Add compiler optimization to get rid of IR-FETCHVARs when unneeded

It replaces the IR-FETCHVAR's result with the variable being
fetched. This only works when the only use is in an operation that
does not require a separate fetch be performed, such as those
implemented as CPU instructions.

wip-duuqnd/user-side-compiler/middle/optimizations.lisp

@@ -169,6 +169,31 @@ though I'm pretty sure it can't anyway.")
           (loop :for (new . old) :in replacements
                 :do (setf (inputs call) (substitute new old (inputs call)))))))))
 
+(defun optim-direct-variable-use (iblock)
+  "Removes unnecessary uses of IR-FETCHVAR.
+Some operations do not require their operands to be copied before use. CPU
+instructions, for example. This optimization pass goes through and removes
+IR-FETCHVAR instructions that would serve no purpose in compiled code."
+  ;; TODO: Add more instructions to the candidates
+  (let ((candidates-type '(or ir-test-equal ir-plus ir-minus))
+        (to-remove '()))
+    (do-instructions (inst iblock)
+      #+(or)
+      (when (equalp (name iblock) "else")
+        (break "~A ~A" inst
+               (if (typep inst 'ir-fetchvar)
+                   (format nil "~A ~A ~A" (input inst) (output inst) (users (output inst)))
+                   "")))
+      (when (typep inst 'ir-fetchvar)
+        (let ((result (output inst))
+              (src (input inst)))
+          (when (and (not (typep (output inst) 'ir-reusable))
+                     (typep (user result) candidates-type))
+            (let ((user (user (output inst))))
+              (setf (inputs user) (substitute src result (inputs user)))
+              (push inst to-remove))))))
+    (mapc #'delete-instruction to-remove)))
+
 (defun compute-lifetime-knowledge (start-iblock)
   (do-iblocks (iblock start-iblock)
     (do-instructions (inst iblock)