[Bug target/39137] [4.4 Regression] -mpreferred-stack-boundary=2 causes lots of dynamic realign

["hubicka at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #26 from hubicka at gcc dot gnu dot org  2009-02-21 13:00 -------
I had somehting along this lines in mind:
Index: config/i386/i386.c
===================================================================
*** config/i386/i386.c  (revision 144352)
--- config/i386/i386.c  (working copy)
*************** unsigned int
*** 19332,19337 ****
--- 19332,19343 ----
  ix86_local_alignment (tree type, enum machine_mode mode,
                      unsigned int align)
  {
+   /* We don't want to align DImode to 64bit for compilation with
+      -mpreferred-stack-boundary=2 to not enforce dynamic stack alignment
+      prologue.  */
+   if (mode == DImode && !TARGET_64BIT && ix86_preferred_stack_boundary < 64)
+     align = 32;
+ 
    /* If TYPE is NULL, we are allocating a stack slot for caller-save
       register in MODE.  We will return the largest alignment of XF
       and DF.  */

this will reduce alignment of long long as stack local variable. Because we
want to keep it aligned for DI->DF, I can do that only for
-mpreferred-stack-boundary=2 for now and perhaps on mainline we can invent new
macro PREFERRED_LOCAL_ALIGN that will return 64 and cfgexpand will allocate
64byte aligned slot but not increase alignment_needed (i.e. control slot
alignment via PREFERRED_LOCAL_ALIGN and stack alignment otherwise) and we will
increase the alignment when expanding full sized DImode instruction?

Honza


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39137