runtime: switch debuglog from const-toggled to type-toggled

Currently, the debuglog build tag controls the dlogEnabled const, and
all methods of dlogger first check this const and immediately return
if dlog is not enabled. With constant folding and inlining, this makes
the whole dlog implementation compile away if it's not enabled.

However, we want to be able to test debuglog even when the build tag
isn't set. For that to work, we need a different mechanism.

This CL changes this mechanism so the debuglog build tag instead
controls the type alias for dlogger to be either dloggerImpl or
dloggerFake. These two types have the same method set, but one is just
stubs. This way, the methods of dloggerImpl don't need to be
conditional dlogEnabled, which sets us up to use the now
fully-functional dloggerImpl type in the test.

I confirmed that this change has no effect on the final size of the
cmd/go binary. It does increase the size of the runtime.a file by 0.9%
and make the runtime take ever so slightly longer to compile because
the compiler can no longer simply eliminate the bodies of the all of
dlogger during early deadcode. However, this all gets eliminated by
the linker. I consider this worth it to always get build and test
coverage of debuglog.

Change-Id: I81759e9e1411b7d369a23383a18b022ab7451421
Reviewed-on: https://go-review.googlesource.com/c/go/+/600696
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/debuglog.go

@@ -12,6 +12,16 @@
 //
 // This facility can be enabled by passing -tags debuglog when
 // building. Without this tag, dlog calls compile to nothing.
+//
+// Implementation notes
+//
+// There are two implementations of the dlog interface: dloggerImpl and
+// dloggerFake. dloggerFake is a no-op implementation. dlogger is type-aliased
+// to one or the other depending on the debuglog build tag. However, both types
+// always exist and are always built. This helps ensure we compile as much of
+// the implementation as possible in the default build configuration, while also
+// enabling us to achieve good test coverage of the real debuglog implementation
+// even when the debuglog build tag is not set.
 
 package runtime
 
@@ -31,8 +41,6 @@ const debugLogBytes = 16 << 10
 // Above this, the string will be truncated with "..(n more bytes).."
 const debugLogStringLimit = debugLogBytes / 8
 
-type dlogger = dloggerImpl
-
 // dlog returns a debug logger. The caller can use methods on the
 // returned logger to add values, which will be space-separated in the
 // final output, much like println. The caller must call end() to
@@ -50,11 +58,20 @@ type dlogger = dloggerImpl
 //
 //go:nosplit
 //go:nowritebarrierrec
-func dlog() *dloggerImpl {
-	if !dlogEnabled {
-		return nil
-	}
+func dlog() dlogger {
+	// dlog1 is defined to either dlogImpl or dlogFake.
+	return dlog1()
+}
 
+//go:nosplit
+//go:nowritebarrierrec
+func dlogFake() dloggerFake {
+	return dloggerFake{}
+}
+
+//go:nosplit
+//go:nowritebarrierrec
+func dlogImpl() *dloggerImpl {
 	// Get the time.
 	tick, nano := uint64(cputicks()), uint64(nanotime())
 
@@ -142,12 +159,14 @@ type dloggerImpl struct {
 // so it doesn't need to protect against ABA races.
 var allDloggers *dloggerImpl
 
+// A dloggerFake is a no-op implementation of dlogger.
+type dloggerFake struct{}
+
+//go:nosplit
+func (l dloggerFake) end() {}
+
 //go:nosplit
 func (l *dloggerImpl) end() {
-	if !dlogEnabled {
-		return
-	}
-
 	// Fill in framing header.
 	size := l.w.write - l.w.r.end
 	if !l.w.writeFrameAt(l.w.r.end, size) {
@@ -182,11 +201,11 @@ const (
 	debugLogTraceback
 )
 
+//go:nosplit
+func (l dloggerFake) b(x bool) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) b(x bool) *dloggerImpl {
-	if !dlogEnabled {
-		return l
-	}
 	if x {
 		l.w.byte(debugLogBoolTrue)
 	} else {
@@ -195,86 +214,113 @@ func (l *dloggerImpl) b(x bool) *dloggerImpl {
 	return l
 }
 
+//go:nosplit
+func (l dloggerFake) i(x int) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) i(x int) *dloggerImpl {
 	return l.i64(int64(x))
 }
 
+//go:nosplit
+func (l dloggerFake) i8(x int8) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) i8(x int8) *dloggerImpl {
 	return l.i64(int64(x))
 }
 
+//go:nosplit
+func (l dloggerFake) i16(x int16) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) i16(x int16) *dloggerImpl {
 	return l.i64(int64(x))
 }
 
+//go:nosplit
+func (l dloggerFake) i32(x int32) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) i32(x int32) *dloggerImpl {
 	return l.i64(int64(x))
 }
 
