cmd/compile/internal/liveness: introduce "live intervals" utility

Introduce a helper type "Intervals" that contains sets of sorted
disjoint ranges corresponding to live ranges within a function.
Example: the Intervals set "{ [0,1), [4,10) }" would indicate that
something is live starting at instruction 0, then up to but not
including instruction 1, then dead from 1-3, then live again at
instruction 4 up to (but not including) instruction 10.

This patch provides APIs for constructing interval sets, testing to
see whether two sets overlap, and unioning/merging together two
intervals sets.

Updates #62737.
Updates #65532.
Updates #65495.

Change-Id: I7140a5989eba93bf3b8762d9224261f5eba0646d
Reviewed-on: https://go-review.googlesource.com/c/go/+/566177
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: thanm

src/cmd/compile/internal/liveness/intervals.go

@@ -0,0 +1,387 @@
+// Copyright 2024 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.
+
+package liveness
+
+// This file defines an "Intervals" helper type that stores a
+// sorted sequence of disjoint ranges or intervals. An Intervals
+// example: { [0,5) [9-12) [100,101) }, which corresponds to the
+// numbers 0-4, 9-11, and 100. Once an Intervals object is created, it
+// can be tested to see if it has any overlap with another Intervals
+// object, or it can be merged with another Intervals object to form a
+// union of the two.
+//
+// The intended use case for this helper is in describing object or
+// variable lifetime ranges within a linearized program representation
+// where each IR instruction has a slot or index. Example:
+//
+//          b1:
+//  0        VarDef abc
+//  1        memset(abc,0)
+//  2        VarDef xyz
+//  3        memset(xyz,0)
+//  4        abc.f1 = 2
+//  5        xyz.f3 = 9
+//  6        if q goto B4
+//  7 B3:    z = xyz.x
+//  8        goto B5
+//  9 B4:    z = abc.x
+//           // fallthrough
+// 10 B5:    z++
+//
+// To describe the lifetime of the variables above we might use these
+// intervals:
+//
+//    "abc"   [1,7), [9,10)
+//    "xyz"   [3,8)
+//
+// Clients can construct an Intervals object from a given IR sequence
+// using the "IntervalsBuilder" helper abstraction (one builder per
+// candidate variable), by making a
+// backwards sweep and invoking the Live/Kill methods to note the
+// starts and end of a given lifetime. For the example above, we would
+// expect to see this sequence of calls to Live/Kill:
+//
+//    abc:  Live(9), Kill(8), Live(6), Kill(0)
+//    xyz:  Live(8), Kill(2)
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+const debugtrace = false
+
+// Interval hols the range [st,en).
+type Interval struct {
+	st, en int
+}
+
+// Intervals is a sequence of sorted, disjoint intervals.
+type Intervals []Interval
+
+func (i Interval) String() string {
+	return fmt.Sprintf("[%d,%d)", i.st, i.en)
+}
+
+// TEMPORARY until bootstrap version catches up.
+func imin(i, j int) int {
+	if i < j {
+		return i
+	}
+	return j
+}
+
+// TEMPORARY until bootstrap version catches up.
+func imax(i, j int) int {
+	if i > j {
+		return i
+	}
+	return j
+}
+
+// Overlaps returns true if here is any overlap between i and i2.
+func (i Interval) Overlaps(i2 Interval) bool {
+	return (imin(i.en, i2.en) - imax(i.st, i2.st)) > 0
+}
+
+// adjacent returns true if the start of one interval is equal to the
+// end of another interval (e.g. they represent consecutive ranges).
+func (i1 Interval) adjacent(i2 Interval) bool {
+	return i1.en == i2.st || i2.en == i1.st
+}
+
+// MergeInto merges interval i2 into i1. This version happens to
+// require that the two intervals either overlap or are adjacent.
+func (i1 *Interval) MergeInto(i2 Interval) error {
+	if !i1.Overlaps(i2) && !i1.adjacent(i2) {
+		return fmt.Errorf("merge method invoked on non-overlapping/non-adjacent")
