[dev.ssa] cmd/compile/ssa: add nilcheckelim benchmarks

These benchmarks demonstrate that
the nilcheckelim pass is roughly O(n^2):

BenchmarkNilCheckDeep1    	 2000000	       741 ns/op	   1.35 MB/s
BenchmarkNilCheckDeep10   	 1000000	      2237 ns/op	   4.47 MB/s
BenchmarkNilCheckDeep100  	   20000	     60713 ns/op	   1.65 MB/s
BenchmarkNilCheckDeep1000 	     200	   7925198 ns/op	   0.13 MB/s
BenchmarkNilCheckDeep10000	       1	1220104252 ns/op	   0.01 MB/s

Profiling suggests that building the
dominator tree is also O(n^2),
and before size factors take over,
considerably more expensive than nilcheckelim.

Change-Id: If966b38ec52243a25f355dab871300d29db02e16
Reviewed-on: https://go-review.googlesource.com/11520
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 56 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/nilcheck_test.go b/src/cmd/compile/internal/ssa/nilcheck_test.go
new file mode 100644
index 0000000000..2d60957d49
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/nilcheck_test.go
@@ -0,0 +1,56 @@
+package ssa
+
+import (
+	"strconv"
+	"testing"
+)
+
+func BenchmarkNilCheckDeep1(b *testing.B)     { benchmarkNilCheckDeep(b, 1) }
+func BenchmarkNilCheckDeep10(b *testing.B)    { benchmarkNilCheckDeep(b, 10) }
+func BenchmarkNilCheckDeep100(b *testing.B)   { benchmarkNilCheckDeep(b, 100) }
+func BenchmarkNilCheckDeep1000(b *testing.B)  { benchmarkNilCheckDeep(b, 1000) }
+func BenchmarkNilCheckDeep10000(b *testing.B) { benchmarkNilCheckDeep(b, 10000) }
+
+// benchmarkNilCheckDeep is a stress test of nilcheckelim.
+// It uses the worst possible input: A linear string of
+// nil checks, none of which can be eliminated.
+// Run with multiple depths to observe big-O behavior.
+func benchmarkNilCheckDeep(b *testing.B, depth int) {
+	ptrType := &TypeImpl{Size_: 8, Ptr: true, Name: "testptr"} // dummy for testing
+
+	var blocs []bloc
+	blocs = append(blocs,
+		Bloc("entry",
+			Valu("mem", OpArg, TypeMem, 0, ".mem"),
+			Goto(blockn(0)),
+		),
+	)
+	for i := 0; i < depth; i++ {
+		blocs = append(blocs,
+			Bloc(blockn(i),
+				Valu(ptrn(i), OpGlobal, ptrType, 0, nil),
+				Valu(booln(i), OpIsNonNil, TypeBool, 0, nil, ptrn(i)),
+				If(booln(i), blockn(i+1), "exit"),
+			),
+		)
+	}
+	blocs = append(blocs,
+		Bloc(blockn(depth), Goto("exit")),
+		Bloc("exit", Exit("mem")),
+	)
+
+	c := NewConfig("amd64", DummyFrontend{b})
+	fun := Fun(c, "entry", blocs...)
+
+	CheckFunc(fun.f)
+	b.SetBytes(int64(depth)) // helps for eyeballing linearity
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		nilcheckelim(fun.f)
+	}
+}
+
+func blockn(n int) string { return "b" + strconv.Itoa(n) }
+func ptrn(n int) string   { return "p" + strconv.Itoa(n) }
+func booln(n int) string  { return "c" + strconv.Itoa(n) }