cmd/compile: make deadcode pass cheaper

The deadcode pass runs a lot.
I'd like it to run even more.

This change adds dedicated storage for deadcode to ssa.Cache.
In addition to being a nice win now, it makes
deadcode easier to add other places in the future.

name        old time/op       new time/op       delta
Template          210ms ± 3%        209ms ± 2%    ~     (p=0.951 n=93+95)
Unicode          92.2ms ± 3%       93.0ms ± 3%  +0.87%  (p=0.000 n=94+94)
GoTypes           739ms ± 2%        733ms ± 2%  -0.84%  (p=0.000 n=92+94)
Compiler          3.51s ± 2%        3.49s ± 2%  -0.57%  (p=0.000 n=94+91)
SSA               9.80s ± 2%        9.75s ± 2%  -0.57%  (p=0.000 n=95+92)
Flate             132ms ± 2%        132ms ± 3%    ~     (p=0.165 n=94+98)
GoParser          160ms ± 3%        159ms ± 3%  -0.42%  (p=0.005 n=96+94)
Reflect           446ms ± 4%        442ms ± 4%  -0.91%  (p=0.000 n=95+98)
Tar               186ms ± 3%        186ms ± 2%    ~     (p=0.221 n=94+97)
XML               252ms ± 2%        250ms ± 2%  -0.55%  (p=0.000 n=95+94)
[Geo mean]        430ms             429ms       -0.34%

name        old user-time/op  new user-time/op  delta
Template          256ms ± 3%        257ms ± 3%    ~     (p=0.521 n=94+98)
Unicode           120ms ± 9%        121ms ± 9%    ~     (p=0.074 n=99+100)
GoTypes           935ms ± 3%        935ms ± 2%    ~     (p=0.574 n=82+96)
Compiler          4.56s ± 1%        4.55s ± 2%    ~     (p=0.247 n=88+90)
SSA               13.6s ± 2%        13.6s ± 1%    ~     (p=0.277 n=94+95)
Flate             155ms ± 3%        156ms ± 3%    ~     (p=0.181 n=95+100)
GoParser          193ms ± 8%        184ms ± 6%  -4.39%  (p=0.000 n=100+89)
Reflect           549ms ± 3%        552ms ± 3%  +0.45%  (p=0.036 n=94+96)
Tar               230ms ± 4%        230ms ± 4%    ~     (p=0.670 n=97+99)
XML               315ms ± 5%        309ms ±12%  -2.05%  (p=0.000 n=99+99)
[Geo mean]        540ms             538ms       -0.47%

name        old alloc/op      new alloc/op      delta
Template         40.3MB ± 0%       38.9MB ± 0%  -3.36%  (p=0.008 n=5+5)
Unicode          28.6MB ± 0%       28.4MB ± 0%  -0.90%  (p=0.008 n=5+5)
GoTypes           137MB ± 0%        132MB ± 0%  -3.65%  (p=0.008 n=5+5)
Compiler          637MB ± 0%        609MB ± 0%  -4.40%  (p=0.008 n=5+5)
SSA              2.19GB ± 0%       2.07GB ± 0%  -5.63%  (p=0.008 n=5+5)
Flate            25.0MB ± 0%       24.1MB ± 0%  -3.80%  (p=0.008 n=5+5)
GoParser         30.0MB ± 0%       29.1MB ± 0%  -3.17%  (p=0.008 n=5+5)
Reflect          87.1MB ± 0%       84.4MB ± 0%  -3.05%  (p=0.008 n=5+5)
Tar              37.3MB ± 0%       36.0MB ± 0%  -3.31%  (p=0.008 n=5+5)
XML              49.8MB ± 0%       48.0MB ± 0%  -3.69%  (p=0.008 n=5+5)
[Geo mean]       87.6MB            84.6MB       -3.50%

name        old allocs/op     new allocs/op     delta
Template           387k ± 0%         380k ± 0%  -1.76%  (p=0.008 n=5+5)
Unicode            342k ± 0%         341k ± 0%  -0.31%  (p=0.008 n=5+5)
GoTypes           1.39M ± 0%        1.37M ± 0%  -1.64%  (p=0.008 n=5+5)
Compiler          5.68M ± 0%        5.60M ± 0%  -1.41%  (p=0.008 n=5+5)
SSA               17.1M ± 0%        16.8M ± 0%  -1.49%  (p=0.008 n=5+5)
Flate              240k ± 0%         236k ± 0%  -1.99%  (p=0.008 n=5+5)
GoParser           309k ± 0%         304k ± 0%  -1.57%  (p=0.008 n=5+5)
Reflect           1.01M ± 0%        0.99M ± 0%  -2.69%  (p=0.008 n=5+5)
Tar                360k ± 0%         353k ± 0%  -1.91%  (p=0.008 n=5+5)
XML                447k ± 0%         441k ± 0%  -1.26%  (p=0.008 n=5+5)
[Geo mean]         858k              844k       -1.60%

Fixes #15306

Change-Id: I9f558adb911efddead3865542fe2ca71f66fe1da
Reviewed-on: https://go-review.googlesource.com/c/go/+/166718
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -25,6 +25,13 @@ type Cache struct {
 	scrSparseSet  []*sparseSet // scratch sparse sets to be re-used.
 	scrSparseMap  []*sparseMap // scratch sparse maps to be re-used.
 	scrPoset      []*poset     // scratch poset to be reused
+	// deadcode contains reusable slices specifically for the deadcode pass.
+	// It gets special treatment because of the frequency with which it is run.
+	deadcode struct {
+		liveOrderStmts []*Value
+		live           []bool
+		q              []*Value
+	}
 
