cmd/compile: fold IsInBounds of modulo.

In b we only need the division by 0 check.

func b(i uint, v []byte) byte {
    return v[i%uint(len(v))]
}

Updates #15079.

Change-Id: Ic7491e677dd57cd6ba577efbce576dbb6e023cbd
Reviewed-on: https://go-review.googlesource.com/21502
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Ahmed Waheed <oneofone@gmail.com>

src/cmd/compile/internal/gc/testdata/divbyzero_ssa.go

+package main
+
+import (
+	"fmt"
+	"runtime"
+)
+
+var failed = false
+
+func checkDivByZero(f func()) (divByZero bool) {
+	defer func() {
+		if r := recover(); r != nil {
+			if e, ok := r.(runtime.Error); ok && e.Error() == "runtime error: integer divide by zero" {
+				divByZero = true
+			}
+		}
+	}()
+	f()
+	return false
+}
+
+//go:noinline
+func a(i uint, s []int) int {
+	return s[i%uint(len(s))]
+}
+
+//go:noinline
+func b(i uint, j uint) uint {
+	return i / j
+}
+
+func main() {
+	if got := checkDivByZero(func() { b(7, 0) }); !got {
+		fmt.Printf("expected div by zero for b(7, 0), got no error\n")
+		failed = true
+	}
+	if got := checkDivByZero(func() { b(7, 7) }); got {
+		fmt.Printf("expected no error for b(7, 7), got div by zero\n")
+		failed = true
+	}
+	if got := checkDivByZero(func() { a(4, nil) }); !got {
+		fmt.Printf("expected div by zero for a(4, nil), got no error\n")
+		failed = true
+	}
+
+	if failed {
+		panic("tests failed")
+	}
+}

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -109,12 +109,15 @@
 (IsInBounds (ZeroExt8to64 _) (Const64 [c])) && (1 << 8) <= c -> (ConstBool [1])
 (IsInBounds (ZeroExt16to32 _) (Const32 [c])) && (1 << 16) <= c -> (ConstBool [1])
 (IsInBounds (ZeroExt16to64 _) (Const64 [c])) && (1 << 16) <= c -> (ConstBool [1])
-
 (IsInBounds x x) -> (ConstBool [0])
 (IsInBounds (And32 (Const32 [c]) _) (Const32 [d])) && 0 <= c && c < d -> (ConstBool [1])
 (IsInBounds (And64 (Const64 [c]) _) (Const64 [d])) && 0 <= c && c < d -> (ConstBool [1])
 (IsInBounds (Const32 [c]) (Const32 [d])) -> (ConstBool [b2i(0 <= c && c < d)])
 (IsInBounds (Const64 [c]) (Const64 [d])) -> (ConstBool [b2i(0 <= c && c < d)])
+// (Mod64u x y) is always between 0 (inclusive) and y (exclusive).
+(IsInBounds (Mod32u _ y) y) -> (ConstBool [1])
+(IsInBounds (Mod64u _ y) y) -> (ConstBool [1])
+
 (IsSliceInBounds x x) -> (ConstBool [1])
 (IsSliceInBounds (And32 (Const32 [c]) _) (Const32 [d])) && 0 <= c && c <= d -> (ConstBool [1])
 (IsSliceInBounds (And64 (Const64 [c]) _) (Const64 [d])) && 0 <= c && c <= d -> (ConstBool [1])

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

@@ -2815,6 +2815,38 @@ func rewriteValuegeneric_OpIsInBounds(v *Value, config *Config) bool {
 		v.AuxInt = b2i(0 <= c && c < d)
 		return true
 	}
+	// match: (IsInBounds (Mod32u _ y) y)
+	// cond:
+	// result: (ConstBool [1])
+	for {
+		v_0 := v.Args[0]
+		if v_0.Op != OpMod32u {
+			break
+		}
+		y := v_0.Args[1]
+		if y != v.Args[1] {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 1
+		return true
+	}
+	// match: (IsInBounds (Mod64u _ y) y)
+	// cond:
+	// result: (ConstBool [1])
+	for {
+		v_0 := v.Args[0]
+		if v_0.Op != OpMod64u {
+			break
+		}
+		y := v_0.Args[1]
+		if y != v.Args[1] {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 1
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpIsSliceInBounds(v *Value, config *Config) bool {