fix[venom]: fix invalid phis after SCCP (vyperlang#4181)

This commit reduces `phi` nodes which, after SCCP, refer to a
basic block which is unreachable.  This could happen when the
SCCP would replace a `jnz` by a `jmp` instruction, resulting in a
`phi` instruction which refers to a non-existent block, or a `phi`
instruction in a basic block which only has one predecessor.  This
commit reduces such `phi` instructions into `store` instructions.

---------

Co-authored-by: Harry Kalogirou <[email protected]>
Co-authored-by: Charles Cooper <[email protected]>

tests/unit/compiler/venom/test_sccp.py

@@ -211,3 +211,34 @@ def test_cont_phi_const_case():
     assert sccp.lattice[IRVariable("%5", version=1)].value == 106
     assert sccp.lattice[IRVariable("%5", version=2)].value == 97
     assert sccp.lattice[IRVariable("%5")].value == 2
+
+
+def test_phi_reduction_after_unreachable_block():
+    ctx = IRContext()
+    fn = ctx.create_function("_global")
+
+    bb = fn.get_basic_block()
+
+    br1 = IRBasicBlock(IRLabel("then"), fn)
+    fn.append_basic_block(br1)
+    join = IRBasicBlock(IRLabel("join"), fn)
+    fn.append_basic_block(join)
+
+    op = bb.append_instruction("store", 1)
+    true = IRLiteral(1)
+    bb.append_instruction("jnz", true, br1.label, join.label)
+
+    op1 = br1.append_instruction("store", 2)
+
+    br1.append_instruction("jmp", join.label)
+
+    join.append_instruction("phi", bb.label, op, br1.label, op1)
+    join.append_instruction("stop")
+
+    ac = IRAnalysesCache(fn)
+    SCCP(ac, fn).run_pass()
+
+    assert join.instructions[0].opcode == "store", join.instructions[0]
+    assert join.instructions[0].operands == [op1]
+
+    assert join.instructions[1].opcode == "stop"

tests/unit/compiler/venom/test_simplify_cfg.py

@@ -0,0 +1,49 @@
+from vyper.venom.analysis.analysis import IRAnalysesCache
+from vyper.venom.basicblock import IRBasicBlock, IRLabel, IRLiteral
+from vyper.venom.context import IRContext
+from vyper.venom.passes.sccp import SCCP
+from vyper.venom.passes.simplify_cfg import SimplifyCFGPass
+
+
+def test_phi_reduction_after_block_pruning():
+    ctx = IRContext()
+    fn = ctx.create_function("_global")
+
+    bb = fn.get_basic_block()
+
+    br1 = IRBasicBlock(IRLabel("then"), fn)
+    fn.append_basic_block(br1)
+    br2 = IRBasicBlock(IRLabel("else"), fn)
+    fn.append_basic_block(br2)
+
+    join = IRBasicBlock(IRLabel("join"), fn)
+    fn.append_basic_block(join)
+
+    true = IRLiteral(1)
+    bb.append_instruction("jnz", true, br1.label, br2.label)
+
+    op1 = br1.append_instruction("store", 1)
+    op2 = br2.append_instruction("store", 2)
+
+    br1.append_instruction("jmp", join.label)
+    br2.append_instruction("jmp", join.label)
+
+    join.append_instruction("phi", br1.label, op1, br2.label, op2)
+    join.append_instruction("stop")
+
+    ac = IRAnalysesCache(fn)
+    SCCP(ac, fn).run_pass()
+    SimplifyCFGPass(ac, fn).run_pass()
+
+    bbs = list(fn.get_basic_blocks())
+
+    assert len(bbs) == 1
+    final_bb = bbs[0]
+
+    inst0, inst1, inst2 = final_bb.instructions
+
+    assert inst0.opcode == "store"
+    assert inst0.operands == [IRLiteral(1)]
+    assert inst1.opcode == "store"
+    assert inst1.operands == [inst0.output]
+    assert inst2.opcode == "stop"

vyper/venom/passes/sccp/sccp.py

@@ -56,14 +56,18 @@ class SCCP(IRPass):
     uses: dict[IRVariable, OrderedSet[IRInstruction]]
     lattice: Lattice
     work_list: list[WorkListItem]
-    cfg_dirty: bool
     cfg_in_exec: dict[IRBasicBlock, OrderedSet[IRBasicBlock]]
 
