cmd/compile: enhance debug_test for infinite loops

ssa/debug_test.go already had a step limit; this exposes
it to individual tests, and it is then set low for the
infinite loop tests.

That however is not enough; in an infinite loop debuggers
see an unchanging line number, and therefore keep trying
until they see a different one.  To do this, the concept
of a "bogus" line number is introduced, and on output
single-instruction infinite loops are detected and a
hardware nop with correct line number is inserted into
the loop; the branch itself receives a bogus line number.

This breaks up the endless stream of same line number and
causes both gdb and delve to not hang; Delve complains
about the incorrect line number while gdb does
a sort of odd step-to-nowhere that then steps back
to the loop.  Since repeats are suppressed in the reference
file, a single line is shown there.

(The wrong line number mentioned in previous message
was an artifact of debug_test.go, not Delve, and is now
fixed.)

The bogus line number exposed in Delve is less than
wonderful, but compared to hanging, it is better.

Fixes #30664.

Change-Id: I30c927cf8869a84c6c9b84033ee44d7044aab552
Reviewed-on: https://go-review.googlesource.com/c/go/+/168477
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -5315,6 +5315,12 @@ func genssa(f *ssa.Func, pp *Progs) {
 				}
 			}
 		}
+		// If this is an empty infinite loop, stick a hardware NOP in there so that debuggers are less confused.
+		if s.bstart[b.ID] == s.pp.next && len(b.Succs) == 1 && b.Succs[0].Block() == b {
+			p := thearch.Ginsnop(s.pp)
+			p.Pos = p.Pos.WithIsStmt()
+			b.Pos = b.Pos.WithBogusLine() // Debuggers are not good about infinite loops, force a change in line number
+		}
 		// Emit control flow instructions for block
 		var next *ssa.Block
 		if i < len(f.Blocks)-1 && Debug['N'] == 0 {

src/cmd/compile/internal/ssa/debug_test.go

@@ -156,33 +156,34 @@ func TestNexting(t *testing.T) {
 
 	subTest(t, debugger+"-dbg-race", "i22600", dbgFlags, append(moreargs, "-race")...)
 
-	optSubTest(t, debugger+"-opt", "hist", optFlags, moreargs...)
-	optSubTest(t, debugger+"-opt", "scopes", optFlags, moreargs...)
+	optSubTest(t, debugger+"-opt", "hist", optFlags, 1000, moreargs...)
+	optSubTest(t, debugger+"-opt", "scopes", optFlags, 1000, moreargs...)
+	optSubTest(t, debugger+"-opt", "infloop", optFlags, 10, moreargs...)
 }
 
 // subTest creates a subtest that compiles basename.go with the specified gcflags and additional compiler arguments,
 // then runs the debugger on the resulting binary, with any comment-specified actions matching tag triggered.
 func subTest(t *testing.T, tag string, basename string, gcflags string, moreargs ...string) {
 	t.Run(tag+"-"+basename, func(t *testing.T) {
-		testNexting(t, basename, tag, gcflags, moreargs...)
+		testNexting(t, basename, tag, gcflags, 1000, moreargs...)
 	})
 }
 
 // optSubTest is the same as subTest except that it skips the test if the runtime and libraries
 // were not compiled with optimization turned on.  (The skip may not be necessary with Go 1.10 and later)
-func optSubTest(t *testing.T, tag string, basename string, gcflags string, moreargs ...string) {
+func optSubTest(t *testing.T, tag string, basename string, gcflags string, count int, moreargs ...string) {
 	// If optimized test is run with unoptimized libraries (compiled with -N -l), it is very likely to fail.
 	// This occurs in the noopt builders (for example).
 	t.Run(tag+"-"+basename, func(t *testing.T) {
 		if *force || optimizedLibs {
-			testNexting(t, basename, tag, gcflags, moreargs...)
+			testNexting(t, basename, tag, gcflags, count, moreargs...)
 		} else {
 			t.Skip("skipping for unoptimized stdlib/runtime")
 		}
 	})
 }
 
-func testNexting(t *testing.T, base, tag, gcflags string, moreArgs ...string) {
+func testNexting(t *testing.T, base, tag, gcflags string, count int, moreArgs ...string) {
 	// (1) In testdata, build sample.go into test-sample.<tag>
 	// (2) Run debugger gathering a history
 	// (3) Read expected history from testdata/sample.<tag>.nexts
@@ -219,7 +220,7 @@ func testNexting(t *testing.T, base, tag, gcflags string, moreArgs ...string) {
 	} else {
 		dbg = newGdb(tag, exe)
 	}
-	h1 := runDbgr(dbg, 1000)
+	h1 := runDbgr(dbg, count)
 	if *dryrun {
 		fmt.Printf("# Tag for above is %s\n", dbg.tag())
 		return
@@ -261,6 +262,7 @@ func runDbgr(dbg dbgr, maxNext int) *nextHist {
 			break
 		}
 	}
+	dbg.quit()
 	h := dbg.hist()
 	return h
 }
@@ -298,7 +300,7 @@ func (t tstring) String() string {
 }
 
 type pos struct {
-	line uint16
+	line uint32
 	file uint8 // Artifact of plans to implement differencing instead of calling out to diff.
 }
 
