Eric Botcazou <ebotca...@adacore.com> writes:
>> expand_assignment calls:
>>
>> if (MEM_P (to_rtx))
>> {
>> /* If the field is at offset zero, we could have been given
>> the DECL_RTX of the parent struct. Don't munge it. */ to_rtx =
>> shallow_copy_rtx (to_rtx);
>>
>> set_mem_attributes_minus_bitpos (to_rtx, to, 0, bitpos);
>> ...
>
> The MEM_KEEP_ALIAS_SET_P line seems to be redundant here (but not the
> MEM_VOLATILE_P line).
Good catch. I've removed that in the patch below.
>> But set_mem_attributes_minus_bitpos has:
>>
>> /* Default values from pre-existing memory attributes if present. */
>> refattrs = MEM_ATTRS (ref);
>> if (refattrs)
>> {
>> /* ??? Can this ever happen? Calling this routine on a MEM that
>> already carries memory attributes should probably be invalid. */
>> attrs.expr = refattrs->expr;
>> attrs.offset_known_p = refattrs->offset_known_p;
>> attrs.offset = refattrs->offset;
>> attrs.size_known_p = refattrs->size_known_p;
>> attrs.size = refattrs->size;
>> attrs.align = refattrs->align;
>> }
>>
>> which of course applies in this case: we have a MEM for g__style.
>> I would expect many calls from this site are for MEMs with an
>> existing MEM_EXPR.
>
> Indeed. Not very clear what to do though.
>
>> Then later:
>>
>> /* If this is a field reference and not a bit-field, record it. */
>> /* ??? There is some information that can be gleaned from bit-fields,
>> such as the word offset in the structure that might be modified.
>> But skip it for now. */
>> else if (TREE_CODE (t) == COMPONENT_REF
>> && ! DECL_BIT_FIELD (TREE_OPERAND (t, 1)))
>>
>> so we leave the offset and expr alone. The end result is:
>>
>> (mem/j/c:SI (symbol_ref:DI ("g__style") [flags 0x4] <var_decl
>> 0x7ffff6e4ee40 g__style>) [0 g__style+0 S1 A64])
>>
>> an SImode reference to the first byte (and only the first byte) of g__style.
>> Then, when we apply adjust_bitfield_address, it looks like we're moving
>> past the end of the region and so we drop the MEM_EXPR.
>>
>> In cases where set_mem_attributes_minus_bitpos does set MEM_EXPR based
>> on the new tree, it also adds the bitpos to the size. But I think we
>> should do that whenever we set the size based on the new tree,
>> regardless of whether we were able to record a MEM_EXPR too.
>>
>> That said, this code has lots of ???s in it, so I'm not exactly
>> confident about this change. Thoughts?
>
> It also seems a little bold to me. Since we now have the new processing of
> MEM_EXPR for bitfields, can't we remove the ! DECL_BIT_FIELD check?
>
> /* If this is a field reference and not a bit-field, record it. */
> /* ??? There is some information that can be gleaned from bit-fields,
> such as the word offset in the structure that might be modified.
> But skip it for now. */
> else if (TREE_CODE (t) == COMPONENT_REF
> && ! DECL_BIT_FIELD (TREE_OPERAND (t, 1)))
> {
> attrs.expr = t;
> attrs.offset_known_p = true;
> attrs.offset = 0;
> apply_bitpos = bitpos;
> /* ??? Any reason the field size would be different than
> the size we got from the type? */
> }
>
> This would mean removing the first ??? comment. As for the second ???
> comment, the answer is easy: because that's pretty much what defines a
> bitfield! The size is DECL_SIZE_UNIT and not TYPE_SIZE_UNIT for them.
I don't know this area well, so I'll have to take your word for it that
handling DECL_BIT_FIELDs is now safe. I certainly won't argue against
removing two ??? comments though :-)
Tested on x86_64-linux-gnu. OK to install?