+    cfg_dirty: bool
+    # list of basic blocks whose cfg_in was modified
+    cfg_in_modified: OrderedSet[IRBasicBlock]
+
     def __init__(self, analyses_cache: IRAnalysesCache, function: IRFunction):
         super().__init__(analyses_cache, function)
         self.lattice = {}
         self.work_list: list[WorkListItem] = []
         self.cfg_dirty = False
+        self.cfg_in_modified = OrderedSet()
 
     def run_pass(self):
         self.fn = self.function
@@ -74,7 +78,9 @@ def run_pass(self):
 
         # self._propagate_variables()
 
-        self.analyses_cache.invalidate_analysis(CFGAnalysis)
+        if self.cfg_dirty:
+            self.analyses_cache.force_analysis(CFGAnalysis)
+            self._fix_phi_nodes()
 
     def _calculate_sccp(self, entry: IRBasicBlock):
         """
@@ -304,7 +310,11 @@ def _replace_constants(self, inst: IRInstruction):
                     target = inst.operands[1]
                 inst.opcode = "jmp"
                 inst.operands = [target]
+
                 self.cfg_dirty = True
+                for bb in inst.parent.cfg_out:
+                    if bb.label == target:
+                        self.cfg_in_modified.add(bb)
 
         elif inst.opcode in ("assert", "assert_unreachable"):
             lat = self._eval_from_lattice(inst.operands[0])
@@ -329,6 +339,34 @@ def _replace_constants(self, inst: IRInstruction):
                 if isinstance(lat, IRLiteral):
                     inst.operands[i] = lat
 
+    def _fix_phi_nodes(self):
+        # fix basic blocks whose cfg in was changed
+        # maybe this should really be done in _visit_phi
+        needs_sort = False
+
+        for bb in self.fn.get_basic_blocks():
+            cfg_in_labels = OrderedSet(in_bb.label for in_bb in bb.cfg_in)
+
+            for inst in bb.instructions:
+                if inst.opcode != "phi":
+                    break
+                needs_sort |= self._fix_phi_inst(inst, cfg_in_labels)
+
+        # move phi instructions to the top of the block
+        if needs_sort:
+            bb.instructions.sort(key=lambda inst: inst.opcode != "phi")
+
+    def _fix_phi_inst(self, inst: IRInstruction, cfg_in_labels: OrderedSet):
+        operands = [op for label, op in inst.phi_operands if label in cfg_in_labels]
+
+        if len(operands) != 1:
+            return False
+
+        assert inst.output is not None
+        inst.opcode = "store"
+        inst.operands = operands
+        return True
+
 
 def _meet(x: LatticeItem, y: LatticeItem) -> LatticeItem:
     if x == LatticeEnum.TOP:

vyper/venom/passes/simplify_cfg.py

@@ -9,23 +9,21 @@ class SimplifyCFGPass(IRPass):
     visited: OrderedSet
 
     def _merge_blocks(self, a: IRBasicBlock, b: IRBasicBlock):
-        a.instructions.pop()
+        a.instructions.pop()  # pop terminating instruction
         for inst in b.instructions:
-            assert inst.opcode != "phi", "Not implemented yet"
-            if inst.opcode == "phi":
-                a.instructions.insert(0, inst)
-            else:
-                inst.parent = a
-                a.instructions.append(inst)
+            assert inst.opcode != "phi", f"Instruction should never be phi {b}"
+            inst.parent = a
+            a.instructions.append(inst)
 
         # Update CFG
         a.cfg_out = b.cfg_out
-        if len(b.cfg_out) > 0:
-            next_bb = b.cfg_out.first()
+
+        for next_bb in a.cfg_out:
             next_bb.remove_cfg_in(b)
             next_bb.add_cfg_in(a)
 
             for inst in next_bb.instructions:
+                # assume phi instructions are at beginning of bb
                 if inst.opcode != "phi":
                     break
                 inst.operands[inst.operands.index(b.label)] = a.label