Add dead code elimination and folding of constant conditionals

user-side-compiler/interface.lisp

@@ -66,8 +66,12 @@
   (do-iblocks (ib start-iblock)
     (optim-reorder-arguments ib)
     (optim-trivial-constant-folding ib)
+    (optim-constant-conditional ib)
     (optim-call-duplicate-args ib)
-    (optim-remove-unused ib)))
+    (optim-remove-unused-instructions ib))
+  (optim-dead-code-elimination start-iblock)
+  (do-iblocks (ib start-iblock)
+    (optim-remove-unused-instructions ib)))
 
 (defun ci-allocation-info (allocs)
   (format *debug-io* "Values: ~D~%" (length allocs))

user-side-compiler/middle/optimizations.lisp

@@ -58,6 +58,22 @@
              inst)
             (delete-instruction inst)))))))
 
+(defun optim-constant-conditional (iblock)
+  (do-instructions (inst iblock)
+    (when (and (typep inst 'ir-if) (ir-constant-p (input inst)))
+      (let ((then-block (first (destinations inst)))
+            (else-block (second (destinations inst)))
+            (falsep (zerop (ir-constant-value (input inst)))))
+        (replace-terminator
+         inst
+         (make-instance 'ir-jump :destinations (list (if falsep
+                                                         else-block
+                                                         then-block))
+                                 :source (source inst)))
+        (dolist (input (inputs inst))
+          (when (typep input 'ir-data)
+            (remove-user input inst)))))))
+
 (defun optim-commutative-constant-folding (iblock)
   "Attempts to replace operations with compile-time computed constants."
   (do-instructions (inst iblock)
@@ -161,7 +177,7 @@ that uses them to assist in generating more direct 6502 code."
 before being cut off. This ensures that it can't get stuck forever, even
 though I'm pretty sure it can't anyway.")
 
-(defun optim-remove-unused (iblock)
+(defun optim-remove-unused-instructions (iblock)
   (let ((to-delete '()))
     (loop :repeat +optim-remove-unused-max-passes+ ; this many times or fewer
           :do (setf to-delete '())
@@ -174,6 +190,16 @@ though I'm pretty sure it can't anyway.")
               (mapc 'delete-instruction to-delete)
           :until (null to-delete))))
 
+(defun optim-dead-code-elimination (start-iblock)
+  (let ((reachable (list start-iblock)))
+    (do-iblocks (iblock start-iblock)
+      (dolist (dest (destinations (end iblock)))
+        (push dest reachable)))
+    (do-iblocks (iblock start-iblock)
+      (unless (member iblock reachable)
+        (setf (next (prev iblock))
+              (next iblock))))))
+
 (defun optim-call-duplicate-args (iblock)
   "Attempts to deduplicate call arguments."
   (let ((calls '()))