On Tue, Dec 10, 2013 at 4:02 PM, Richard Biener
<[email protected]> wrote:
> On Tue, Dec 10, 2013 at 11:53 AM, Eric Botcazou <[email protected]> wrote:
>>> What we support is out of bounds accesses for heap vars if the var's type
>>> has flexible array member or something we treat similarly and there is the
>>> possibility that there could be payload after the heap var that could be
>>> accessed from the flexible array members or similar arrays.
>>
>> My question was about the above similar arrays, i.e. whether we consider all
>> trailing arrays in structures as flexible-like or not. No strong opinion.
>
> Yes we do, even for struct { struct { int a; char a[1] } }; (note the not
> really
> "trailing" as there is padding after the trailing array). We do take
> size limitations from a DECL (if we see one) into account to limit the
> effect of this trailing-array-supporting, so it effectively only applies to
> indirect accesses (and the padding example above, you can use the whole
> padding if DECL_SIZE allows that).
>
>>> So, I don't see what is the big deal with BLKmode, because all the cases
>>> which actually could have flexible array member extra payloads (or similar)
>>> must necessarily live in memory, if it is the compiler that decides whether
>>> to put it into memory or keep in registers etc., then it can't be heap
>>> allocated.
>>
>> The invariant is that types for which objects can effectively have variable
>> size must have BLKmode, otherwise you need to add very ugly code in the RTL
>> expander to mask the lie.
>
> I wonder if we can make the expander more rely on the DECLs mode
> and optimize only the DECLs mode (if we can constrain its size via
> DECL_SIZE) to non-BLKmode instead of doing that for the TYPEs mode.
> Yes, you'd have DECL_MODE != TYPE_MODE that way.
>
> Or rather I wonder if the expander doesn't already work that way
> (looks at DECL_MODE).
To speak in patches (completely untested) - sth like the following ontop
of making more types have BLKmode:
Index: gcc/stor-layout.c
===================================================================
--- gcc/stor-layout.c (revision 205857)
+++ gcc/stor-layout.c (working copy)
@@ -569,8 +569,17 @@ layout_decl (tree decl, unsigned int kno
bitsize_unit_node));
if (code != FIELD_DECL)
- /* For non-fields, update the alignment from the type. */
- do_type_align (type, decl);
+ {
+ /* For non-fields, update the alignment from the type. */
+ do_type_align (type, decl);
+ /* Optimize the mode of the decl.
+ ??? Ensure choosen mode alignment fits decl alignment. */
+ if (DECL_MODE (decl) == BLKmode
+ && DECL_SIZE (decl)
+ && compare_tree_int (DECL_SIZE (t), MAX_FIXED_MODE_SIZE) <= 0)
+ DECL_MODE (decl)
+ = mode_for_size (TREE_INT_CST_LOW (DECL_SIZE (decl)), MODE_INT, 1);
+ }
else
/* For fields, it's a bit more complicated... */
{
> Richard.
>
>> --
>> Eric Botcazou
