> Sorry for the late notice, but this regressed memcpy-1.c for n32 and n64
> on mips64-linux-gnu. I realise memcpy-1.c isn't viewed as being a proper
> SRA test, but in this case I think it really is showing a genuine problem.
> We have:
>
> struct a {int a,b,c;} a;
> int test(struct a a)
> {
> struct a nasty_local;
> __builtin_memcpy (&nasty_local,&a, sizeof(a));
> return nasty_local.a;
> }
>
> We apply LOCAL_ALIGNMENT to nasty_local during estimated_stack_frame_size,
> so we have a VAR_DECL (nasty_local) with 64-bit alignment and a PARM_DECL
> (a) with 32-bit alignment. This fails the condition:
>
> if (STRICT_ALIGNMENT
> && tree_non_aligned_mem_p (rhs, get_object_alignment (lhs)))
> lacc->grp_unscalarizable_region = 1;
>
> because LHS has 64-bit alignment but RHS has 32-bit alignment.
Do you mean that the patch pessimizes this case? If so, yes, this is known, I
ran into similar cases in Ada (we do this kind of local alignment promotion).
The patch is conservative but, given the number of problems we have with SRA on
strict-alignment platforms, we need to draw a line somewhere. But if you think
that optimizing this kind of memcpy calls is worth the hassle, then let's try
harder. We have:
MEM[(char * {ref-all})&nasty_local] = MEM[(char * {ref-all})&a];
and SRA now refuses to scalarize this on the ground that the LHS has 64-bit
alignment so SRA could generate a 64-bit aligned load-store, which would break
since the RHS only has 32-bit alignment. This is meant for cases like:
struct a {int a,b,c;} a;
struct __attribute__((packed)) b { struct a f; }
int test(struct b b)
{
struct a nasty_local;
__builtin_memcpy (&nasty_local, &b.f, sizeof(a));
return nasty_local.a;
}
where you cannot scalarize (even without alignment promotion). I can propose
an alignment cap to that of the access type:
Index: tree-sra.c
===================================================================
--- tree-sra.c (revision 182780)
+++ tree-sra.c (working copy)
@@ -1124,7 +1124,9 @@ build_accesses_from_assign (gimple stmt)
{
lacc->grp_assignment_write = 1;
if (STRICT_ALIGNMENT
- && tree_non_aligned_mem_p (rhs, get_object_alignment (lhs)))
+ && tree_non_aligned_mem_p (rhs,
+ MIN (TYPE_ALIGN (lacc->type),
+ get_object_alignment (lhs))))
lacc->grp_unscalarizable_region = 1;
}
@@ -1135,7 +1137,9 @@ build_accesses_from_assign (gimple stmt)
&& !is_gimple_reg_type (racc->type))
bitmap_set_bit (should_scalarize_away_bitmap, DECL_UID (racc->base));
if (STRICT_ALIGNMENT
- && tree_non_aligned_mem_p (lhs, get_object_alignment (rhs)))
+ && tree_non_aligned_mem_p (lhs,
+ MIN (TYPE_ALIGN (racc->type),
+ get_object_alignment (rhs))))
racc->grp_unscalarizable_region = 1;
}
on the grounds that sub-accesses shouldn't be more aligned. I think this
should be OK but, well...
--
Eric Botcazou
