runtime: implement GODEBUG=inittrace=1 support

Setting inittrace=1 causes the runtime to emit a single line to standard error for
each package with init work, summarizing the execution time and memory allocation.

The emitted debug information for init functions can be used to find bottlenecks
or regressions in Go startup performance.

Packages with no init function work (user defined or compiler generated) are omitted.

Tracing plugin inits is not supported as they can execute concurrently. This would
make the implementation of tracing more complex while adding support for a very rare
use case. Plugin inits can be traced separately by testing a main package importing
the plugins package imports explicitly.

$ GODEBUG=inittrace=1 go test
init internal/bytealg @0.008 ms, 0 ms clock, 0 bytes, 0 allocs
init runtime @0.059 ms, 0.026 ms clock, 0 bytes, 0 allocs
init math @0.19 ms, 0.001 ms clock, 0 bytes, 0 allocs
init errors @0.22 ms, 0.004 ms clock, 0 bytes, 0 allocs
init strconv @0.24 ms, 0.002 ms clock, 32 bytes, 2 allocs
init sync @0.28 ms, 0.003 ms clock, 16 bytes, 1 allocs
init unicode @0.44 ms, 0.11 ms clock, 23328 bytes, 24 allocs
...

Inspired by stapelberg@google.com who instrumented doInit
in a prototype to measure init times with GDB.

Fixes #41378

Change-Id: Ic37c6a0cfc95488de9e737f5e346b8dbb39174e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/254659
Trust: Martin Möhrmann <moehrmann@google.com>
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 11 insertions, 2 deletions

diff --git a/src/runtime/runtime1.go b/src/runtime/runtime1.go
index 7c893aa25c..0f182ac58e 100644
--- a/src/runtime/runtime1.go
+++ b/src/runtime/runtime1.go
@@ -300,7 +300,6 @@ type dbgVar struct {
 // existing int var for that value, which may
 // already have an initial value.
 var debug struct {
-	allocfreetrace     int32
 	cgocheck           int32
 	clobberfree        int32
 	efence             int32
@@ -311,13 +310,20 @@ var debug struct {
 	gctrace            int32
 	invalidptr         int32
 	madvdontneed       int32 // for Linux; issue 28466
-	sbrk               int32
 	scavenge           int32
 	scavtrace          int32
 	scheddetail        int32
 	schedtrace         int32
 	tracebackancestors int32
 	asyncpreemptoff    int32
+
+	// debug.malloc is used as a combined debug check
+	// in the malloc function and should be set
+	// if any of the below debug options is != 0.
+	malloc         bool
+	allocfreetrace int32
+	inittrace      int32
+	sbrk           int32
 }
 
 var dbgvars = []dbgVar{
@@ -339,6 +345,7 @@ var dbgvars = []dbgVar{
 	{"schedtrace", &debug.schedtrace},
 	{"tracebackancestors", &debug.tracebackancestors},
 	{"asyncpreemptoff", &debug.asyncpreemptoff},
+	{"inittrace", &debug.inittrace},
 }
 
 func parsedebugvars() {
@@ -378,6 +385,8 @@ func parsedebugvars() {
 		}
 	}
 
+	debug.malloc = (debug.allocfreetrace | debug.inittrace | debug.sbrk) != 0
+
 	setTraceback(gogetenv("GOTRACEBACK"))
 	traceback_env = traceback_cache
 }