+	}
+	i1.st = imin(i1.st, i2.st)
+	i1.en = imax(i1.en, i2.en)
+	return nil
+}
+
+// IntervalsBuilder is a helper for constructing intervals based on
+// live dataflow sets for a series of BBs where we're making a
+// backwards pass over each BB looking for uses and kills. The
+// expected use case is:
+//
+//   - invoke MakeIntervalsBuilder to create a new object "b"
+//   - series of calls to b.Live/b.Kill based on a backwards reverse layout
+//     order scan over instructions
+//   - invoke b.Finish() to produce final set
+//
+// See the Live method comment for an IR example.
+type IntervalsBuilder struct {
+	s Intervals
+	// index of last instruction visited plus 1
+	lidx int
+}
+
+func (c *IntervalsBuilder) last() int {
+	return c.lidx - 1
+}
+
+func (c *IntervalsBuilder) setLast(x int) {
+	c.lidx = x + 1
+}
+
+func (c *IntervalsBuilder) Finish() (Intervals, error) {
+	// Reverse intervals list and check.
+	// FIXME: replace with slices.Reverse once the
+	// bootstrap version supports it.
+	for i, j := 0, len(c.s)-1; i < j; i, j = i+1, j-1 {
+		c.s[i], c.s[j] = c.s[j], c.s[i]
+	}
+	if err := check(c.s); err != nil {
+		return Intervals{}, err
+	}
+	r := c.s
+	return r, nil
+}
+
+// Live method should be invoked on instruction at position p if instr
+// contains an upwards-exposed use of a resource. See the example in
+// the comment at the beginning of this file for an example.
+func (c *IntervalsBuilder) Live(pos int) error {
+	if pos < 0 {
+		return fmt.Errorf("bad pos, negative")
+	}
+	if c.last() == -1 {
+		c.setLast(pos)
+		if debugtrace {
+			fmt.Fprintf(os.Stderr, "=-= begin lifetime at pos=%d\n", pos)
+		}
+		c.s = append(c.s, Interval{st: pos, en: pos + 1})
+		return nil
+	}
+	if pos >= c.last() {
+		return fmt.Errorf("pos not decreasing")
+	}
+	// extend lifetime across this pos
+	c.s[len(c.s)-1].st = pos
+	c.setLast(pos)
+	return nil
+}
+
+// Kill method should be invoked on instruction at position p if instr
+// should be treated as as having a kill (lifetime end) for the
+// resource. See the example in the comment at the beginning of this
+// file for an example. Note that if we see a kill at position K for a
+// resource currently live since J, this will result in a lifetime
+// segment of [K+1,J+1), the assumption being that the first live
+// instruction will be the one after the kill position, not the kill
+// position itself.
+func (c *IntervalsBuilder) Kill(pos int) error {
+	if pos < 0 {
+		return fmt.Errorf("bad pos, negative")
+	}
+	if c.last() == -1 {
+		return nil
+	}
+	if pos >= c.last() {
+		return fmt.Errorf("pos not decreasing")
+	}
+	c.s[len(c.s)-1].st = pos + 1
+	// terminate lifetime
+	c.setLast(-1)
+	if debugtrace {
+		fmt.Fprintf(os.Stderr, "=-= term lifetime at pos=%d\n", pos)
+	}
+	return nil
+}
+
+// check examines the intervals in "is" to try to find internal
+// inconsistencies or problems.
+func check(is Intervals) error {
+	for i := 0; i < len(is); i++ {
+		st := is[i].st
+		en := is[i].en
+		if en <= st {
+			return fmt.Errorf("bad range elem %d:%d, en<=st", st, en)
+		}
+		if i == 0 {
+			continue
+		}
+		// check for badly ordered starts
+		pst := is[i-1].st
+		pen := is[i-1].en
+		if pst >= st {
+			return fmt.Errorf("range start not ordered %d:%d less than prev %d:%d", st, en,
+				pst, pen)
+		}
+		// check end of last range against start of this range
+		if pen > st {
+			return fmt.Errorf("bad range elem %d:%d overlaps prev %d:%d", st, en,
+				pst, pen)
+		}
+	}
+	return nil
+}
+
+func (is *Intervals) String() string {
+	var sb strings.Builder
+	for i := range *is {
+		if i != 0 {
+			sb.WriteString(" ")
+		}
+		sb.WriteString((*is)[i].String())
+	}
+	return sb.String()
+}
+
+// intWithIdx holds an interval i and an index pairIndex storing i's
+// position (either 0 or 1) within some previously specified interval
