From fdfcd8bd40eef875b2fb298117b59aa08599eab1 Mon Sep 17 00:00:00 2001
From: Benedikt Meurer <benedikt.meurer@googlemail.com>
Date: Sat, 20 Aug 2011 19:15:28 +0200
Subject: [PATCH] Improve Cmm code generator for "for" loops.

The code generated by cmmgen for Ufor loops doesn't optimize in case of
constant low and high expressions. This is fixed by a special case where
the initial comparison can be skipped in case of constant low and high
subexpressions.

In addition, this commit also reorders the generated code by moving the
initial comparison out of the catch block in the general case. This may
enable certain optimizations in later phases.
---
 asmcomp/cmmgen.ml |   49 ++++++++++++++++++++++++++++---------------------
 1 files changed, 28 insertions(+), 21 deletions(-)

diff --git a/asmcomp/cmmgen.ml b/asmcomp/cmmgen.ml
index b8589d1..3ee5c5f 100644
--- a/asmcomp/cmmgen.ml
+++ b/asmcomp/cmmgen.ml
@@ -1028,27 +1028,34 @@ let rec transl = function
   | Ufor(id, low, high, dir, body) ->
       let tst = match dir with Upto -> Cgt   | Downto -> Clt in
       let inc = match dir with Upto -> Caddi | Downto -> Csubi in
-      let raise_num = next_raise_count () in
-      let id_prev = Ident.rename id in
-      return_unit
-        (Clet
-           (id, transl low,
-            bind_nonvar "bound" (transl high) (fun high ->
-              Ccatch
-                (raise_num, [],
-                 Cifthenelse
-                   (Cop(Ccmpi tst, [Cvar id; high]), Cexit (raise_num, []),
-                    Cloop
-                      (Csequence
-                         (remove_unit(transl body),
-                         Clet(id_prev, Cvar id,
-                          Csequence
-                            (Cassign(id,
-                               Cop(inc, [Cvar id; Cconst_int 2])),
-                             Cifthenelse
-                               (Cop(Ccmpi Ceq, [Cvar id_prev; high]),
-                                Cexit (raise_num,[]), Ctuple [])))))),
-                 Ctuple []))))
+      let loop high =
+        let raise_num = next_raise_count () in
+        Ccatch
+          (raise_num, [],
+           Cloop
+             (Csequence
+                (remove_unit (transl body),
+                   (Cifthenelse
+                      (Cop(Ccmpi Ceq, [Cvar id; high]),
+                       Cexit(raise_num, []),
+                       Cassign(id, Cop(inc, [Cvar id; Cconst_int 2])))))),
+           Ctuple [])
+      in begin match transl low, transl high with
+        (Cconst_int l as low), (Cconst_int h as high) ->
+          if (dir = Upto && l <= h) || (dir = Downto && l >= h) then
+            return_unit (Clet(id, low, loop high))
+          else
+            return_unit (Ctuple [])
+      | low, high ->
+          return_unit
+            (Clet
+               (id, low,
+                bind_nonvar "bound" high (fun high ->
+                  Cifthenelse
+                    (Cop(Ccmpi tst, [Cvar id; high]),
+                     Ctuple [],
+                     loop high))))
+      end
   | Uassign(id, exp) ->
       return_unit(Cassign(id, transl exp))
 
-- 
1.7.4.4