http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55882
--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> 2013-01-09
11:35:33 UTC ---
(In reply to comment #10)
> Eric, I am double-checking my patch and I believe all this 'bitpos' handling
> in set_mem_attributes_minus_bitpos (and/or MEM_OFFSET in general) looks
> suspicious.
>
> /* For a MEM rtx, the offset from the start of MEM_EXPR. */
> #define MEM_OFFSET(RTX) (get_mem_attrs (RTX)->offset)
>
> I read from this that the XEXP (RTX, 0) points to MEM_EXPR + MEM_OFFSET
> (if MEM_OFFSET_KNOWN_P, and if !MEM_OFFSET_KNOWN_P we don't know how
> XEXP (RTX, 0) and MEM_EXPR relate).
>
> Now, in expand_assignment we do
>
> tem = get_inner_reference (to, &bitsize, &bitpos, &offset, &mode1,
> &unsignedp, &volatilep, true);
>
> if (TREE_CODE (to) == COMPONENT_REF
> && DECL_BIT_FIELD_TYPE (TREE_OPERAND (to, 1)))
> get_bit_range (&bitregion_start, &bitregion_end, to, &bitpos,
> &offset);
>
> ...
> to_rtx = expand_expr (tem, NULL_RTX, VOIDmode, EXPAND_WRITE);
> ...
> offset it with variable parts from 'offset'
> ...
> set_mem_attributes_minus_bitpos (to_rtx, to, 0, bitpos);
>
> but bitpos here is _not_ ontop of 'to' but extracted from 'to' (and
> eventually modified by get_bit_range which may shift things to 'offset').
>
> The only 'bitpos' that should be passed to set_mem_attributes_minus_bitpos
> is one that reflects - in the bitfield access case - that we actually
> access things at a different position. But at this point we don't know
> what optimize_bitfield_assignment_op or store_field will eventually do.
> Also MEM_OFFSET is in bytes while I believe 'bitpos' can end up as
> an actual bit position, so
>
> /* 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. */
> if (apply_bitpos)
> {
> gcc_assert (attrs.offset_known_p);
> attrs.offset -= apply_bitpos / BITS_PER_UNIT;
> if (attrs.size_known_p)
> attrs.size += apply_bitpos / BITS_PER_UNIT;
> }
>
> (whatever this comment means). I believe my fix is correct following
> the MEM_OFFSET description and guessing at what the argument to
> set_mem_attributes_minus_bitpos means by looking at its use. But I
> believe that expand_assignment should pass zero as bitpos to
> set_mem_attributes_minus_bitpos (making the only caller that calls this
> function with bitpos != 0 go).
>
> In this testcase we want to access sth at offset 8 bytes, 0 bits but
> the memory model tells us the bitregion to consider is
> everything from offset 6 to offset 14 so instead of the correct
> initial mem
>
> (mem/j:HI (plus:SI (reg/f:SI 206)
> (const_int 8 [0x6])) [0 dmfe[i_1].use_alt_rd_dqs S2 A32])
>
> (with 4 byte alignemnt!) we create
>
> (mem/j:BLK (plus:SI (reg/f:SI 206)
> (const_int 6 [0x6])) [0 dmfe[i_1].use_alt_rd_dqs+-6 S8 A32])
>
> where the alignment is bogus.
>
> Thus, given the above general MEM_OFFSET analysis I'd say the following
> (ontop of my previous patch) should fix things more correctly:
>
> Index: gcc/expr.c
> ===================================================================
> --- gcc/expr.c (revision 195014)
> +++ gcc/expr.c (working copy)
> @@ -4669,7 +4669,7 @@ expand_assignment (tree to, tree from, b
> || TREE_CODE (TREE_TYPE (to)) == ARRAY_TYPE)
> {
> enum machine_mode mode1;
> - HOST_WIDE_INT bitsize, bitpos;
> + HOST_WIDE_INT bitsize, bitpos, bitpos_adj;
> unsigned HOST_WIDE_INT bitregion_start = 0;
> unsigned HOST_WIDE_INT bitregion_end = 0;
> tree offset;
> @@ -4683,9 +4683,15 @@ expand_assignment (tree to, tree from, b
> tem = get_inner_reference (to, &bitsize, &bitpos, &offset, &mode1,
> &unsignedp, &volatilep, true);
>
> + bitpos_adj = 0;
> if (TREE_CODE (to) == COMPONENT_REF
> && DECL_BIT_FIELD_TYPE (TREE_OPERAND (to, 1)))
> - get_bit_range (&bitregion_start, &bitregion_end, to, &bitpos,
> &offset);
> + {
> + HOST_WIDE_INT orig_bitpos = bitpos;
> + get_bit_range (&bitregion_start, &bitregion_end,
> + to, &bitpos, &offset);
> + bitpos_adj = orig_bitpos - bitpos;
> + }
>
> /* If we are going to use store_bit_field and extract_bit_field,
> make sure to_rtx will be safe for multiple use. */
> @@ -4839,7 +4845,7 @@ expand_assignment (tree to, tree from, b
> 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);
> + set_mem_attributes_minus_bitpos (to_rtx, to, 0, bitpos_adj);
>
> /* Deal with volatile and readonly fields. The former is only
> done for MEM. Also set MEM_KEEP_ALIAS_SET_P if needed. */
>
> that is, we always pass zero to set_mem_attributes_minus_bitpos unless
> the original offset was modified.
>
> Hmm, but we really pass in a MEM that only covers tem + offset and not
> bitpos. But MEM_EXPR does not reflect that - we pass in the original
> 'to', not sth like *(&tem + offset). Maybe in very distant times
> all the stripping of component refs from MEM_EXPR compensated for that?
>
> What a mess ...
Forget all that - it seems to be correct. With the fix from
comment #11. Apart from the bit-align issue.
