cmd/compile/internal/escape: add hash for bisecting stack allocation of variable-sized makeslice

CL 653856 enabled stack allocation of variable-sized makeslice results.

This CL adds debug hashing of that change, plus a debug flag
to control the byte threshold used.

The debug hashing machinery means we also now have a way to disable just
the CL 653856 optimization by doing -gcflags='all=-d=variablemakehash=n'
or similar, though the stderr output will then typically have many
lines of debug hash output.

Using this CL plus the bisect command, I was able to retroactively
find one of the lines of code responsible for #73199:

  $ bisect -compile=variablemake go test -skip TestListWireGuardDrivers
  [...]
  bisect: FOUND failing change set
  --- change set #1 (enabling changes causes failure)
  ./security_windows.go:1321:38 (variablemake)
  ./security_windows.go:1321:38 (variablemake)
  ---

Previously, I had tracked down those lines by diffing '-gcflags=-m=1'
output and brief code inspection, but seeing the bisect was very nice.

This CL also adds a compiler debug flag to control the threshold for
stack allocation of variably sized make results. This can help
us identify more code that is relying on certain stack allocations.
This might be a temporary flag that we delete prior to Go 1.25
(given we would not want people to rely on it), or maybe it
might make sense to keep it for some period of time beyond the release
of Go 1.25 to help the ecosystem shake out other bugs.

Using these two flags together (and picking a threshold of 64 rather
than the default of 32), it looks for example like this
x/sys/windows code might be relying on stack allocation of
a byte slice:

  $ bisect -compile=variablemake go test -gcflags=-d=variablemakethreshold=64 -skip TestListWireGuardDrivers
  [...]
  bisect: FOUND failing change set
  --- change set #1 (enabling changes causes failure)
  ./syscall_windows_test.go:1178:16 (variablemake)

Updates #73199
Fixes #73253

Change-Id: I160179a0e3c148c3ea86be5c9b6cea8a52c3e5b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/663795
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: thepudds

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

@@ -72,6 +72,8 @@ type DebugFlags struct {
 	PGOInlineBudget       int    `help:"inline budget for hot functions" concurrent:"ok"`
 	PGODevirtualize       int    `help:"enable profile-guided devirtualization; 0 to disable, 1 to enable interface devirtualization, 2 to enable function devirtualization" concurrent:"ok"`
 	RangeFuncCheck        int    `help:"insert code to check behavior of range iterator functions" concurrent:"ok"`
+	VariableMakeHash      string `help:"hash value for debugging stack allocation of variable-sized make results" concurrent:"ok"`
+	VariableMakeThreshold int    `help:"threshold in bytes for possible stack allocation of variable-sized make results" concurrent:"ok"`
 	WrapGlobalMapDbg      int    `help:"debug trace output for global map init wrapping"`
 	WrapGlobalMapCtl      int    `help:"global map init wrap control (0 => default, 1 => off, 2 => stress mode, no size cutoff)"`
 	ZeroCopy              int    `help:"enable zero-copy string->[]byte conversions" concurrent:"ok"`

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

@@ -183,7 +183,8 @@ func ParseFlags() {
 	Debug.InlStaticInit = 1
 	Debug.PGOInline = 1
 	Debug.PGODevirtualize = 2
-	Debug.SyncFrames = -1 // disable sync markers by default
+	Debug.SyncFrames = -1            // disable sync markers by default
+	Debug.VariableMakeThreshold = 32 // 32 byte default for stack allocated make results
 	Debug.ZeroCopy = 1
 	Debug.RangeFuncCheck = 1
 	Debug.MergeLocals = 1
@@ -270,6 +271,9 @@ func ParseFlags() {
 	if Debug.MergeLocalsHash != "" {
 		MergeLocalsHash = NewHashDebug("mergelocals", Debug.MergeLocalsHash, nil)
 	}
+	if Debug.VariableMakeHash != "" {
+		VariableMakeHash = NewHashDebug("variablemake", Debug.VariableMakeHash, nil)
+	}
 
 	if Flag.MSan && !platform.MSanSupported(buildcfg.GOOS, buildcfg.GOARCH) {
 		log.Fatalf("%s/%s does not support -msan", buildcfg.GOOS, buildcfg.GOARCH)

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

@@ -53,10 +53,11 @@ func (d *HashDebug) SetInlineSuffixOnly(b bool) *HashDebug {
 // The default compiler-debugging HashDebug, for "-d=gossahash=..."
 var hashDebug *HashDebug
 
-var FmaHash *HashDebug         // for debugging fused-multiply-add floating point changes
-var LoopVarHash *HashDebug     // for debugging shared/private loop variable changes
-var PGOHash *HashDebug         // for debugging PGO optimization decisions
-var MergeLocalsHash *HashDebug // for debugging local stack slot merging changes
+var FmaHash *HashDebug          // for debugging fused-multiply-add floating point changes
+var LoopVarHash *HashDebug      // for debugging shared/private loop variable changes
+var PGOHash *HashDebug          // for debugging PGO optimization decisions
+var MergeLocalsHash *HashDebug  // for debugging local stack slot merging changes
+var VariableMakeHash *HashDebug // for debugging variable-sized make optimizations
 
 // DebugHashMatchPkgFunc reports whether debug variable Gossahash
 //

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

@@ -568,7 +568,8 @@ func walkMakeSlice(n *ir.MakeExpr, init *ir.Nodes) ir.Node {
 			// The conv is necessary in case n.Type is named.
 			return walkExpr(typecheck.Expr(typecheck.Conv(s, n.Type())), init)
 		}
-		tryStack = base.Flag.N == 0
+		// Check that this optimization is enabled in general and for this node.
+		tryStack = base.Flag.N == 0 && base.VariableMakeHash.MatchPos(n.Pos(), nil)
 	}
 
 	// The final result is assigned to this variable.
@@ -582,7 +583,7 @@ func walkMakeSlice(n *ir.MakeExpr, init *ir.Nodes) ir.Node {
 		// } else {
 		//     slice = makeslice(elemType, len, cap)
 		// }
-		const maxStackSize = 32
+		maxStackSize := int64(base.Debug.VariableMakeThreshold)
 		K := maxStackSize / t.Elem().Size() // rounds down
 		if K > 0 {                          // skip if elem size is too big.
 			nif := ir.NewIfStmt(base.Pos, ir.NewBinaryExpr(base.Pos, ir.OLE, typecheck.Conv(cap, types.Types[types.TUINT64]), ir.NewInt(base.Pos, K)), nil, nil)