strings: fix Contains on amd64

The 17-31 byte code is broken.  Disabled it.

Added a bunch of tests to at least cover the cases
in indexShortStr.  I'll channel Brad and wonder why
this CL ever got in without any tests.

Fixes #15679

Change-Id: I84a7b283a74107db865b9586c955dcf5f2d60161
Reviewed-on: https://go-review.googlesource.com/23106
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/asm_amd64.s

@@ -1800,6 +1800,7 @@ loop16:
 	CMPQ DI,DX
 	JB loop16
 	JMP fail
+//TODO: the code below is wrong.  Fix it.  See #15679.
 _17_to_31:
 	LEAQ 1(DI)(DX*1), DX
 	SUBQ AX, DX

src/strings/strings_amd64.go

@@ -7,7 +7,7 @@ package strings
 // indexShortStr returns the index of the first instance of c in s, or -1 if c is not present in s.
 // indexShortStr requires 2 <= len(c) <= shortStringLen
 func indexShortStr(s, c string) int // ../runtime/asm_$GOARCH.s
-const shortStringLen = 31
+const shortStringLen = 16           // TODO: restore to 31 when #15679 is fixed
 
 // Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
 func Index(s, sep string) int {

src/strings/strings_test.go

@@ -1036,6 +1036,70 @@ var ContainsTests = []struct {
 	{"abc", "bcd", false},
 	{"abc", "", true},
 	{"", "a", false},
+
+	// cases to cover code in runtime/asm_amd64.s:indexShortStr
+	// 2-byte needle
+	{"xxxxxx", "01", false},
+	{"01xxxx", "01", true},
+	{"xx01xx", "01", true},
+	{"xxxx01", "01", true},
+	{"01xxxxx"[1:], "01", false},
+	{"xxxxx01"[:6], "01", false},
+	// 3-byte needle
+	{"xxxxxxx", "012", false},
+	{"012xxxx", "012", true},
+	{"xx012xx", "012", true},
+	{"xxxx012", "012", true},
+	{"012xxxxx"[1:], "012", false},
+	{"xxxxx012"[:7], "012", false},
+	// 4-byte needle
+	{"xxxxxxxx", "0123", false},
+	{"0123xxxx", "0123", true},
+	{"xx0123xx", "0123", true},
+	{"xxxx0123", "0123", true},
+	{"0123xxxxx"[1:], "0123", false},
+	{"xxxxx0123"[:8], "0123", false},
+	// 5-7-byte needle
+	{"xxxxxxxxx", "01234", false},
+	{"01234xxxx", "01234", true},
+	{"xx01234xx", "01234", true},
+	{"xxxx01234", "01234", true},
+	{"01234xxxxx"[1:], "01234", false},
+	{"xxxxx01234"[:9], "01234", false},
+	// 8-byte needle
+	{"xxxxxxxxxxxx", "01234567", false},
+	{"01234567xxxx", "01234567", true},
+	{"xx01234567xx", "01234567", true},
+	{"xxxx01234567", "01234567", true},
+	{"01234567xxxxx"[1:], "01234567", false},
+	{"xxxxx01234567"[:12], "01234567", false},
+	// 9-15-byte needle
+	{"xxxxxxxxxxxxx", "012345678", false},
+	{"012345678xxxx", "012345678", true},
+	{"xx012345678xx", "012345678", true},
+	{"xxxx012345678", "012345678", true},
+	{"012345678xxxxx"[1:], "012345678", false},
+	{"xxxxx012345678"[:13], "012345678", false},
+	// 16-byte needle
+	{"xxxxxxxxxxxxxxxxxxxx", "0123456789ABCDEF", false},
+	{"0123456789ABCDEFxxxx", "0123456789ABCDEF", true},
+	{"xx0123456789ABCDEFxx", "0123456789ABCDEF", true},
+	{"xxxx0123456789ABCDEF", "0123456789ABCDEF", true},
+	{"0123456789ABCDEFxxxxx"[1:], "0123456789ABCDEF", false},
+	{"xxxxx0123456789ABCDEF"[:20], "0123456789ABCDEF", false},
+	// 17-31-byte needle
+	{"xxxxxxxxxxxxxxxxxxxxx", "0123456789ABCDEFG", false},
+	{"0123456789ABCDEFGxxxx", "0123456789ABCDEFG", true},
+	{"xx0123456789ABCDEFGxx", "0123456789ABCDEFG", true},
+	{"xxxx0123456789ABCDEFG", "0123456789ABCDEFG", true},
+	{"0123456789ABCDEFGxxxxx"[1:], "0123456789ABCDEFG", false},
+	{"xxxxx0123456789ABCDEFG"[:21], "0123456789ABCDEFG", false},
+
+	// partial match cases
+	{"xx01x", "012", false},                             // 3
+	{"xx0123x", "01234", false},                         // 5-7
+	{"xx01234567x", "012345678", false},                 // 9-15
+	{"xx0123456789ABCDEFx", "0123456789ABCDEFG", false}, // 17-31, issue 15679
 }
 
 func TestContains(t *testing.T) {