cmd/compile/internal: remove incorrect riscv64 SLTI rule

The rule

(SLTI  [x] (ORI  [y] _)) && y >= 0 && int64(y) >= int64(x) => (MOVDconst [0])

is incorrect as it only generates correct code if the unknown value
being compared is >= 0. If the unknown value is < 0 the rule will
incorrectly produce a constant value of 0, whereas the code optimized
away by the rule would have produced a value of 1.

A new test that causes the faulty rule to generate incorrect code
is also added to ensure that the error does not return.

Change-Id: I69224e0776596f1b9538acf9dacf9009d305f966
Reviewed-on: https://go-review.googlesource.com/c/go/+/720220
Reviewed-by: Meng Zhuo <[email protected]>
Reviewed-by: Junyang Shao <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Joel Sing <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: markdryan

src/cmd/compile/internal/ssa/_gen/RISCV64.rules

@@ -792,7 +792,6 @@
 // SLTI/SLTIU with known outcomes.
 (SLTI  [x] (ANDI [y] _)) && y >= 0 && int64(y) < int64(x) => (MOVDconst [1])
 (SLTIU [x] (ANDI [y] _)) && y >= 0 && uint64(y) < uint64(x) => (MOVDconst [1])
-(SLTI  [x] (ORI  [y] _)) && y >= 0 && int64(y) >= int64(x) => (MOVDconst [0])
 (SLTIU [x] (ORI  [y] _)) && y >= 0 && uint64(y) >= uint64(x) => (MOVDconst [0])
 
 // SLT/SLTU with known outcomes.

src/cmd/compile/internal/ssa/rewriteRISCV64.go

@@ -444,6 +444,19 @@ func testBitwiseRshU_ssa(a uint32, b, c uint32) uint32 {
 	return a >> b >> c
 }
 
+//go:noinline
+func orLt_ssa(x int) bool {
+	y := x - x
+	return (x | 2) < y
+}
+
+// test riscv64 SLTI rules
+func testSetIfLessThan(t *testing.T) {
+	if want, got := true, orLt_ssa(-7); got != want {
+		t.Errorf("orLt_ssa(-7) = %t want %t", got, want)
+	}
+}
+
 //go:noinline
 func testShiftCX_ssa() int {
 	v1 := uint8(3)
@@ -977,6 +990,7 @@ func TestArithmetic(t *testing.T) {
 	testRegallocCVSpill(t)
 	testSubqToNegq(t)
 	testBitwiseLogic(t)
+	testSetIfLessThan(t)
 	testOcom(t)
 	testLrot(t)
 	testShiftCX(t)