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

[jakub at gcc dot gnu dot org]

------- Comment #32 from jakub at gcc dot gnu dot org  2009-03-03 10:36 -------
I don't see the reason for && optimize_function_for_size_p (cfun), care to back
up with benchmarks that forcing dynamic realignment for long long variables
with -mpreferred-stack-boundary=2 improves performance rather than slows things
down (because of the dynamic realignment)?

Also, I fail to see why 2 hunks in ix86_local_alignment are needed instead of
just one.  The second hunk won't catch !type case, where we have just mode (but
no need to test type && there, type is always non-NULL).

I think:
--- i386.c2009-03-02 09:45:43.000000000 +0100
+++ i386.c2009-03-03 11:35:21.000000000 +0100
@@ -19351,6 +19351,14 @@ unsigned int
 ix86_local_alignment (tree type, enum machine_mode mode,
       unsigned int align)
 {
+  /* Don't do dynamic stack realignment for long long objects with
+     -mpreferred-stack-boundary=2.  */
+  if (!TARGET_64BIT
+      && align == 64
+      && ix86_preferred_stack_boundary < 64 
+      && (mode == DImode || (type && TYPE_MODE (type) == DImode)))
+    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.  */

should be sufficient.


-- 


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