+// pair <I1,I2>; a pairIndex of -1 is used to signal "end of
+// iteration". Used for Intervals operations, not expected to be
+// exported.
+type intWithIdx struct {
+	i         Interval
+	pairIndex int
+}
+
+func (iwi intWithIdx) done() bool {
+	return iwi.pairIndex == -1
+}
+
+// pairVisitor provides a way to visit (iterate through) each interval
+// within a pair of Intervals in order of increasing start time. Expected
+// usage model:
+//
+//	func example(i1, i2 Intervals) {
+//	  var pairVisitor pv
+//	  cur := pv.init(i1, i2);
+//	  for !cur.done() {
+//	     fmt.Printf("interval %s from i%d", cur.i.String(), cur.pairIndex+1)
+//	     cur = pv.nxt()
+//	  }
+//	}
+//
+// Used internally for Intervals operations, not expected to be exported.
+type pairVisitor struct {
+	cur    intWithIdx
+	i1pos  int
+	i2pos  int
+	i1, i2 Intervals
+}
+
+// init initializes a pairVisitor for the specified pair of intervals
+// i1 and i2 and returns an intWithIdx object that points to the first
+// interval by start position within i1/i2.
+func (pv *pairVisitor) init(i1, i2 Intervals) intWithIdx {
+	pv.i1, pv.i2 = i1, i2
+	pv.cur = pv.sel()
+	return pv.cur
+}
+
+// nxt advances the pairVisitor to the next interval by starting
+// position within the pair, returning an intWithIdx that describes
+// the interval.
+func (pv *pairVisitor) nxt() intWithIdx {
+	if pv.cur.pairIndex == 0 {
+		pv.i1pos++
+	} else {
+		pv.i2pos++
+	}
+	pv.cur = pv.sel()
+	return pv.cur
+}
+
+// sel is a helper function used by 'init' and 'nxt' above; it selects
+// the earlier of the two intervals at the current positions within i1
+// and i2, or a degenerate (pairIndex -1) intWithIdx if we have no
+// more intervals to visit.
+func (pv *pairVisitor) sel() intWithIdx {
+	var c1, c2 intWithIdx
+	if pv.i1pos >= len(pv.i1) {
+		c1.pairIndex = -1
+	} else {
+		c1 = intWithIdx{i: pv.i1[pv.i1pos], pairIndex: 0}
+	}
+	if pv.i2pos >= len(pv.i2) {
+		c2.pairIndex = -1
+	} else {
+		c2 = intWithIdx{i: pv.i2[pv.i2pos], pairIndex: 1}
+	}
+	if c1.pairIndex == -1 {
+		return c2
+	}
+	if c2.pairIndex == -1 {
+		return c1
+	}
+	if c1.i.st <= c2.i.st {
+		return c1
+	}
+	return c2
+}
+
+// Overlaps returns whether any of the component ranges in is overlaps
+// with some range in is2.
+func (is Intervals) Overlaps(is2 Intervals) bool {
+	// check for empty intervals
+	if len(is) == 0 || len(is2) == 0 {
+		return false
+	}
+	li := len(is)
+	li2 := len(is2)
+	// check for completely disjoint ranges
+	if is[li-1].en <= is2[0].st ||
+		is[0].st >= is2[li2-1].en {
+		return false
+	}
+	// walk the combined sets of intervals and check for piecewise
+	// overlap.
+	var pv pairVisitor
+	first := pv.init(is, is2)
+	for {
+		second := pv.nxt()
+		if second.done() {
+			break
+		}
+		if first.pairIndex == second.pairIndex {
+			first = second
+			continue
+		}
+		if first.i.Overlaps(second.i) {
+			return true
+		}
+		first = second
+	}
+	return false
+}
+
+// Merge combines the intervals from "is" and "is2" and returns
+// a new Intervals object containing all combined ranges from the
+// two inputs.
+func (is Intervals) Merge(is2 Intervals) Intervals {
+	if len(is) == 0 {
+		return is2
+	} else if len(is2) == 0 {
+		return is
+	}
+	// walk the combined set of intervals and merge them together.
+	var ret Intervals
+	var pv pairVisitor
+	cur := pv.init(is, is2)
+	for {
+		second := pv.nxt()
+		if second.done() {
+			break
+		}
+
+		// Check for overlap between cur and second. If no overlap
+		// then add cur to result and move on.
+		if !cur.i.Overlaps(second.i) && !cur.i.adjacent(second.i) {
+			ret = append(ret, cur.i)
+			cur = second
+			continue
+		}
+		// cur overlaps with second; merge second into cur
+		cur.i.MergeInto(second.i)
+	}
+	ret = append(ret, cur.i)
+	return ret
+}