cmd/compile: tighten bounds for induction variables in strided loops

for i := 0; i < 9; i += 3

Currently we compute bounds of [0,8]. Really we know that it is [0,6].

CL 415874 computed the better bound as part of overflow detection.
This CL just incorporates that better info to the prove pass.

R=go1.20

Change-Id: Ife82cc415321f6652c2b5d132a40ec23e3385766
Reviewed-on: https://go-review.googlesource.com/c/go/+/415937
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Keith Randall <khr@golang.org>

diff --git a/src/cmd/compile/internal/ssa/loopbce.go b/src/cmd/compile/internal/ssa/loopbce.go
index 22fb5118ce30402be3bd2b7eaf25f217b03adf23..d92566f2d3eb456b690f5a70e79b1d45878ec4e9 100644 (file)
--- a/src/cmd/compile/internal/ssa/loopbce.go
+++ b/src/cmd/compile/internal/ssa/loopbce.go
@@ -200,8 +200,15 @@ func findIndVar(f *Func) []indVar {
                                                }
                                                v = addU(init.AuxInt, diff(v, init.AuxInt)/uint64(step)*uint64(step))
                                        }
-                                       // It is ok if we can't overflow when incrementing from the largest value.
-                                       return !addWillOverflow(v, step)
+                                       if addWillOverflow(v, step) {
+                                               return false
+                                       }
+                                       if inclusive && v != limit.AuxInt || !inclusive && v+1 != limit.AuxInt {
+                                               // We know a better limit than the programmer did. Use our limit instead.
+                                               limit = f.ConstInt64(f.Config.Types.Int64, v)
+                                               inclusive = true
+                                       }
+                                       return true
                                }
                                if step == 1 && !inclusive {
                                        // Can't overflow because maxint is never a possible value.
@@ -238,8 +245,15 @@ func findIndVar(f *Func) []indVar {
                                                }
                                                v = subU(init.AuxInt, diff(init.AuxInt, v)/uint64(-step)*uint64(-step))
                                        }
-                                       // It is ok if we can't underflow when decrementing from the smallest value.
-                                       return !subWillUnderflow(v, -step)
+                                       if subWillUnderflow(v, -step) {
+                                               return false
+                                       }
+                                       if inclusive && v != limit.AuxInt || !inclusive && v-1 != limit.AuxInt {
+                                               // We know a better limit than the programmer did. Use our limit instead.
+                                               limit = f.ConstInt64(f.Config.Types.Int64, v)
+                                               inclusive = true
+                                       }
+                                       return true
                                }
                                if step == -1 && !inclusive {
                                        // Can't underflow because minint is never a possible value.

diff --git a/test/loopbce.go b/test/loopbce.go
index 4ae9a6a630b91208057cacb31d0b4f5cfddedd18..db830daf5cc191cdacd3a5452d0d3c8783f082fd 100644 (file)
--- a/test/loopbce.go
+++ b/test/loopbce.go
@@ -49,7 +49,7 @@ func f2(a []int) int {
 
 func f4(a [10]int) int {
        x := 0
-       for i := 0; i < len(a); i += 2 { // ERROR "Induction variable: limits \[0,10\), increment 2$"
+       for i := 0; i < len(a); i += 2 { // ERROR "Induction variable: limits \[0,8\], increment 2$"
                x += a[i] // ERROR "(\([0-9]+\) )?Proved IsInBounds$"
        }
        return x
@@ -57,7 +57,7 @@ func f4(a [10]int) int {
 
 func f5(a [10]int) int {
        x := 0
-       for i := -10; i < len(a); i += 2 { // ERROR "Induction variable: limits \[-10,10\), increment 2$"
+       for i := -10; i < len(a); i += 2 { // ERROR "Induction variable: limits \[-10,8\], increment 2$"
                x += a[i]
        }
        return x
@@ -121,7 +121,7 @@ func g0f(a string) int {
 func g1() int {
        a := "evenlength"
        x := 0
-       for i := 0; i < len(a); i += 2 { // ERROR "Induction variable: limits \[0,10\), increment 2$"
+       for i := 0; i < len(a); i += 2 { // ERROR "Induction variable: limits \[0,8\], increment 2$"
                x += int(a[i]) // ERROR "(\([0-9]+\) )?Proved IsInBounds$"
        }
        return x
@@ -130,7 +130,7 @@ func g1() int {
 func g2() int {
        a := "evenlength"
        x := 0
-       for i := 0; i < len(a); i += 2 { // ERROR "Induction variable: limits \[0,10\), increment 2$"
+       for i := 0; i < len(a); i += 2 { // ERROR "Induction variable: limits \[0,8\], increment 2$"
                j := i
                if a[i] == 'e' { // ERROR "(\([0-9]+\) )?Proved IsInBounds$"
                        j = j + 1
@@ -142,9 +142,11 @@ func g2() int {
 
 func g3a() {
        a := "this string has length 25"
-       for i := 0; i < len(a); i += 5 { // ERROR "Induction variable: limits \[0,25\), increment 5$"
+       for i := 0; i < len(a); i += 5 { // ERROR "Induction variable: limits \[0,20\], increment 5$"
                useString(a[i:]) // ERROR "(\([0-9]+\) )?Proved IsSliceInBounds$"
-               useString(a[:i+3])
+               useString(a[:i+3]) // ERROR "(\([0-9]+\) )?Proved IsSliceInBounds$"
+               useString(a[:i+5]) // ERROR "(\([0-9]+\) )?Proved IsSliceInBounds$"
+               useString(a[:i+6])
        }
 }
 
