cmd/compile: add -smallframes gc flag for GC latency diagnosis

Shrinks the size of things that can be stack allocated from
10M to 128k for declared variables and from 64k to 16k for
implicit allocations (new(T), &T{}, etc).

Usage: "go build -gcflags -smallframes hello.go"

An earlier GOEXPERIMENT version of this caused only one
problem, when a gc-should-detect-oversize-stack test no
longer had an oversized stack to detect.  The change was
converted to a flag to make it easier to access (for
diagnosing "long" GC-related single-thread pauses) and to
remove interference with the test.

Includes test to verify behavior.

Updates #27732.

Change-Id: I1255d484331e77185e07c78389a8b594041204c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/180817
Run-TryBot: David Chase <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

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

@@ -14,17 +14,21 @@ import (
 
 const (
 	BADWIDTH = types.BADWIDTH
+)
 
+var (
 	// maximum size variable which we will allocate on the stack.
 	// This limit is for explicit variable declarations like "var x T" or "x := ...".
-	maxStackVarSize = 10 * 1024 * 1024
+	// Note: the flag smallframes can update this value.
+	maxStackVarSize = int64(10 * 1024 * 1024)
 
 	// maximum size of implicit variables that we will allocate on the stack.
 	//   p := new(T)          allocating T on the stack
 	//   p := &T{}            allocating T on the stack
 	//   s := make([]T, n)    allocating [n]T on the stack
 	//   s := []byte("...")   allocating [n]byte on the stack
-	maxImplicitStackVarSize = 64 * 1024
+	// Note: the flag smallframes can update this value.
+	maxImplicitStackVarSize = int64(64 * 1024)
 )
 
 // isRuntimePkg reports whether p is package runtime.

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

@@ -190,6 +190,10 @@ func Main(archInit func(*Arch)) {
 	Nacl = objabi.GOOS == "nacl"
 	Wasm := objabi.GOARCH == "wasm"
 
+	// Whether the limit for stack-allocated objects is much smaller than normal.
+	// This can be helpful for diagnosing certain causes of GC latency. See #27732.
+	smallFrames := false
+
 	flag.BoolVar(&compiling_runtime, "+", false, "compiling runtime")
 	flag.BoolVar(&compiling_std, "std", false, "compiling standard library")
 	objabi.Flagcount("%", "debug non-static initializers", &Debug['%'])
@@ -261,13 +265,19 @@ func Main(archInit func(*Arch)) {
 	flag.StringVar(&mutexprofile, "mutexprofile", "", "write mutex profile to `file`")
 	flag.StringVar(&benchfile, "bench", "", "append benchmark times to `file`")
 	flag.BoolVar(&newescape, "newescape", true, "enable new escape analysis")
+	flag.BoolVar(&smallFrames, "smallframes", false, "reduce the size limit for stack allocated objects")
 	flag.BoolVar(&Ctxt.UseBASEntries, "dwarfbasentries", Ctxt.UseBASEntries, "use base address selection entries in DWARF")
 	objabi.Flagparse(usage)
 
 	// Record flags that affect the build result. (And don't
 	// record flags that don't, since that would cause spurious
 	// changes in the binary.)
-	recordFlags("B", "N", "l", "msan", "race", "shared", "dynlink", "dwarflocationlists", "newescape", "dwarfbasentries")
+	recordFlags("B", "N", "l", "msan", "race", "shared", "dynlink", "dwarflocationlists", "newescape", "dwarfbasentries", "smallFrames")
+
+	if smallFrames {
+		maxStackVarSize = 128 * 1024
+		maxImplicitStackVarSize = 16 * 1024
+	}
 
 	Ctxt.Flag_shared = flag_dynlink || flag_shared
 	Ctxt.Flag_dynlink = flag_dynlink

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

@@ -1393,7 +1393,7 @@ opswitch:
 			// Allocate a [n]byte of the right size.
 			t := types.NewArray(types.Types[TUINT8], int64(len(sc)))
 			var a *Node
-			if n.Esc == EscNone && len(sc) <= maxImplicitStackVarSize {
+			if n.Esc == EscNone && len(sc) <= int(maxImplicitStackVarSize) {
 				a = nod(OADDR, temp(t), nil)
 			} else {
 				a = callnew(t)

test/fixedbugs/issue27732a.go

@@ -0,0 +1,23 @@
+// errorcheck -0 -m -l -smallframes -newescape=true
+
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This checks that the -smallframes flag forces a large variable to heap.
+
+package main
+
+const (
+	bufferLen = 200000
+)
+
+type kbyte []byte
+type circularBuffer [bufferLen]kbyte
+
+var sink byte
+
+func main() {
+	var c circularBuffer // ERROR "moved to heap: c$"
+	sink = c[0][0]
+}