+//go:nosplit
+func (l dloggerFake) i64(x int64) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) i64(x int64) *dloggerImpl {
-	if !dlogEnabled {
-		return l
-	}
 	l.w.byte(debugLogInt)
 	l.w.varint(x)
 	return l
 }
 
+//go:nosplit
+func (l dloggerFake) u(x uint) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) u(x uint) *dloggerImpl {
 	return l.u64(uint64(x))
 }
 
+//go:nosplit
+func (l dloggerFake) uptr(x uintptr) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) uptr(x uintptr) *dloggerImpl {
 	return l.u64(uint64(x))
 }
 
+//go:nosplit
+func (l dloggerFake) u8(x uint8) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) u8(x uint8) *dloggerImpl {
 	return l.u64(uint64(x))
 }
 
+//go:nosplit
+func (l dloggerFake) u16(x uint16) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) u16(x uint16) *dloggerImpl {
 	return l.u64(uint64(x))
 }
 
+//go:nosplit
+func (l dloggerFake) u32(x uint32) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) u32(x uint32) *dloggerImpl {
 	return l.u64(uint64(x))
 }
 
+//go:nosplit
+func (l dloggerFake) u64(x uint64) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) u64(x uint64) *dloggerImpl {
-	if !dlogEnabled {
-		return l
-	}
 	l.w.byte(debugLogUint)
 	l.w.uvarint(x)
 	return l
 }
 
+//go:nosplit
+func (l dloggerFake) hex(x uint64) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) hex(x uint64) *dloggerImpl {
-	if !dlogEnabled {
-		return l
-	}
 	l.w.byte(debugLogHex)
 	l.w.uvarint(x)
 	return l
 }
 
+//go:nosplit
+func (l dloggerFake) p(x any) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) p(x any) *dloggerImpl {
-	if !dlogEnabled {
-		return l
-	}
 	l.w.byte(debugLogPtr)
 	if x == nil {
 		l.w.uvarint(0)
@@ -291,11 +337,10 @@ func (l *dloggerImpl) p(x any) *dloggerImpl {
 }
 
 //go:nosplit
-func (l *dloggerImpl) s(x string) *dloggerImpl {
-	if !dlogEnabled {
-		return l
-	}
+func (l dloggerFake) s(x string) dloggerFake { return l }
 
+//go:nosplit
+func (l *dloggerImpl) s(x string) *dloggerImpl {
 	strData := unsafe.StringData(x)
 	datap := &firstmoduledata
 	if len(x) > 4 && datap.etext <= uintptr(unsafe.Pointer(strData)) && uintptr(unsafe.Pointer(strData)) < datap.end {
@@ -326,21 +371,21 @@ func (l *dloggerImpl) s(x string) *dloggerImpl {
 	return l
 }
 
+//go:nosplit
+func (l dloggerFake) pc(x uintptr) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) pc(x uintptr) *dloggerImpl {
-	if !dlogEnabled {
-		return l
-	}
 	l.w.byte(debugLogPC)
 	l.w.uvarint(uint64(x))
 	return l
 }
 
+//go:nosplit
+func (l dloggerFake) traceback(x []uintptr) dloggerFake { return l }
+
 //go:nosplit
 func (l *dloggerImpl) traceback(x []uintptr) *dloggerImpl {
-	if !dlogEnabled {
-		return l
-	}
 	l.w.byte(debugLogTraceback)
 	l.w.uvarint(uint64(len(x)))
 	for _, pc := range x {

src/runtime/debuglog_off.go

@@ -8,6 +8,12 @@ package runtime
 
 const dlogEnabled = false
 
+type dlogger = dloggerFake
+
+func dlog1() dloggerFake {
+	return dlogFake()
+}
+
 type dlogPerM struct{}
 
 func getCachedDlogger() *dloggerImpl {

src/runtime/debuglog_on.go

@@ -8,6 +8,17 @@ package runtime
 
 const dlogEnabled = true
 
+// dlogger is the underlying implementation of the dlogger interface, selected
+// at build time.
+//
+// We use a type alias instead of struct embedding so that the dlogger type is
+// identical to the type returned by method chaining on the methods of this type.
+type dlogger = *dloggerImpl
+
+func dlog1() *dloggerImpl {
+	return dlogImpl()
+}
+
 // dlogPerM is the per-M debug log data. This is embedded in the m
 // struct.
 type dlogPerM struct {

src/runtime/export_debuglog_test.go

@@ -12,7 +12,11 @@ const DebugLogBytes = debugLogBytes
 
 const DebugLogStringLimit = debugLogStringLimit
 
-var Dlog = dlog
+type Dlogger = dloggerImpl
+
+func Dlog() *Dlogger {
+	return dlogImpl()
+}
 
 func (l *dloggerImpl) End()                      { l.end() }
 func (l *dloggerImpl) B(x bool) *dloggerImpl     { return l.b(x) }