cmd/compile: avoid generating some dead blocks

We generate a lot of pointless dead blocks
during the AST to SSA conversion.
There are a few commonly occurring kinds
of statements that contain neither variables
nor code and that switch to a new block themselves.
Stop making dead blocks for them.

For the code in #19379, this reduces compilation
wall time by 36% and max rss by 28%.

This also helps a little for regular code,
particularly code heavy on switch statements.

name       old time/op      new time/op      delta
Template        231ms ± 3%       230ms ± 5%    ~     (p=0.402 n=17+16)
Unicode         101ms ± 4%       103ms ± 5%    ~     (p=0.221 n=19+18)
GoTypes         635ms ± 5%       625ms ± 4%    ~     (p=0.063 n=20+18)
Compiler        2.93s ± 2%       2.89s ± 2%  -1.22%  (p=0.003 n=20+19)
SSA             4.53s ± 3%       4.52s ± 3%    ~     (p=0.380 n=20+19)
Flate           132ms ± 4%       133ms ± 5%    ~     (p=0.647 n=20+19)
GoParser        161ms ± 3%       161ms ± 4%    ~     (p=0.749 n=20+19)
Reflect         403ms ± 4%       397ms ± 3%  -1.53%  (p=0.030 n=20+19)
Tar             121ms ± 2%       121ms ± 8%    ~     (p=0.544 n=19+19)
XML             225ms ± 3%       224ms ± 4%    ~     (p=0.396 n=20+19)

name       old user-ns/op   new user-ns/op   delta
Template   302user-ms ± 1%  297user-ms ± 7%  -1.49%  (p=0.048 n=15+18)
Unicode    142user-ms ± 3%  143user-ms ± 5%    ~     (p=0.363 n=19+17)
GoTypes    852user-ms ± 5%  851user-ms ± 3%    ~     (p=0.851 n=20+18)
Compiler   4.11user-s ± 6%  3.98user-s ± 3%  -3.08%  (p=0.000 n=20+19)
SSA        6.91user-s ± 5%  6.82user-s ± 7%    ~     (p=0.113 n=20+19)
Flate      164user-ms ± 4%  168user-ms ± 4%  +2.42%  (p=0.001 n=18+19)
GoParser   207user-ms ± 4%  206user-ms ± 4%    ~     (p=0.176 n=20+18)
Reflect    509user-ms ± 4%  505user-ms ± 4%    ~     (p=0.113 n=20+19)
Tar        153user-ms ± 7%  151user-ms ± 9%    ~     (p=0.283 n=20+19)
XML        284user-ms ± 4%  282user-ms ± 4%    ~     (p=0.270 n=20+19)

name       old alloc/op     new alloc/op     delta
Template       42.6MB ± 0%      41.9MB ± 0%  -1.55%  (p=0.000 n=19+19)
Unicode        31.7MB ± 0%      31.7MB ± 0%    ~     (p=0.828 n=20+18)
GoTypes         124MB ± 0%       121MB ± 0%  -2.11%  (p=0.000 n=20+17)
Compiler        534MB ± 0%       523MB ± 0%  -2.06%  (p=0.000 n=20+19)
SSA             989MB ± 0%       977MB ± 0%  -1.28%  (p=0.000 n=20+19)
Flate          27.8MB ± 0%      27.5MB ± 0%  -0.98%  (p=0.000 n=20+19)
GoParser       34.3MB ± 0%      34.0MB ± 0%  -0.81%  (p=0.000 n=20+19)
Reflect        84.6MB ± 0%      82.9MB ± 0%  -2.00%  (p=0.000 n=17+18)
Tar            28.8MB ± 0%      28.3MB ± 0%  -1.52%  (p=0.000 n=16+18)
XML            47.2MB ± 0%      45.8MB ± 0%  -2.99%  (p=0.000 n=20+19)

name       old allocs/op    new allocs/op    delta
Template         421k ± 1%        419k ± 1%  -0.41%  (p=0.001 n=20+19)
Unicode          338k ± 1%        338k ± 1%    ~     (p=0.478 n=20+19)
GoTypes         1.28M ± 0%       1.28M ± 0%  -0.36%  (p=0.000 n=20+18)
Compiler        5.06M ± 0%       5.03M ± 0%  -0.63%  (p=0.000 n=20+19)
SSA             9.14M ± 0%       9.11M ± 0%  -0.34%  (p=0.000 n=20+19)
Flate            267k ± 1%        266k ± 1%    ~     (p=0.149 n=20+19)
GoParser         347k ± 0%        347k ± 1%    ~     (p=0.103 n=19+19)
Reflect         1.07M ± 0%       1.07M ± 0%  -0.42%  (p=0.000 n=16+18)
Tar              274k ± 0%        273k ± 1%    ~     (p=0.116 n=19+19)
XML              449k ± 0%        446k ± 1%  -0.60%  (p=0.000 n=20+19)

Updates #19379

Change-Id: Ie798c347a0c081f5e349e1529880bebaae290967
Reviewed-on: https://go-review.googlesource.com/37760Reviewed-by: David Chase <drchase@google.com>

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

@@ -496,8 +496,14 @@ func (s *state) stmt(n *Node) {
 	// Provide a block for the dead code so that we don't have
 	// to add special cases everywhere else.
 	if s.curBlock == nil {
-		dead := s.f.NewBlock(ssa.BlockPlain)
-		s.startBlock(dead)
+		switch n.Op {
+		case OLABEL, OBREAK, OCONTINUE:
+			// These statements don't need a block,
+			// and they commonly occur without one.
+		default:
+			dead := s.f.NewBlock(ssa.BlockPlain)
+			s.startBlock(dead)
+		}
 	}
 
 	s.stmtList(n.Ninit)
@@ -604,9 +610,12 @@ func (s *state) stmt(n *Node) {
 			lab.target = s.f.NewBlock(ssa.BlockPlain)
 		}
 
-		// go to that label (we pretend "label:" is preceded by "goto label")
-		b := s.endBlock()
-		b.AddEdgeTo(lab.target)
+		// Go to that label.
+		// (We pretend "label:" is preceded by "goto label", unless the predecessor is unreachable.)
+		if s.curBlock != nil {
+			b := s.endBlock()
+			b.AddEdgeTo(lab.target)
+		}
 		s.startBlock(lab.target)
 
 	case OGOTO:
@@ -826,8 +835,10 @@ func (s *state) stmt(n *Node) {
 			}
 		}
 
-		b := s.endBlock()
-		b.AddEdgeTo(to)
+		if s.curBlock != nil {
+			b := s.endBlock()
+			b.AddEdgeTo(to)
+		}
 
 	case OFOR:
 		// OFOR: for Ninit; Left; Right { Nbody }