmd/compile: preallocate storage for three Value args

benchstat master2 arg3b
name      old time/op    new time/op    delta
Template     441ms ± 4%     444ms ± 6%    ~     (p=0.335 n=22+25)
GoTypes      1.51s ± 2%     1.51s ± 2%    ~     (p=0.129 n=25+21)
Compiler     5.59s ± 1%     5.56s ± 2%  -0.65%  (p=0.001 n=24+21)

name      old alloc/op   new alloc/op   delta
Template    85.6MB ± 0%    85.3MB ± 0%  -0.40%  (p=0.000 n=25+24)
GoTypes      307MB ± 0%     305MB ± 0%  -0.38%  (p=0.000 n=25+25)
Compiler    1.06GB ± 0%    1.05GB ± 0%  -0.43%  (p=0.000 n=25+25)

name      old allocs/op  new allocs/op  delta
Template     1.10M ± 0%     1.09M ± 0%  -1.04%  (p=0.000 n=25+25)
GoTypes      3.36M ± 0%     3.32M ± 0%  -1.13%  (p=0.000 n=25+24)
Compiler     13.0M ± 0%     12.9M ± 0%  -1.12%  (p=0.000 n=25+25)

Change-Id: I1280b846e895c00b95bb6664958a7765bd819610
Reviewed-on: https://go-review.googlesource.com/20296Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/TODO

@@ -29,7 +29,6 @@ Optimizations (better compiled code)
 
 Optimizations (better compiler)
 -------------------------------
-- OpStore uses 3 args.  Increase the size of Value.argstorage to 3?
 - Handle signed division overflow and sign extension earlier
 
 Regalloc

src/cmd/compile/internal/ssa/sizeof_test.go

@@ -22,7 +22,7 @@ func TestSizeof(t *testing.T) {
 		_32bit uintptr     // size on 32bit platforms
 		_64bit uintptr     // size on 64bit platforms
 	}{
-		{Value{}, 60, 104},
+		{Value{}, 64, 112},
 		{Block{}, 124, 232},
 	}
 

src/cmd/compile/internal/ssa/value.go

@@ -38,8 +38,8 @@ type Value struct {
 	// Source line number
 	Line int32
 
-	// Storage for the first two args
-	argstorage [2]*Value
+	// Storage for the first three args
+	argstorage [3]*Value
 }
 
 // Examples: