[PATCH, i386]: Avoid store forwarding stalls in assign_i386_stack_local

[PATCH, i386]: Avoid store forwarding stalls in assign_i386_stack_local

[Uros Bizjak]

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Hello!

This patch avoids store forwarding stalls in stack slots, created by 
assign_i386_stack_local. We use DImode slots to load DImode values into 
x87 reg usign fildll insn through the stack. When stored value crosses 
16byte boundary this causes store forwarding stall.

2008-03-22  Uros Bizjak  <ubizjak@gmail.com>

        * config/i386/i386.c (assign_386_stack_local): Align DImode slots
        to their natural alignment to avoid store forwarding stalls.

Patch is bootstrapped and regression tested on x86_64-pc-linux-gnu, 
committed to SVN.

Uros.

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Index: i386.c
===================================================================
--- i386.c	(revision 133445)
+++ i386.c	(working copy)
@@ -16315,6 +16315,7 @@ rtx
 assign_386_stack_local (enum machine_mode mode, enum ix86_stack_slot n)
 {
   struct stack_local_entry *s;
+  int align;
 
   gcc_assert (n < MAX_386_STACK_LOCALS);
 
@@ -16325,11 +16326,19 @@ assign_386_stack_local (enum machine_mod
     if (s->mode == mode && s->n == n)
       return copy_rtx (s->rtl);
 
+  /* Align DImode slots to their natural alignment
+     to avoid store forwarding stalls.  */
+  if (mode == DImode
+      && (GET_MODE_BITSIZE (mode) > GET_MODE_ALIGNMENT (mode)))
+    align = GET_MODE_BITSIZE (mode);
+  else
+    align = 0;
+
   s = (struct stack_local_entry *)
     ggc_alloc (sizeof (struct stack_local_entry));
   s->n = n;
   s->mode = mode;
-  s->rtl = assign_stack_local (mode, GET_MODE_SIZE (mode), 0);
+  s->rtl = assign_stack_local (mode, GET_MODE_SIZE (mode), align);
 
   s->next = ix86_stack_locals;
   ix86_stack_locals = s;