@@ -293,22 +295,22 @@ func d3(a [100]int) [100]int {
 }
 
 func d4() {
-       for i := int64(math.MaxInt64 - 9); i < math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775798,9223372036854775805\), increment 4$"
+       for i := int64(math.MaxInt64 - 9); i < math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775798,9223372036854775802\], increment 4$"
                useString("foo")
        }
-       for i := int64(math.MaxInt64 - 8); i < math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775799,9223372036854775805\), increment 4$"
+       for i := int64(math.MaxInt64 - 8); i < math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775799,9223372036854775803\], increment 4$"
                useString("foo")
        }
        for i := int64(math.MaxInt64 - 7); i < math.MaxInt64-2; i += 4 {
                useString("foo")
        }
-       for i := int64(math.MaxInt64 - 6); i < math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775801,9223372036854775805\), increment 4$"
+       for i := int64(math.MaxInt64 - 6); i < math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775801,9223372036854775801\], increment 4$"
                useString("foo")
        }
-       for i := int64(math.MaxInt64 - 9); i <= math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775798,9223372036854775805\], increment 4$"
+       for i := int64(math.MaxInt64 - 9); i <= math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775798,9223372036854775802\], increment 4$"
                useString("foo")
        }
-       for i := int64(math.MaxInt64 - 8); i <= math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775799,9223372036854775805\], increment 4$"
+       for i := int64(math.MaxInt64 - 8); i <= math.MaxInt64-2; i += 4 { // ERROR "Induction variable: limits \[9223372036854775799,9223372036854775803\], increment 4$"
                useString("foo")
        }
        for i := int64(math.MaxInt64 - 7); i <= math.MaxInt64-2; i += 4 {
@@ -320,22 +322,22 @@ func d4() {
 }
 
 func d5() {
-       for i := int64(math.MinInt64 + 9); i > math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \(-9223372036854775806,-9223372036854775799\], increment 4"
+       for i := int64(math.MinInt64 + 9); i > math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \[-9223372036854775803,-9223372036854775799\], increment 4"
                useString("foo")
        }
-       for i := int64(math.MinInt64 + 8); i > math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \(-9223372036854775806,-9223372036854775800\], increment 4"
+       for i := int64(math.MinInt64 + 8); i > math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \[-9223372036854775804,-9223372036854775800\], increment 4"
                useString("foo")
        }
        for i := int64(math.MinInt64 + 7); i > math.MinInt64+2; i -= 4 {
                useString("foo")
        }
-       for i := int64(math.MinInt64 + 6); i > math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \(-9223372036854775806,-9223372036854775802\], increment 4"
+       for i := int64(math.MinInt64 + 6); i > math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \[-9223372036854775802,-9223372036854775802\], increment 4"
                useString("foo")
        }
-       for i := int64(math.MinInt64 + 9); i >= math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \[-9223372036854775806,-9223372036854775799\], increment 4"
+       for i := int64(math.MinInt64 + 9); i >= math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \[-9223372036854775803,-9223372036854775799\], increment 4"
                useString("foo")
        }
-       for i := int64(math.MinInt64 + 8); i >= math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \[-9223372036854775806,-9223372036854775800\], increment 4"
+       for i := int64(math.MinInt64 + 8); i >= math.MinInt64+2; i -= 4 { // ERROR "Induction variable: limits \[-9223372036854775804,-9223372036854775800\], increment 4"
                useString("foo")
        }
        for i := int64(math.MinInt64 + 7); i >= math.MinInt64+2; i -= 4 {
@@ -356,7 +358,7 @@ func bce1() {
                panic("invalid test: modulos should differ")
        }
 
-       for i := b; i < a; i += z { // ERROR "Induction variable: limits \[-1547,9223372036854774057\), increment 1337"
+       for i := b; i < a; i += z { // ERROR "Induction variable: limits \[-1547,9223372036854772720\], increment 1337"
                useString("foobar")
        }
 }
@@ -400,6 +402,22 @@ func issue26116a(a []int) {
        }
 }
 
+func stride1(x *[7]int) int {
+       s := 0
+       for i := 0; i <= 8; i += 3 { // ERROR "Induction variable: limits \[0,6\], increment 3"
+               s += x[i] // ERROR "Proved IsInBounds"
+       }
+       return s
+}
+
+func stride2(x *[7]int) int {
+       s := 0
+       for i := 0; i < 9; i += 3 { // ERROR "Induction variable: limits \[0,6\], increment 3"
+               s += x[i] // ERROR "Proved IsInBounds"
+       }
+       return s
+}
+
 //go:noinline
 func useString(a string) {
 }