[PATCH] Fix IRA register preferencing

["Wilco Dijkstra" <Wilco dot Dijkstra at arm dot com>]

Ping of https://gcc.gnu.org/ml/gcc-patches/2014-12/msg00829.html:



This fixes a bug in register preferencing. When live range splitting creates
a new register from
another, it copies most fields except for the register preferences. The
preference GENERAL_REGS is
used as reg_pref[i].prefclass is initialized with GENERAL_REGS in
allocate_reg_info () and
resize_reg_info ().

This initialization value is not innocuous like the comment suggests - if a
new register has a
non-integer mode, it is forced to prefer GENERAL_REGS. This changes the
register costs in pass 2 so
that they are incorrect. As a result the liverange is either spilled or
allocated to an integer
register:

void g(double);
void f(double x) 
{ 
  if (x == 0) 
    return; 
  g (x);
  g (x); 
}

f:
        fcmp    d0, #0.0
        bne     .L6
        ret
.L6:
        stp     x19, x30, [sp, -16]!
        fmov    x19, d0
        bl      g
        fmov    d0, x19
        ldp     x19, x30, [sp], 16
        b       g

With the fix it uses a floating point register as expected. Given a similar
issue in
https://gcc.gnu.org/ml/gcc-patches/2014-11/msg02253.html, would it not be
better to change the
initialization values of reg_pref to illegal register classes so this kind
of issue can be trivially
found with an assert? Also would it not be a good idea to have a single
register copy function that
ensures all data is copied?


ChangeLog: 2014-12-09  Wilco Dijkstra  wdijkstr@arm.com

        * gcc/ira-emit.c (ira_create_new_reg): Copy preference classes.

---
 gcc/ira-emit.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/gcc/ira-emit.c b/gcc/ira-emit.c
index d246b7f..d736836 100644
--- a/gcc/ira-emit.c
+++ b/gcc/ira-emit.c
@@ -348,6 +348,7 @@ rtx
 ira_create_new_reg (rtx original_reg)
 {
   rtx new_reg;
+  int original_regno = REGNO (original_reg);
 
   new_reg = gen_reg_rtx (GET_MODE (original_reg));
   ORIGINAL_REGNO (new_reg) = ORIGINAL_REGNO (original_reg);
@@ -356,8 +357,16 @@ ira_create_new_reg (rtx original_reg)
   REG_ATTRS (new_reg) = REG_ATTRS (original_reg);
   if (internal_flag_ira_verbose > 3 && ira_dump_file != NULL)
     fprintf (ira_dump_file, "      Creating newreg=%i from oldreg=%i\n",
-            REGNO (new_reg), REGNO (original_reg));
+            REGNO (new_reg), original_regno);
   ira_expand_reg_equiv ();
+
+  /* Copy the preference classes to new_reg.  */
+  resize_reg_info ();
+  setup_reg_classes (REGNO (new_reg),
+                   reg_preferred_class (original_regno),
+                   reg_alternate_class (original_regno),
+                   reg_allocno_class (original_regno));
+
   return new_reg;
 }
 
-- 
1.9.1