[Bug tree-optimization/108199] Bitfields, unions and SRA and storage_order_attribute

[ebotcazou at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108199

--- Comment #11 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> Here is a testcase for the trunk on x86_64-linux-gnu:

Thanks.  The problem is indeed the BIT_FIELD_REF of a scalar, which is an
undocumented extension of GENERIC:

/* Reference to a group of bits within an object.  Similar to COMPONENT_REF
   except the position is given explicitly rather than via a FIELD_DECL.
   Operand 0 is the structure or union expression;
   operand 1 is a tree giving the constant number of bits being referenced;
   operand 2 is a tree giving the constant position of the first referenced
bit.
   The result type width has to match the number of bits referenced.
   If the result type is integral, its signedness specifies how it is extended
   to its mode width.  */
DEFTREECODE (BIT_FIELD_REF, "bit_field_ref", tcc_reference, 3)

How are the bits numbered in there, IOW is bit 0 always the LSB or not?