venom produces unneeded stack traffic

[charles-cooper]

Version Information

• vyper Version (output of vyper --version): 074073f

What's your issue about?

sometimes, venom produces some unneeded stack traffic. this can't be fixed by improved dataflow analysis, because it is an issue in the stack scheduler rather than instruction reordering. what happens is a stack item is produced, but it has a longer lifespan than other things on the stack. i think a lot of it can be solved with a simple heuristic: at the time of stack item production, check the live list for something with "shorter" liveness we can swap it with.

How can it be fixed?

i think something like this does what we want (but in its current form it is borked, it causes some larger contracts to puke):

diff --git a/vyper/venom/venom_to_assembly.py b/vyper/venom/venom_to_assembly.py
index dd4901aaa..8104c205e 100644
--- a/vyper/venom/venom_to_assembly.py
+++ b/vyper/venom/venom_to_assembly.py
@@ -16,6 +16,7 @@ from vyper.venom.analysis import (
     calculate_dup_requirements,
     calculate_liveness,
     input_vars_from,
+    DFG,
 )
 from vyper.venom.basicblock import (
     IRBasicBlock,
@@ -156,6 +157,7 @@ class VenomCompiler:
             calculate_cfg(ctx)
             calculate_liveness(ctx)
             calculate_dup_requirements(ctx)
+            self._dfg = DFG.build_dfg(ctx)
 
             assert ctx.normalized, "Non-normalized CFG!"
 
@@ -532,8 +534,16 @@ class VenomCompiler:
 
         # Step 6: Emit instructions output operands (if any)
         if inst.output is not None:
-            if "call" in inst.opcode and inst.output not in next_liveness:
+            if inst.output not in next_liveness:
                 self.pop(assembly, stack)
+            else:
+                # peek at next_liveness to find the next scheduled item,
+                # and optimistically swap with it
+                bb = inst.parent
+                for s in reversed(list(next_liveness)):
+                    if all(t.parent == bb for t in self._dfg.get_uses(s)):
+                        self.swap_op(assembly, stack, s)
+                        break
 
         return apply_line_numbers(inst, assembly)

[charles-cooper]

completed in #3951