cmd/compile: tighten for huge functions in -N mode

Currently, in -N mode we skip the tighten pass. However, for very
large functions, many values live across blocks can cause
pathological behavior in the register allocator, which could use
a huge amount of memory or cause the program to hang. For
functions that large, debugging using a debugger is unlikely to be
very useful (the function is probably generated anyway). So we do
a little optimization to make fewer values live across blocks and
make it easier for the compiler.

Fixes #52180.

Change-Id: I355fe31bb87ea5d0870bb52dd06405dd5d791dab
Reviewed-on: https://go-review.googlesource.com/c/go/+/475339
Reviewed-by: Cuong Manh Le <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Cherry Mui <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: cherrymui

src/cmd/compile/internal/ssa/compile.go

@@ -496,7 +496,7 @@ var passes = [...]pass{
 	{name: "checkLower", fn: checkLower, required: true},
 	{name: "late phielim", fn: phielim},
 	{name: "late copyelim", fn: copyelim},
-	{name: "tighten", fn: tighten}, // move values closer to their uses
+	{name: "tighten", fn: tighten, required: true}, // move values closer to their uses
 	{name: "late deadcode", fn: deadcode},
 	{name: "critical", fn: critical, required: true}, // remove critical edges
 	{name: "phi tighten", fn: phiTighten},            // place rematerializable phi args near uses to reduce value lifetimes

src/cmd/compile/internal/ssa/tighten.go

@@ -4,12 +4,21 @@
 
 package ssa
 
+import "cmd/compile/internal/base"
+
 // tighten moves Values closer to the Blocks in which they are used.
 // This can reduce the amount of register spilling required,
 // if it doesn't also create more live values.
 // A Value can be moved to any block that
 // dominates all blocks in which it is used.
 func tighten(f *Func) {
+	if base.Flag.N != 0 && len(f.Blocks) < 10000 {
+		// Skip the optimization in -N mode, except for huge functions.
+		// Too many values live across blocks can cause pathological
+		// behavior in the register allocator (see issue 52180).
+		return
+	}
+
 	canMove := f.Cache.allocBoolSlice(f.NumValues())
 	defer f.Cache.freeBoolSlice(canMove)
 	for _, b := range f.Blocks {