cmd/compile/internal/gc: speed-up small array comparison

Currently we inline array comparisons for arrays with at most 4 elements.
Compare arrays with small size, but more than 4 elements (e. g. [16]byte)
with larger compares. This provides very slightly smaller binaries,
and results in faster code.

ArrayEqual-6  7.41ns ± 0%  3.17ns ± 0%  -57.15%  (p=0.000 n=10+10)

For go tool:
global text (code) = -559 bytes (-0.014566%)

This also helps mapaccess1_faststr, and maps in general:

MapDelete/Str/1-6               195ns ± 1%     186ns ± 2%   -4.47%  (p=0.000 n=10+10)
MapDelete/Str/2-6               211ns ± 1%     177ns ± 1%  -16.01%  (p=0.000 n=10+10)
MapDelete/Str/4-6               225ns ± 1%     183ns ± 1%  -18.49%  (p=0.000 n=8+10)
MapStringKeysEight_16-6        31.3ns ± 0%    28.6ns ± 0%   -8.63%  (p=0.000 n=6+9)
MapStringKeysEight_32-6        29.2ns ± 0%    27.6ns ± 0%   -5.45%  (p=0.000 n=10+10)
MapStringKeysEight_64-6        29.1ns ± 1%    27.5ns ± 0%   -5.46%  (p=0.000 n=10+10)
MapStringKeysEight_1M-6        29.1ns ± 1%    27.6ns ± 0%   -5.49%  (p=0.000 n=10+10)

Change-Id: I9ec98e41b233031e0e96c4e13d86a324f628ed4a
Reviewed-on: https://go-review.googlesource.com/40771
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -166,7 +166,7 @@ var allAsmTests = []*asmTests{
 	{
 		arch:    "amd64",
 		os:      "linux",
-		imports: []string{"encoding/binary", "math/bits"},
+		imports: []string{"encoding/binary", "math/bits", "unsafe"},
 		tests:   linuxAMD64Tests,
 	},
 	{
@@ -869,6 +869,35 @@ var linuxAMD64Tests = []*asmTest{
 		}`,
 		[]string{"\tRORB\t"},
 	},
+	// Check that array compare uses 2/4/8 byte compares
+	{
+		`
+		func f68(a,b [2]byte) bool {
+		    return a == b
+		}`,
+		[]string{"\tCMPW\t[A-Z]"},
+	},
+	{
+		`
+		func f69(a,b [3]uint16) bool {
+		    return a == b
+		}`,
+		[]string{"\tCMPL\t[A-Z]"},
+	},
+	{
+		`
+		func f70(a,b [15]byte) bool {
+		    return a == b
+		}`,
+		[]string{"\tCMPQ\t[A-Z]"},
+	},
+	{
+		`
+		func f71(a,b unsafe.Pointer) bool { // This was a TODO in mapaccess1_faststr
+		    return *((*[4]byte)(a)) != *((*[4]byte)(b))
+		}`,
+		[]string{"\tCMPL\t[A-Z]"},
+	},
 }
 
 var linux386Tests = []*asmTest{

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

@@ -3243,11 +3243,25 @@ func walkcompare(n *Node, init *Nodes) *Node {
 	// inline or call an eq alg.
 	t := n.Left.Type
 	var inline bool
+
+	maxcmpsize := int64(4)
+	unalignedLoad := false
+	switch thearch.LinkArch.Family {
+	case sys.AMD64, sys.ARM64, sys.S390X:
+		// Keep this low enough, to generate less code than function call.
+		maxcmpsize = 16
+		unalignedLoad = true
+	case sys.I386:
+		maxcmpsize = 8
+		unalignedLoad = true
+	}
+
 	switch t.Etype {
 	default:
 		return n
 	case TARRAY:
-		inline = t.NumElem() <= 1 || (t.NumElem() <= 4 && issimple[t.Elem().Etype])
+		// We can compare several elements at once with 2/4/8 byte integer compares
+		inline = t.NumElem() <= 1 || (issimple[t.Elem().Etype] && (t.NumElem() <= 4 || t.Elem().Width*t.NumElem() <= maxcmpsize))
 	case TSTRUCT:
 		inline = t.NumFields() <= 4
 	}
@@ -3333,11 +3347,54 @@ func walkcompare(n *Node, init *Nodes) *Node {
 			)
 		}
 	} else {
-		for i := 0; int64(i) < t.NumElem(); i++ {
-			compare(
-				nod(OINDEX, cmpl, nodintconst(int64(i))),
-				nod(OINDEX, cmpr, nodintconst(int64(i))),
-			)
+		step := int64(1)
+		remains := t.NumElem() * t.Elem().Width
+		combine64bit := unalignedLoad && Widthreg == 8 && t.Elem().Width <= 4 && t.Elem().IsInteger()
+		combine32bit := unalignedLoad && t.Elem().Width <= 2 && t.Elem().IsInteger()
+		combine16bit := unalignedLoad && t.Elem().Width == 1 && t.Elem().IsInteger()
+		for i := int64(0); remains > 0; {
+			var convType *types.Type
+			switch {
+			case remains >= 8 && combine64bit:
+				convType = types.Types[TINT64]
+				step = 8 / t.Elem().Width
+			case remains >= 4 && combine32bit:
+				convType = types.Types[TUINT32]
+				step = 4 / t.Elem().Width
+			case remains >= 2 && combine16bit:
+				convType = types.Types[TUINT16]
+				step = 2 / t.Elem().Width
+			default:
+				step = 1
+			}
+			if step == 1 {
+				compare(
+					nod(OINDEX, cmpl, nodintconst(int64(i))),
+					nod(OINDEX, cmpr, nodintconst(int64(i))),
+				)
+				i++
+				remains -= t.Elem().Width
+			} else {
+				cmplw := nod(OINDEX, cmpl, nodintconst(int64(i)))
+				cmplw = conv(cmplw, convType)
+				cmprw := nod(OINDEX, cmpr, nodintconst(int64(i)))
+				cmprw = conv(cmprw, convType)
+				// For code like this:  uint32(s[0]) | uint32(s[1])<<8 | uint32(s[2])<<16 ...
+				// ssa will generate a single large load.
+				for offset := int64(1); offset < step; offset++ {
+					lb := nod(OINDEX, cmpl, nodintconst(int64(i+offset)))
+					lb = conv(lb, convType)
+					lb = nod(OLSH, lb, nodintconst(int64(8*t.Elem().Width*offset)))
+					cmplw = nod(OOR, cmplw, lb)
+					rb := nod(OINDEX, cmpr, nodintconst(int64(i+offset)))
+					rb = conv(rb, convType)
+					rb = nod(OLSH, rb, nodintconst(int64(8*t.Elem().Width*offset)))
+					cmprw = nod(OOR, cmprw, rb)
+				}
+				compare(cmplw, cmprw)
+				i += step
+				remains -= step * t.Elem().Width
+			}
 		}
 	}
 	if expr == nil {

src/runtime/hashmap_fast.go

@@ -252,8 +252,6 @@ func mapaccess1_faststr(t *maptype, h *hmap, ky string) unsafe.Pointer {
 				return add(unsafe.Pointer(b), dataOffset+bucketCnt*2*sys.PtrSize+i*uintptr(t.valuesize))
 			}
 			// check first 4 bytes
-			// TODO: on amd64/386 at least, make this compile to one 4-byte comparison instead of
-			// four 1-byte comparisons.
 			if *((*[4]byte)(key.str)) != *((*[4]byte)(k.str)) {
 				continue
 			}