internal/godebug: add bisect support

CL 491875 introduces a new bisect command, which we plan to
document for use by end users to debug semantic changes in
the compiler and in GODEBUGs.

This CL adds bisect support to GODEBUGs, at least the ones
used via internal/godebug. Support for runtime-internal
GODEBUGs like panicnil will take a bit more work in followup CLs.

The new API in internal/bisect to support stack-based bisecting
should be easily reusable in non-GODEBUG settings as well,
once we finalize and export the API.

Change-Id: I6cf779c775329aceb3f3b2b2b2f221ce8a67deee
Reviewed-on: https://go-review.googlesource.com/c/go/+/491975
Reviewed-by: David Chase <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Russ Cox <[email protected]>

Author: rsc

src/cmd/compile/internal/base/hashdebug.go

@@ -296,7 +296,7 @@ func (d *HashDebug) DebugHashMatchParam(pkgAndName string, param uint64) bool {
 
 	hash := hashOf(pkgAndName, param)
 	if d.bisect != nil {
-		if d.bisect.ShouldReport(hash) {
+		if d.bisect.ShouldPrint(hash) {
 			d.logDebugHashMatch(d.name, pkgAndName, hash, param)
 		}
 		return d.bisect.ShouldEnable(hash)
@@ -324,14 +324,14 @@ func (d *HashDebug) DebugHashMatchPos(pos src.XPos) bool {
 func (d *HashDebug) debugHashMatchPos(ctxt *obj.Link, pos src.XPos) bool {
 	// TODO: When we remove the old d.match code, we can use
 	// d.bisect.Hash instead of the locked buffer, and we can
-	// use d.bisect.Visible to decide whether to format a string.
+	// use d.bisect.MarkerOnly to decide whether to format a string.
 	d.mu.Lock()
 	defer d.mu.Unlock()
 
 	b := d.bytesForPos(ctxt, pos)
 	hash := hashOfBytes(b, 0)
 	if d.bisect != nil {
-		if d.bisect.ShouldReport(hash) {
+		if d.bisect.ShouldPrint(hash) {
 			d.logDebugHashMatchLocked(d.name, string(b), hash, 0)
 		}
 		return d.bisect.ShouldEnable(hash)

src/go/build/deps_test.go

@@ -68,6 +68,7 @@ var depsRules = `
 	< sync/atomic
 	< internal/race
 	< sync
+	< internal/bisect
 	< internal/godebug
 	< internal/reflectlite
 	< errors