compiler: fixed dead code eliminator

Instead of removing basic blocks with no predecessor, we will now mark
and remove all blocks that are unreachable from the entry block. This
can handle loops that are dead code. This is needed as we will now
generate more complicated code for exception handling which the old dead
code eliminator failed to handle.

artiq/compiler/ir.py

@@ -135,6 +135,7 @@ class NamedValue(Value):
     def __init__(self, typ, name):
         super().__init__(typ)
         self.name, self.function = name, None
+        self.is_removed = False
 
     def set_name(self, new_name):
         if self.function is not None:
@@ -235,7 +236,7 @@ class Instruction(User):
         self.drop_references()
         # Check this after drop_references in case this
         # is a self-referencing phi.
-        assert not any(self.uses)
+        assert all(use.is_removed for use in self.uses)
 
     def replace_with(self, value):
         self.replace_all_uses_with(value)
@@ -370,7 +371,7 @@ class BasicBlock(NamedValue):
         self.remove_from_parent()
         # Check this after erasing instructions in case the block
         # loops into itself.
-        assert not any(self.uses)
+        assert all(use.is_removed for use in self.uses)
 
     def prepend(self, insn):
         assert isinstance(insn, Instruction)
@@ -1360,14 +1361,6 @@ class LandingPad(Terminator):
     def cleanup(self):
         return self.operands[0]
 
-    def erase(self):
-        self.remove_from_parent()
-        # we should erase all clauses as well
-        for block in set(self.operands):
-            block.uses.remove(self)
-            block.erase()
-        assert not any(self.uses)
-
     def clauses(self):
         return zip(self.operands[1:], self.types)
 

artiq/compiler/transforms/dead_code_eliminator.py

@@ -15,13 +15,26 @@ class DeadCodeEliminator:
             self.process_function(func)
 
     def process_function(self, func):
-        modified = True
-        while modified:
-            modified = False
-            for block in list(func.basic_blocks):
-                if not any(block.predecessors()) and block != func.entry():
-                    self.remove_block(block)
-                    modified = True
+        # defer removing those blocks, so our use checks will ignore deleted blocks
+        preserve = [func.entry()]
+        work_list = [func.entry()]
+        while any(work_list):
+            block = work_list.pop()
+            for succ in block.successors():
+                if succ not in preserve:
+                    preserve.append(succ)
+                    work_list.append(succ)
+
+        to_be_removed = []
+        for block in func.basic_blocks:
+            if block not in preserve:
+                block.is_removed = True
+                to_be_removed.append(block)
+                for insn in block.instructions:
+                    insn.is_removed = True
+
+        for block in to_be_removed:
+            self.remove_block(block)
 
         modified = True
         while modified:
@@ -42,6 +55,8 @@ class DeadCodeEliminator:
     def remove_block(self, block):
         # block.uses are updated while iterating
         for use in set(block.uses):
+            if use.is_removed:
+                continue
             if isinstance(use, ir.Phi):
                 use.remove_incoming_block(block)
                 if not any(use.operands):
@@ -56,6 +71,8 @@ class DeadCodeEliminator:
 
     def remove_instruction(self, insn):
         for use in set(insn.uses):
+            if use.is_removed:
+                continue
             if isinstance(use, ir.Phi):
                 use.remove_incoming_value(insn)
                 if not any(use.operands):