cmd/compile: correct sparseSet probes in regalloc to avoid index error

In regalloc, a sparse map is preallocated for later use by
spill-in-loop sinking.  However, variables (spills) are added
during register allocation before spill sinking, and a map
query involving any of these new variables will index out of
bounds in the map.

To fix:
1) fix the queries to use s.orig[v.ID].ID instead, to ensure
proper indexing.  Note that s.orig will be nil for values
that are not eligible for spilling (like memory and flags).

2) add a test.

Fixes #15585.

Change-Id: I8f2caa93b132a0f2a9161d2178320d5550583075
Reviewed-on: https://go-review.googlesource.com/22911
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1311,20 +1311,25 @@ func (s *regAllocState) regalloc(f *Func) {
 						// Start with live at end.
 						for _, li := range s.live[ss.ID] {
 							if s.isLoopSpillCandidate(loop, s.orig[li.ID]) {
+								// s.live contains original IDs, use s.orig above to map back to *Value
 								entryCandidates.setBit(li.ID, uint(whichExit))
 							}
 						}
 						// Control can also be live.
-						if ss.Control != nil && s.isLoopSpillCandidate(loop, ss.Control) {
-							entryCandidates.setBit(ss.Control.ID, uint(whichExit))
+						if ss.Control != nil && s.orig[ss.Control.ID] != nil && s.isLoopSpillCandidate(loop, s.orig[ss.Control.ID]) {
+							entryCandidates.setBit(s.orig[ss.Control.ID].ID, uint(whichExit))
 						}
 						// Walk backwards, filling in locally live values, removing those defined.
 						for i := len(ss.Values) - 1; i >= 0; i-- {
 							v := ss.Values[i]
-							entryCandidates.remove(v.ID) // Cannot be an issue, only keeps the sets smaller.
+							vorig := s.orig[v.ID]
+							if vorig != nil {
+								entryCandidates.remove(vorig.ID) // Cannot be an issue, only keeps the sets smaller.
+							}
 							for _, a := range v.Args {
-								if s.isLoopSpillCandidate(loop, a) {
-									entryCandidates.setBit(a.ID, uint(whichExit))
+								aorig := s.orig[a.ID]
+								if aorig != nil && s.isLoopSpillCandidate(loop, aorig) {
+									entryCandidates.setBit(aorig.ID, uint(whichExit))
 								}
 							}
 						}

test/fixedbugs/issue15585.go

@@ -0,0 +1,45 @@
+// compile
+
+// Copyright 2016 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 bug
+
+func example(n int) (rc int) {
+	var cc, ll, pp, rr [27]int
+	for q0 := 0; q0 < n-2; q0++ {
+		for q1 := q0 + 2; q1 < n; q1++ {
+			var c, d, l, p, r int
+			b0 := 1 << uint(q0)
+			b1 := 1 << uint(q1)
+			l = ((b0 << 1) | b1) << 1
+			c = b0 | b1 | (-1 << uint(n))
+			r = ((b0 >> 1) | b1) >> 1
+		E:
+			if c != -1 {
+				p = ^(l | c | r)
+			} else {
+				rc++
+				goto R
+			}
+		L:
+			if p != 0 {
+				lsb := p & -p
+				p &^= lsb
+				ll[d], cc[d], rr[d], pp[d] = l, c, r, p
+				l, c, r = (l|lsb)<<1, c|lsb, (r|lsb)>>1
+				d++
+				goto E
+			}
+		R:
+			d--
+			if d >= 0 {
+				l, c, r, p = ll[d], cc[d], rr[d], pp[d]
+				goto L
+			}
+		}
+	}
+	rc <<= 1
+	return
+}