@@ -386,7 +388,7 @@ func (h *nextHist) add(file, line, text string) bool {
 		}
 	}
 	l := len(h.ps)
-	p := pos{line: uint16(li), file: fi}
+	p := pos{line: uint32(li), file: fi}
 
 	if l == 0 || *repeats || h.ps[l-1] != p {
 		h.ps = append(h.ps, p)
@@ -721,6 +723,15 @@ func varsToPrint(line, lookfor string) []string {
 func (s *gdbState) quit() {
 	response := s.ioState.writeRead("q\n")
 	if strings.Contains(response.o, "Quit anyway? (y or n)") {
+		defer func() {
+			if r := recover(); r != nil {
+				if s, ok := r.(string); !(ok && strings.Contains(s, "'Y\n'")) {
+					// Not the panic that was expected.
+					fmt.Printf("Expected a broken pipe panic, but saw the following panic instead")
+					panic(r)
+				}
+			}
+		}()
 		s.ioState.writeRead("Y\n")
 	}
 }

src/cmd/compile/internal/ssa/testdata/infloop.dlv-opt.nexts

+  ./testdata/infloop.go
+6:	func test() {
+8:		go func() {}()
+10:		for {
+1048575:
+10:		for {
+1048575:
+10:		for {
+1048575:
+10:		for {
+1048575:
+10:		for {

src/cmd/compile/internal/ssa/testdata/infloop.gdb-opt.nexts

+  src/cmd/compile/internal/ssa/testdata/infloop.go
+6:	func test() {
+8:		go func() {}()
+10:		for {

src/cmd/compile/internal/ssa/testdata/infloop.go

+package main
+
+var sink int
+
+//go:noinline
+func test() {
+	// This is for #30167, incorrect line numbers in an infinite loop
+	go func() {}()
+
+	for {
+	}
+}
+
+func main() {
+	test()
+}

src/cmd/internal/src/pos.go

@@ -304,7 +304,8 @@ type lico uint32
 // TODO: Prologue and epilogue are perhaps better handled as psuedoops for the assembler,
 // because they have almost no interaction with other uses of the position.
 const (
-	lineBits, lineMax     = 20, 1<<lineBits - 1
+	lineBits, lineMax     = 20, 1<<lineBits - 2
+	bogusLine             = 1<<lineBits - 1 // Not a line number; used to disruopt infinite loops
 	isStmtBits, isStmtMax = 2, 1<<isStmtBits - 1
 	xlogueBits, xlogueMax = 2, 1<<xlogueBits - 1
 	colBits, colMax       = 32 - lineBits - xlogueBits - isStmtBits, 1<<colBits - 1
@@ -355,6 +356,16 @@ const (
 	PosEpilogueBegin
 )
 
+func makeLicoRaw(line, col uint) lico {
+	return lico(line<<lineShift | col<<colShift)
+}
+
+// This is a not-position that will not be elided.
+// Depending on the debugger (gdb or delve) it may or may not be displayed.
+func makeBogusLico() lico {
+	return makeLicoRaw(bogusLine, 0).withIsStmt()
+}
+
 func makeLico(line, col uint) lico {
 	if line > lineMax {
 		// cannot represent line, use max. line so we have some information
@@ -365,7 +376,7 @@ func makeLico(line, col uint) lico {
 		col = colMax
 	}
 	// default is not-sure-if-statement
-	return lico(line<<lineShift | col<<colShift)
+	return makeLicoRaw(line, col)
 }
 
 func (x lico) Line() uint { return uint(x) >> lineShift }

src/cmd/internal/src/xpos.go

@@ -60,6 +60,16 @@ func (p XPos) WithIsStmt() XPos {
 	return p
 }
 
+// WithBogusLine returns a bogus line that won't match any recorded for the source code.
+// Its use is to disrupt the statements within an infinite loop so that the debugger
+// will not itself loop infinitely waiting for the line number to change.
+// gdb chooses not to display the bogus line; delve shows it with a complaint, but the
+// alternative behavior is to hang.
+func (p XPos) WithBogusLine() XPos {
+	p.lico = makeBogusLico()
+	return p
+}
+
 // WithXlogue returns the same location but marked with DWARF function prologue/epilogue
 func (p XPos) WithXlogue(x PosXlogue) XPos {
 	p.lico = p.lico.withXlogue(x)