Add addi-zero assembly peephole rule (Topic 13)

benchmarks/fixtures/peephole_demo.asm

@@ -0,0 +1,13 @@
+# Peephole optimizer demo (Topic 13)
+# Run: python3 -m scratchv.backend.asm_peephole benchmarks/fixtures/peephole_demo.asm --report
+
+.text
+main:
+    li a0, 5
+    addi a0, a0, 3          # li+addi fusion -> li a0, 8
+    addi a0, a0, 0          # addi zero elimination -> deleted
+    mv t0, t1
+    mv t2, t0
+    beq x0, x0, .Ldone       # beq zero-zero -> j .Ldone
+.Ldone:
+    ret

scratchv/backend/asm_peephole.py

@@ -192,8 +192,16 @@ def _lines_to_asm(lines: list[AsmLine]) -> str:
 # ---------------------------------------------------------------------------
 
 def _default_rules() -> list[PeepholeRule]:
-    """Return the set of five default peephole optimization rules."""
+    """Return the set of default peephole optimization rules."""
     return [
+        # Rule 0: addi x, x, 0 -> delete (no-op increment)
+        PeepholeRule(
+            name="addi zero elimination",
+            pattern=["addi"],
+            replacement=[],
+            register_constraints=[(0, 0, 1)],
+        ),
+
         # Rule 1: addi x, x, a; addi x, x, b -> addi x, x, a+b
         PeepholeRule(
             name="addi+addi fusion",
@@ -326,6 +334,12 @@ def _match_rule(
         if ops[0] not in ("x0", "zero") or ops[1] not in ("x0", "zero"):
             return None
 
+    # addi rd, rd, 0 is a no-op
+    if rule.name == "addi zero elimination":
+        ops = window[0].operands
+        if len(ops) < 3 or ops[2] != "0":
+            return None
+
     return bindings
 
 
@@ -339,7 +353,7 @@ class AsmPeepholeOptimizer:
     Parameters
     ----------
     rules:
-        List of peephole rules. If None, uses the five default rules.
+        List of peephole rules. If None, uses the default rule set.
 
     Usage::
 

tests/test_asm_peephole.py

@@ -35,7 +35,22 @@ def test_roundtrip(self):
 
 
 class TestDefaultRules:
-    """Tests for the five default peephole rules."""
+    """Tests for the default peephole rules."""
+
+    def test_addi_zero_elimination(self):
+        optimizer = AsmPeepholeOptimizer()
+        asm = "  addi t0, t0, 0\n  add t1, t2, t3\n"
+        result, changes = optimizer.optimize(asm)
+        assert changes >= 1
+        assert "addi" not in result
+        assert "add" in result
+
+    def test_addi_zero_elimination_preserves_nonzero(self):
+        optimizer = AsmPeepholeOptimizer()
+        asm = "  addi t0, t0, 1\n  add t1, t2, t3\n"
+        result, changes = optimizer.optimize(asm)
+        assert "addi t0, t0, 1" in result or "addi" in result
+        assert "add" in result
 
     def test_addi_addi_fusion(self):
         optimizer = AsmPeepholeOptimizer()