genconfig.c (main): Disable HAVE_conditional_arithmetic.

        * genconfig.c (main): Disable HAVE_conditional_arithmetic.
        * jump.c (jump_optimize_1): Document why.

From-SVN: r29841

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 2bba07c..67ad315 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+Wed Oct  6 10:21:15 1999  Richard Henderson  <rth@cygnus.com>
+
+       * genconfig.c (main): Disable HAVE_conditional_arithmetic.
+       * jump.c (jump_optimize_1): Document why.
+
 Wed Oct  6 10:41:56 1999  Kaveh R. Ghazi  <ghazi@caip.rutgers.edu>
 
        * collect2.c (scan_prog_file, scan_libraries): Prototype function

diff --git a/gcc/genconfig.c b/gcc/genconfig.c
index 23bffea..bd31d73 100644 (file)
--- a/gcc/genconfig.c
+++ b/gcc/genconfig.c
@@ -348,8 +348,11 @@ from the machine description file `md'.  */\n\n");
   if (have_cmove_flag)
     printf ("#define HAVE_conditional_move\n");
 
+#if 0
+  /* Disabled.  See the discussion in jump.c.  */
   if (have_cond_arith_flag)
     printf ("#define HAVE_conditional_arithmetic\n");
+#endif
 
   if (have_lo_sum_flag)
     printf ("#define HAVE_lo_sum\n");

diff --git a/gcc/jump.c b/gcc/jump.c
index dcd1a6a..f6e973e 100644 (file)
--- a/gcc/jump.c
+++ b/gcc/jump.c
@@ -943,6 +943,37 @@ jump_optimize_1 (f, cross_jump, noop_moves, after_regscan, mark_labels_only)
 #endif /* HAVE_cc0 */
 
 #ifdef HAVE_conditional_arithmetic
+         /* ??? This is disabled in genconfig, as this simple-minded
+            transformation can incredibly lengthen register lifetimes.
+
+            Consider this example from cexp.c's yyparse:
+
+               234 (set (pc)
+                     (if_then_else (ne (reg:DI 149) (const_int 0 [0x0]))
+                       (label_ref 248) (pc)))
+               237 (set (reg/i:DI 0 $0) (const_int 1 [0x1]))
+               239 (set (pc) (label_ref 2382))
+               248 (code_label ("yybackup"))
+
+            This will be transformed to:
+
+               237 (set (reg/i:DI 0 $0)
+                     (if_then_else:DI (eq (reg:DI 149) (const_int 0 [0x0]))
+                       (const_int 1 [0x1]) (reg/i:DI 0 $0)))
+               239 (set (pc)
+                     (if_then_else (eq (reg:DI 149) (const_int 0 [0x0]))
+                       (label_ref 2382) (pc)))
+
+            which, from this narrow viewpoint looks fine.  Except that
+            between this and 3 other ocurrences of the same pattern, $0
+            is now live for basically the entire function, and we'll 
+            get an abort in caller_save.
+
+            Any replacement for this code should recall that a set of
+            a register that is not live need not, and indeed should not,
+            be conditionalized.  Either that, or delay the transformation
+            until after register allocation.  */
+
          /* See if this is a conditional jump around a small number of
             instructions that we can conditionalize.  Don't do this before
             the initial CSE pass or after reload.