dialects: (riscv) Add rewrite pattern to optimize bitwise xor by zero (…

…#3197)

Add a rewrite pattern to the risv dialect for to optimise bitwise xor by
zero (x^0 -> x)

---------

Co-authored-by: emmau678 <eu233@Emma-laptop>

GETTING_STARTED.md

@@ -47,7 +47,6 @@ You're welcome to come up with your own, or do one of the following:
 - `x * 2ⁱ -> x << i`
 - `x & 0 -> 0`
 - `x | 0 -> x`
-- `x ^ 0 -> x`
 
 The patterns are defined in
 [xdsl/transforms/canonicalization_patterns/riscv.py](xdsl/transforms/canonicalization_patterns/riscv.py).

tests/filecheck/backend/riscv/canonicalize.mlir

@@ -111,6 +111,12 @@ builtin.module {
   %xor_lhs_rhs = riscv.xor %i1, %i1 : (!riscv.reg<a1>, !riscv.reg<a1>) -> !riscv.reg<a0>
   "test.op"(%xor_lhs_rhs) : (!riscv.reg<a0>) -> ()
 
+  %xor_bitwise_zero_l0 = riscv.xor %c1, %c0 : (!riscv.reg, !riscv.reg) -> !riscv.reg<a0>
+  "test.op"(%xor_bitwise_zero_l0) : (!riscv.reg<a0>) -> ()
+
+  %xor_bitwise_zero_r0 = riscv.xor %c0, %c1 : (!riscv.reg, !riscv.reg) -> !riscv.reg<a0>
+  "test.op"(%xor_bitwise_zero_r0) : (!riscv.reg<a0>) -> ()
+
   // scfgw immediates
   riscv_snitch.scfgw %i1, %c1 : (!riscv.reg<a1>, !riscv.reg) -> ()
 }
@@ -221,6 +227,12 @@ builtin.module {
 // CHECK-NEXT:   %xor_lhs_rhs_1 = riscv.mv %xor_lhs_rhs : (!riscv.reg<zero>) -> !riscv.reg<a0>
 // CHECK-NEXT:   "test.op"(%xor_lhs_rhs_1) : (!riscv.reg<a0>) -> ()
 
+// CHECK-NEXT:   %xor_bitwise_zero_l0 = riscv.mv %c1 : (!riscv.reg) -> !riscv.reg<a0>
+// CHECK-NEXT:   "test.op"(%xor_bitwise_zero_l0) : (!riscv.reg<a0>) -> ()
+
+// CHECK-NEXT:   %xor_bitwise_zero_r0 = riscv.mv %c1 : (!riscv.reg) -> !riscv.reg<a0>
+// CHECK-NEXT:   "test.op"(%xor_bitwise_zero_r0) : (!riscv.reg<a0>) -> ()
+
 // CHECK-NEXT:   riscv_snitch.scfgwi %i1, 1 : (!riscv.reg<a1>) -> ()
 
 // CHECK-NEXT: }

xdsl/dialects/riscv.py

@@ -1777,9 +1777,12 @@ class OrOp(RdRsRsOperation[IntRegisterType, IntRegisterType, IntRegisterType]):
 class BitwiseXorHasCanonicalizationPatternsTrait(HasCanonicalizationPatternsTrait):
     @classmethod
     def get_canonicalization_patterns(cls) -> tuple[RewritePattern, ...]:
-        from xdsl.transforms.canonicalization_patterns.riscv import XorBySelf
+        from xdsl.transforms.canonicalization_patterns.riscv import (
+            BitwiseXorByZero,
+            XorBySelf,
+        )
 
-        return (XorBySelf(),)
+        return (XorBySelf(), BitwiseXorByZero())
 
 
 @irdl_op_definition

xdsl/transforms/canonicalization_patterns/riscv.py

@@ -419,6 +419,21 @@ def match_and_rewrite(self, op: riscv.XorOp, rewriter: PatternRewriter):
             )
 
 
+class BitwiseXorByZero(RewritePattern):
+    @op_type_rewrite_pattern
+    def match_and_rewrite(self, op: riscv.XorOp, rewriter: PatternRewriter):
+        """
+        x ^ 0 = x
+        """
+        if (rs1 := get_constant_value(op.rs1)) is not None and rs1.value.data == 0:
+            rd = cast(riscv.IntRegisterType, op.rd.type)
+            rewriter.replace_matched_op(riscv.MVOp(op.rs2, rd=rd))
+
+        if (rs2 := get_constant_value(op.rs2)) is not None and rs2.value.data == 0:
+            rd = cast(riscv.IntRegisterType, op.rd.type)
+            rewriter.replace_matched_op(riscv.MVOp(op.rs1, rd=rd))
+
+
 class ScfgwOpUsingImmediate(RewritePattern):
     @op_type_rewrite_pattern
     def match_and_rewrite(