cmd/compile: optimize signed non-negative div/mod by a power of 2

This CL optimizes assembly for len() or cap() division
by a power of 2 constants:

    func lenDiv(s []int) int {
        return len(s) / 16
    }

amd64 assembly before the CL:

    MOVQ    "".s+16(SP), AX
    MOVQ    AX, CX
    SARQ    $63, AX
    SHRQ    $60, AX
    ADDQ    CX, AX
    SARQ    $4, AX
    MOVQ    AX, "".~r1+32(SP)
    RET

amd64 assembly after the CL:

    MOVQ    "".s+16(SP), AX
    SHRQ    $4, AX
    MOVQ    AX, "".~r1+32(SP)
    RET

The CL relies on the fact that len() and cap() result cannot
be negative.

Trigger stats for the added SSA rules on linux/amd64 when running
make.bash:

     46 Div64
     12 Mod64

The added SSA rules may trigger on more cases in the future
when SSA values will be populated with the info on their
lower bounds.

For instance:

    func f(i int16) int16 {
        if i < 3 {
            return -1
        }

        // Lower bound of i is 3 here -> i is non-negative,
        // so unsigned arithmetics may be used here.
        return i % 16
    }

Change-Id: I8bc6be5a03e71157ced533c01416451ff6f1a7f0
Reviewed-on: https://go-review.googlesource.com/65530
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1140,6 +1140,58 @@ var linuxAMD64Tests = []*asmTest{
 		`,
 		pos: []string{"\tSETHI\t\\("},
 	},
+	// Check that len() and cap() div by a constant power of two
+	// are compiled into SHRQ.
+	{
+		fn: `
+		func $(a []int) int {
+			return len(a) / 1024
+		}
+		`,
+		pos: []string{"\tSHRQ\t\\$10,"},
+	},
+	{
+		fn: `
+		func $(s string) int {
+			return len(s) / (4097 >> 1)
+		}
+		`,
+		pos: []string{"\tSHRQ\t\\$11,"},
+	},
+	{
+		fn: `
+		func $(a []int) int {
+			return cap(a) / ((1 << 11) + 2048)
+		}
+		`,
+		pos: []string{"\tSHRQ\t\\$12,"},
+	},
+	// Check that len() and cap() mod by a constant power of two
+	// are compiled into ANDQ.
+	{
+		fn: `
+		func $(a []int) int {
+			return len(a) % 1024
+		}
+		`,
+		pos: []string{"\tANDQ\t\\$1023,"},
+	},
+	{
+		fn: `
+		func $(s string) int {
+			return len(s) % (4097 >> 1)
+		}
+		`,
+		pos: []string{"\tANDQ\t\\$2047,"},
+	},
+	{
+		fn: `
+		func $(a []int) int {
+			return cap(a) % ((1 << 11) + 2048)
+		}
+		`,
+		pos: []string{"\tANDQ\t\\$4095,"},
+	},
 }
 
 var linux386Tests = []*asmTest{
@@ -1219,6 +1271,58 @@ var linux386Tests = []*asmTest{
 		}`,
 		pos: []string{"\tADDL\t[$]-19", "\tIMULL"}, // (n-19)*a
 	},
+	// Check that len() and cap() div by a constant power of two
+	// are compiled into SHRL.
+	{
+		fn: `
+		func $(a []int) int {
+			return len(a) / 1024
+		}
+		`,
+		pos: []string{"\tSHRL\t\\$10,"},
+	},
+	{
+		fn: `
+		func $(s string) int {
+			return len(s) / (4097 >> 1)
+		}
+		`,
+		pos: []string{"\tSHRL\t\\$11,"},
+	},
+	{
+		fn: `
+		func $(a []int) int {
+			return cap(a) / ((1 << 11) + 2048)
+		}
+		`,
+		pos: []string{"\tSHRL\t\\$12,"},
+	},
+	// Check that len() and cap() mod by a constant power of two
+	// are compiled into ANDL.
+	{
+		fn: `
+		func $(a []int) int {
+			return len(a) % 1024
+		}
+		`,
+		pos: []string{"\tANDL\t\\$1023,"},
+	},
+	{
+		fn: `
+		func $(s string) int {
+			return len(s) % (4097 >> 1)
+		}
+		`,
+		pos: []string{"\tANDL\t\\$2047,"},
+	},
+	{
+		fn: `
+		func $(a []int) int {
+			return cap(a) % ((1 << 11) + 2048)
+		}
+		`,
+		pos: []string{"\tANDL\t\\$4095,"},
+	},
 }
 
 var linuxS390XTests = []*asmTest{