fix[venom]: fix store elimination pass (#4428)

this commit fixes the store elimination pass by updating the dfg
in-place instead of relying on a stale dfg.

this currently results in no bytecode changes. previously this
was undetected because the order of items in the dfg happens to be
"well-behaved", but if the dfg is built using a traversal of basic
blocks in a different order (as may happen in upcoming passes), it can
result in store instructions failing to be eliminated.

note that we haven't rebuilt the dfg properly because `dfg.outputs`
is invalid after this pass. we could modify `dfg.outputs` in place, but
that results in a bytecode regression.

this commit also removes the dependency on CFGAnalysis as it is not
actually needed by the pass.

---------

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

vyper/venom/passes/store_elimination.py

@@ -1,4 +1,4 @@
-from vyper.venom.analysis import CFGAnalysis, DFGAnalysis, LivenessAnalysis
+from vyper.venom.analysis import DFGAnalysis, LivenessAnalysis
 from vyper.venom.basicblock import IRVariable
 from vyper.venom.passes.base_pass import IRPass
 
@@ -13,31 +13,33 @@ class StoreElimination(IRPass):
     # with LoadElimination
 
     def run_pass(self):
-        self.analyses_cache.request_analysis(CFGAnalysis)
-        dfg = self.analyses_cache.request_analysis(DFGAnalysis)
+        self.dfg = self.analyses_cache.request_analysis(DFGAnalysis)
 
-        for var, inst in dfg.outputs.items():
+        for var, inst in self.dfg.outputs.items():
             if inst.opcode != "store":
                 continue
-            self._process_store(dfg, inst, var, inst.operands[0])
+            self._process_store(inst, var, inst.operands[0])
 
         self.analyses_cache.invalidate_analysis(LivenessAnalysis)
         self.analyses_cache.invalidate_analysis(DFGAnalysis)
 
-    def _process_store(self, dfg, inst, var: IRVariable, new_var: IRVariable):
+    def _process_store(self, inst, var: IRVariable, new_var: IRVariable):
         """
         Process store instruction. If the variable is only used by a load instruction,
         forward the variable to the load instruction.
         """
-        if any([inst.opcode == "phi" for inst in dfg.get_uses(new_var)]):
+        if any([inst.opcode == "phi" for inst in self.dfg.get_uses(new_var)]):
             return
 
-        uses = dfg.get_uses(var)
+        uses = self.dfg.get_uses(var)
         if any([inst.opcode == "phi" for inst in uses]):
             return
-        for use_inst in uses:
+        for use_inst in uses.copy():
             for i, operand in enumerate(use_inst.operands):
                 if operand == var:
                     use_inst.operands[i] = new_var
 
+            self.dfg.add_use(new_var, use_inst)
+            self.dfg.remove_use(var, use_inst)
+
         inst.parent.remove_instruction(inst)