> Now, back to PR53970, where #pragma pack() is used to pack a
> struct. With #pragma pack() no part of the type or field-decls
> have a hint that packing took place (well, their align information
> tell you), which means the vectorizers use of contains_packed_reference
> is not conservative enough, nor is expand_exprs use:
>
> case BIT_FIELD_REF:
> case ARRAY_RANGE_REF:
> normal_inner_ref:
> {
> ...
> if (TYPE_PACKED (TREE_TYPE (TREE_OPERAND (exp, 0)))
>
> || (TREE_CODE (TREE_OPERAND (exp, 1)) == FIELD_DECL
>
> && DECL_PACKED (TREE_OPERAND (exp, 1))))
> packedp = true;
>
> I'm not sure if this flag is required for correctness - it's only
> passed to extract_bit_field - but if it is the above code does
> not work for #pragma packed structs. I suppose what should be
> checked is (a few lines below the above test, after we expanded tem)
>
> if (MEM_P (op0)
> && GET_MODE (op0) != BLKmode
> && MEM_ALIGN (op0) < GET_MODE_ALIGNMENT (GET_MODE (op0)))
> packedp = true;
>
> ? I suppose packedp was computed for STRICT_ALIGNMENT targets only.
> I'm not changing the above, but Eric, you should be able to produce a
> #pragma packed testcase that fails on a STRICT_ALIGNMENT target?
This is the -fstrict-volatile-bitfields business though, and its documentation
explicitly refers to the packed attribute:
If the target requires strict alignment, and honoring the field
type would require violating this alignment, a warning is issued.
If the field has `packed' attribute, the access is done without
honoring the field type. If the field doesn't have `packed'
attribute, the access is done honoring the field type. In both
cases, GCC assumes that the user knows something about the target
hardware that it is unaware of.
so I'm a little reluctant to touch that. But, yes, generally speaking, testing
TYPE_PACKED or DECL_PACKED to drive code generation is wrong.
> Oh, and this does not yet fix PR53970 - but I hope that I can
> remove contains_packed_reference ;)
Right, it should definitely go away.
--
Eric Botcazou
