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 <mengzhuo1203@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/test/testdata/arith_test.go b/src/cmd/compile/internal/test/testdata/arith_test.go
index 8984cd3e26..34ac73c068 100644
--- a/src/cmd/compile/internal/test/testdata/arith_test.go
+++ b/src/cmd/compile/internal/test/testdata/arith_test.go
@@ -445,6 +445,19 @@ func testBitwiseRshU_ssa(a uint32, b, c uint32) uint32 {
 }
 
 //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)
 	v4 := (v1 * v1) ^ v1 | v1 - v1 - v1&v1 ^ uint8(3+2) + v1*1>>0 - v1 | 1 | v1<<(2*3|0-0*0^1)
@@ -977,6 +990,7 @@ func TestArithmetic(t *testing.T) {
 	testRegallocCVSpill(t)
 	testSubqToNegq(t)
 	testBitwiseLogic(t)
+	testSetIfLessThan(t)
 	testOcom(t)
 	testLrot(t)
 	testShiftCX(t)