Richard
gcc/
* expr.c (expand_assignment): Don't set MEM_KEEP_ALIAS_SET_P here.
* emit-rtl.c (set_mem_attributes_minus_bitpos): Handle DECL_BIT_FIELDs,
using their size instead of the COMPONENT_REF's.
Index: gcc/emit-rtl.c
===================================================================
--- gcc/emit-rtl.c 2012-11-10 22:17:24.797602770 +0000
+++ gcc/emit-rtl.c 2012-11-10 22:19:17.779228832 +0000
@@ -1679,11 +1679,7 @@ set_mem_attributes_minus_bitpos (rtx ref
attrs.align = MAX (attrs.align, TYPE_ALIGN (type));
/* If the size is known, we can set that. */
- if (TYPE_SIZE_UNIT (type) && host_integerp (TYPE_SIZE_UNIT (type), 1))
- {
- attrs.size_known_p = true;
- attrs.size = tree_low_cst (TYPE_SIZE_UNIT (type), 1);
- }
+ tree new_size = TYPE_SIZE_UNIT (type);
/* If T is not a type, we may be able to deduce some more information about
the expression. */
@@ -1742,13 +1738,7 @@ set_mem_attributes_minus_bitpos (rtx ref
attrs.offset_known_p = true;
attrs.offset = 0;
apply_bitpos = bitpos;
- if (DECL_SIZE_UNIT (t) && host_integerp (DECL_SIZE_UNIT (t), 1))
- {
- attrs.size_known_p = true;
- attrs.size = tree_low_cst (DECL_SIZE_UNIT (t), 1);
- }
- else
- attrs.size_known_p = false;
+ new_size = DECL_SIZE_UNIT (t);
attrs.align = DECL_ALIGN (t);
align_computed = true;
}
@@ -1763,19 +1753,15 @@ set_mem_attributes_minus_bitpos (rtx ref
align_computed = true;
}
- /* If this is a field reference and not a bit-field, record it. */
- /* ??? There is some information that can be gleaned from bit-fields,
- such as the word offset in the structure that might be modified.
- But skip it for now. */
- else if (TREE_CODE (t) == COMPONENT_REF
- && ! DECL_BIT_FIELD (TREE_OPERAND (t, 1)))
+ /* If this is a field reference, record it. */
+ else if (TREE_CODE (t) == COMPONENT_REF)
{
attrs.expr = t;
attrs.offset_known_p = true;
attrs.offset = 0;
apply_bitpos = bitpos;
- /* ??? Any reason the field size would be different than
- the size we got from the type? */
+ if (DECL_BIT_FIELD (TREE_OPERAND (t, 1)))
+ new_size = DECL_SIZE_UNIT (TREE_OPERAND (t, 1));
}
/* If this is an array reference, look for an outer field reference. */
@@ -1861,6 +1847,12 @@ set_mem_attributes_minus_bitpos (rtx ref
else
as = TYPE_ADDR_SPACE (type);
+ if (host_integerp (new_size, 1))
+ {
+ attrs.size_known_p = true;
+ attrs.size = tree_low_cst (new_size, 1);
+ }
+
/* If we modified OFFSET based on T, then subtract the outstanding
bit position offset. Similarly, increase the size of the accessed
object to contain the negative offset. */
Index: gcc/expr.c
===================================================================
--- gcc/expr.c 2012-11-10 22:19:14.000000000 +0000
+++ gcc/expr.c 2012-11-10 22:21:17.615919793 +0000
@@ -4821,8 +4821,6 @@ expand_assignment (tree to, tree from, b
done for MEM. Also set MEM_KEEP_ALIAS_SET_P if needed. */
if (volatilep)
MEM_VOLATILE_P (to_rtx) = 1;
- if (component_uses_parent_alias_set (to))
- MEM_KEEP_ALIAS_SET_P (to_rtx) = 1;
}
if (optimize_bitfield_assignment_op (bitsize, bitpos,
