cmd/compile: bce when max and limit are consts

Removes 49 more bound checks in make.bash. For example:

var a[100]int
for i := 0; i < 50; i++ {
  use a[i+25]
}

Change-Id: I85e0130ee5d07f0ece9b17044bba1a2047414ce7
Reviewed-on: https://go-review.googlesource.com/21379Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -240,6 +240,37 @@ func removeBoundsChecks(f *Func, sdom sparseTree, m map[*Value]indVar) {
 		}
 	skip2:
 
+		// Simplify
+		// (IsInBounds (Add64 ind) (Const64 [c])) where 0 <= min <= ind < max <= (Const64 [c])
+		// (IsSliceInBounds ind (Const64 [c])) where 0 <= min <= ind < max <= (Const64 [c])
+		if v.Op == OpIsInBounds || v.Op == OpIsSliceInBounds {
+			ind, add := dropAdd64(v.Args[0])
+			if ind.Op != OpPhi {
+				goto skip3
+			}
+
+			// ind + add >= 0 <-> min + add >= 0 <-> min >= -add
+			if iv, has := m[ind]; has && sdom.isAncestorEq(iv.entry, b) && isGreaterOrEqualThan(iv.min, -add) {
+				if !v.Args[1].isGenericIntConst() || !iv.max.isGenericIntConst() {
+					goto skip3
+				}
+
+				limit := v.Args[1].AuxInt
+				if v.Op == OpIsSliceInBounds {
+					// If limit++ overflows signed integer then 0 <= max && max <= limit will be false.
+					limit++
+				}
+
+				if max := iv.max.AuxInt + add; 0 <= max && max <= limit { // handle overflow
+					if f.pass.debug > 0 {
+						f.Config.Warnl(b.Line, "Found redundant (%s ind %d), ind < %d", v.Op, v.Args[1].AuxInt, iv.max.AuxInt+add)
+					}
+					goto simplify
+				}
+			}
+		}
+	skip3:
+
 		continue
 
 	simplify:
@@ -258,3 +289,13 @@ func dropAdd64(v *Value) (*Value, int64) {
 	}
 	return v, 0
 }
+
+func isGreaterOrEqualThan(v *Value, c int64) bool {
+	if c == 0 {
+		return isNonNegative(v)
+	}
+	if v.isGenericIntConst() && v.AuxInt >= c {
+		return true
+	}
+	return false
+}

test/checkbce.go

@@ -57,7 +57,7 @@ func f6(a [32]int, b [64]int, i int) {
 	useInt(b[uint64(i*0x07C4ACDD)>>58])
 	useInt(a[uint(i*0x07C4ACDD)>>59])
 
-	// The following bounds should removed as they can overflow.
+	// The following bounds should not be removed because they can overflow.
 	useInt(a[uint32(i*0x106297f105d0cc86)>>26]) // ERROR "Found IsInBounds$"
 	useInt(b[uint64(i*0x106297f105d0cc86)>>57]) // ERROR "Found IsInBounds$"
 	useInt(a[int32(i*0x106297f105d0cc86)>>26])  // ERROR "Found IsInBounds$"
@@ -89,6 +89,19 @@ func g3(a []int) {
 	}
 }
 
+func g4(a [100]int) {
+	for i := 10; i < 50; i++ {
+		useInt(a[i-10])
+		useInt(a[i])
+		useInt(a[i+25])
+		useInt(a[i+50])
+
+		// The following are out of bounds.
+		useInt(a[i-11]) // ERROR "Found IsInBounds$"
+		useInt(a[i+51]) // ERROR "Found IsInBounds$"
+	}
+}
+
 //go:noinline
 func useInt(a int) {
 }

