From a0e738c657d33e2a648838546812fb50cf42e41d Mon Sep 17 00:00:00 2001
From: Mark Ryan <markdryan@rivosinc.com>
Date: Thu, 13 Nov 2025 12:28:29 +0100
Subject: [PATCH] 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>
---
 src/cmd/compile/internal/ssa/_gen/RISCV64.rules  |  1 -
 src/cmd/compile/internal/ssa/rewriteRISCV64.go   | 16 ----------------
 .../compile/internal/test/testdata/arith_test.go | 14 ++++++++++++++
 3 files changed, 14 insertions(+), 17 deletions(-)

diff --git a/src/cmd/compile/internal/ssa/_gen/RISCV64.rules b/src/cmd/compile/internal/ssa/_gen/RISCV64.rules
index 6166bd5584d..13a8cab3b52 100644
--- a/src/cmd/compile/internal/ssa/_gen/RISCV64.rules
+++ b/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.
diff --git a/src/cmd/compile/internal/ssa/rewriteRISCV64.go b/src/cmd/compile/internal/ssa/rewriteRISCV64.go
index 24fef3fe724..284d88967ba 100644
--- a/src/cmd/compile/internal/ssa/rewriteRISCV64.go
+++ b/src/cmd/compile/internal/ssa/rewriteRISCV64.go
@@ -7362,22 +7362,6 @@ func rewriteValueRISCV64_OpRISCV64SLTI(v *Value) bool {
 		v.AuxInt = int64ToAuxInt(1)
 		return true
 	}
-	// match: (SLTI [x] (ORI [y] _))
-	// cond: y >= 0 && int64(y) >= int64(x)
-	// result: (MOVDconst [0])
-	for {
-		x := auxIntToInt64(v.AuxInt)
-		if v_0.Op != OpRISCV64ORI {
-			break
-		}
-		y := auxIntToInt64(v_0.AuxInt)
-		if !(y >= 0 && int64(y) >= int64(x)) {
-			break
-		}
-		v.reset(OpRISCV64MOVDconst)
-		v.AuxInt = int64ToAuxInt(0)
-		return true
-	}
 	return false
 }
 func rewriteValueRISCV64_OpRISCV64SLTIU(v *Value) bool {
diff --git a/src/cmd/compile/internal/test/testdata/arith_test.go b/src/cmd/compile/internal/test/testdata/arith_test.go
index 8984cd3e268..34ac73c068d 100644
--- a/src/cmd/compile/internal/test/testdata/arith_test.go
+++ b/src/cmd/compile/internal/test/testdata/arith_test.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)