cmd/compile: eliminate stores to unread auto variables

This is a crude compiler pass to eliminate stores to auto variables
that are only ever written to.

Eliminates an unnecessary store to x from the following code:

func f() int {
	var x := 1
	return *(&x)
}

Fixes #19765.

Change-Id: If2c63a8ae67b8c590b6e0cc98a9610939a3eeffa
Reviewed-on: https://go-review.googlesource.com/38746
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -973,13 +973,23 @@ var linuxAMD64Tests = []*asmTest{
 		// make sure assembly output has matching offset and base register.
 		`
 		func f72(a, b int) int {
-			var x [16]byte // use some frame
-			_ = x
+			//go:noinline
+			func() {_, _ = a, b} () // use some frame
 			return b
 		}
 		`,
 		[]string{"b\\+40\\(SP\\)"},
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]0-8"},
+	},
 }
 
 var linux386Tests = []*asmTest{
@@ -1015,6 +1025,16 @@ var linux386Tests = []*asmTest{
 		}`,
 		[]string{"\tADDL\t[$]19", "\tIMULL"}, // (n+19)*a
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]0-4"},
+	},
 }
 
 var linuxS390XTests = []*asmTest{
@@ -1293,6 +1313,16 @@ var linuxS390XTests = []*asmTest{
 		`,
 		[]string{"\tFLOGR\t"},
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]0-8"},
+	},
 }
 
 var linuxARMTests = []*asmTest{
@@ -1404,13 +1434,23 @@ var linuxARMTests = []*asmTest{
 		// make sure assembly output has matching offset and base register.
 		`
 		func f13(a, b int) int {
-			var x [16]byte // use some frame
-			_ = x
+			//go:noinline
+			func() {_, _ = a, b} () // use some frame
 			return b
 		}
 		`,
 		[]string{"b\\+4\\(FP\\)"},
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]-4-4"},
+	},
 }
 
 var linuxARM64Tests = []*asmTest{
@@ -1584,6 +1624,16 @@ var linuxARM64Tests = []*asmTest{
 		`,
 		[]string{"\tMOVD\t\"\"\\.a\\+[0-9]+\\(FP\\), R[0-9]+", "\tMOVD\tR[0-9]+, \"\"\\.b\\+[0-9]+\\(FP\\)"},
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]-8-8"},
+	},
 }
 
 var linuxMIPSTests = []*asmTest{
@@ -1667,6 +1717,16 @@ var linuxMIPSTests = []*asmTest{
 		`,
 		[]string{"\tCLZ\t"},
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]-4-4"},
+	},
 }
 
 var linuxPPC64LETests = []*asmTest{
@@ -1751,6 +1811,16 @@ var linuxPPC64LETests = []*asmTest{
 		`,
 		[]string{"\tROTL\t"},
 	},
+	{
+		// check that stack store is optimized away
+		`
+		func $() int {
+			var x int
+			return *(&x)
+		}
+		`,
+		[]string{"TEXT\t.*, [$]0-8"},
+	},
 }
 
 // TestLineNumber checks to make sure the generated assembly has line numbers