test/loopbce.go

@@ -139,6 +139,70 @@ func h2(a []byte) {
 	}
 }
 
+func k0(a [100]int) [100]int {
+	for i := 10; i < 90; i++ { // ERROR "Induction variable with minimum 10 and increment 1$"
+		a[i-11] = i
+		a[i-10] = i // ERROR "Found redundant \(IsInBounds ind 100\), ind < 80$"
+		a[i-5] = i  // ERROR "Found redundant \(IsInBounds ind 100\), ind < 85$"
+		a[i] = i    // ERROR "Found redundant \(IsInBounds ind 100\), ind < 90$"
+		a[i+5] = i  // ERROR "Found redundant \(IsInBounds ind 100\), ind < 95$"
+		a[i+10] = i // ERROR "Found redundant \(IsInBounds ind 100\), ind < 100$"
+		a[i+11] = i
+	}
+	return a
+}
+
+func k1(a [100]int) [100]int {
+	for i := 10; i < 90; i++ { // ERROR "Induction variable with minimum 10 and increment 1$"
+		useSlice(a[:i-11])
+		useSlice(a[:i-10]) // ERROR "Found redundant \(IsSliceInBounds ind 100\), ind < 80$"
+		useSlice(a[:i-5])  // ERROR "Found redundant \(IsSliceInBounds ind 100\), ind < 85$"
+		useSlice(a[:i])    // ERROR "Found redundant \(IsSliceInBounds ind 100\), ind < 90$"
+		useSlice(a[:i+5])  // ERROR "Found redundant \(IsSliceInBounds ind 100\), ind < 95$"
+		useSlice(a[:i+10]) // ERROR "Found redundant \(IsSliceInBounds ind 100\), ind < 100$"
+		useSlice(a[:i+11]) // ERROR "Found redundant \(IsSliceInBounds ind 100\), ind < 101$"
+
+	}
+	return a
+}
+
+func k2(a [100]int) [100]int {
+	for i := 10; i < 90; i++ { // ERROR "Induction variable with minimum 10 and increment 1$"
+		useSlice(a[i-11:])
+		useSlice(a[i-10:]) // ERROR "Found redundant \(IsSliceInBounds ind 100\), ind < 80$"
+		useSlice(a[i-5:])  // ERROR "Found redundant \(IsSliceInBounds ind 100\), ind < 85$"
+		useSlice(a[i:])    // ERROR "Found redundant \(IsSliceInBounds ind 100\), ind < 90$"
+		useSlice(a[i+5:])  // ERROR "Found redundant \(IsSliceInBounds ind 100\), ind < 95$"
+		useSlice(a[i+10:]) // ERROR "Found redundant \(IsSliceInBounds ind 100\), ind < 100$"
+		useSlice(a[i+11:]) // ERROR "Found redundant \(IsSliceInBounds ind 100\), ind < 101$"
+	}
+	return a
+}
+
+func k3(a [100]int) [100]int {
+	for i := -10; i < 90; i++ { // ERROR "Induction variable with minimum -10 and increment 1$"
+		a[i+10] = i // ERROR "Found redundant \(IsInBounds ind 100\), ind < 100$"
+	}
+	return a
+}
+
+func k4(a [100]int) [100]int {
+	min := (-1) << 63
+	for i := min; i < min+50; i++ { // ERROR "Induction variable with minimum -9223372036854775808 and increment 1$"
+		a[i-min] = i // ERROR "Found redundant \(IsInBounds ind 100\), ind < 50$"
+	}
+	return a
+}
+
+func k5(a [100]int) [100]int {
+	max := (1 << 63) - 1
+	for i := max - 50; i < max; i++ { // ERROR "Induction variable with minimum 9223372036854775757 and increment 1$"
+		a[i-max+50] = i
+		a[i-(max-70)] = i // ERROR "Found redundant \(IsInBounds ind 100\), ind < 70$"
+	}
+	return a
+}
+
 func nobce1() {
 	// tests overflow of max-min
 	a := int64(9223372036854774057)
@@ -168,9 +232,22 @@ func nobce2(a string) {
 	}
 }
 
+func nobce3(a [100]int64) [100]int64 {
+	min := int64((-1) << 63)
+	max := int64((1 << 63) - 1)
+	for i := min; i < max; i++ { // ERROR "Induction variable with minimum -9223372036854775808 and increment 1$"
+		a[i] = i
+	}
+	return a
+}
+
 //go:noinline
 func useString(a string) {
 }
 
+//go:noinline
+func useSlice(a []int) {
+}
+
 func main() {
 }