 	ValueToProgAfter []*obj.Prog
 	debugState       debugState
@@ -49,4 +56,18 @@ func (c *Cache) Reset() {
 		xl[i] = nil
 	}
 
+	// liveOrderStmts gets used multiple times during compilation of a function.
+	// We don't know where the high water mark was, so reslice to cap and search.
+	c.deadcode.liveOrderStmts = c.deadcode.liveOrderStmts[:cap(c.deadcode.liveOrderStmts)]
+	no := sort.Search(len(c.deadcode.liveOrderStmts), func(i int) bool { return c.deadcode.liveOrderStmts[i] == nil })
+	xo := c.deadcode.liveOrderStmts[:no]
+	for i := range xo {
+		xo[i] = nil
+	}
+	c.deadcode.q = c.deadcode.q[:cap(c.deadcode.q)]
+	nq := sort.Search(len(c.deadcode.q), func(i int) bool { return c.deadcode.q[i] == nil })
+	xq := c.deadcode.q[:nq]
+	for i := range xq {
+		xq[i] = nil
+	}
 }

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

@@ -9,9 +9,12 @@ import (
 )
 
 // findlive returns the reachable blocks and live values in f.
+// The caller should call f.retDeadcodeLive(live) when it is done with it.
 func findlive(f *Func) (reachable []bool, live []bool) {
 	reachable = ReachableBlocks(f)
-	live, _ = liveValues(f, reachable)
+	var order []*Value
+	live, order = liveValues(f, reachable)
+	f.retDeadcodeLiveOrderStmts(order)
 	return
 }
 
@@ -48,8 +51,21 @@ func ReachableBlocks(f *Func) []bool {
 // to be statements in reversed data flow order.
 // The second result is used to help conserve statement boundaries for debugging.
 // reachable is a map from block ID to whether the block is reachable.
+// The caller should call f.retDeadcodeLive(live) and f.retDeadcodeLiveOrderStmts(liveOrderStmts)
+// when they are done with the return values.
 func liveValues(f *Func, reachable []bool) (live []bool, liveOrderStmts []*Value) {
-	live = make([]bool, f.NumValues())
+	live = f.newDeadcodeLive()
+	if cap(live) < f.NumValues() {
+		live = make([]bool, f.NumValues())
+	} else {
+		live = live[:f.NumValues()]
+		for i := range live {
+			live[i] = false
+		}
+	}
+
+	liveOrderStmts = f.newDeadcodeLiveOrderStmts()
+	liveOrderStmts = liveOrderStmts[:0]
 
 	// After regalloc, consider all values to be live.
 	// See the comment at the top of regalloc.go and in deadcode for details.
@@ -61,7 +77,8 @@ func liveValues(f *Func, reachable []bool) (live []bool, liveOrderStmts []*Value
 	}
 
 	// Find all live values
-	q := make([]*Value, 0, 64) // stack-like worklist of unscanned values
+	q := f.Cache.deadcode.q[:0]
+	defer func() { f.Cache.deadcode.q = q }()
 
 	// Starting set: all control values of reachable blocks are live.
 	// Calls are live (because callee can observe the memory state).
@@ -163,6 +180,8 @@ func deadcode(f *Func) {
 
 	// Find live values.
 	live, order := liveValues(f, reachable)
+	defer f.retDeadcodeLive(live)
+	defer f.retDeadcodeLiveOrderStmts(order)
 
 	// Remove dead & duplicate entries from namedValues map.
 	s := f.newSparseSet(f.NumValues())

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

@@ -153,6 +153,33 @@ func (f *Func) retPoset(po *poset) {
 	f.Cache.scrPoset = append(f.Cache.scrPoset, po)
 }
 
+// newDeadcodeLive returns a slice for the
+// deadcode pass to use to indicate which values are live.
+func (f *Func) newDeadcodeLive() []bool {
+	r := f.Cache.deadcode.live
+	f.Cache.deadcode.live = nil
+	return r
+}
+
+// retDeadcodeLive returns a deadcode live value slice for re-use.
+func (f *Func) retDeadcodeLive(live []bool) {
+	f.Cache.deadcode.live = live
+}
+
+// newDeadcodeLiveOrderStmts returns a slice for the
+// deadcode pass to use to indicate which values
+// need special treatment for statement boundaries.
+func (f *Func) newDeadcodeLiveOrderStmts() []*Value {
+	r := f.Cache.deadcode.liveOrderStmts
+	f.Cache.deadcode.liveOrderStmts = nil
+	return r
+}
+
+// retDeadcodeLiveOrderStmts returns a deadcode liveOrderStmts slice for re-use.
+func (f *Func) retDeadcodeLiveOrderStmts(liveOrderStmts []*Value) {
+	f.Cache.deadcode.liveOrderStmts = liveOrderStmts
+}
+
 // newValue allocates a new Value with the given fields and places it at the end of b.Values.
 func (f *Func) newValue(op Op, t *types.Type, b *Block, pos src.XPos) *Value {
 	var v *Value

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

@@ -83,6 +83,7 @@ func (p stringFuncPrinter) named(n LocalSlot, vals []*Value) {
 
 func fprintFunc(p funcPrinter, f *Func) {
 	reachable, live := findlive(f)
+	defer f.retDeadcodeLive(live)
 	p.header(f)
 	printed := make([]bool, f.NumValues())
 	for _, b := range f.Blocks {