Re: [PATCH] MIPS bit field failures in gdb.base/store.exp

[Luis Machado <lgustavo@codesourcery.com>]

On 09/12/2014 05:10 PM, Luis Machado wrote:
> Hi,
>
> I've been chasing this one for a while, trying to come up with the best
> solution while not having to do many changes to GDB's type system.
>
> On MIPS64 little endian, attempting an assignment to a bit field that
> lives in a register yields the wrong result. It just corrupts the data
> in the register depending on the specific position of the bit field
> inside the structure.
>
> FAIL: gdb.base/store.exp: f_1.j
> FAIL: gdb.base/store.exp: f_1.k
> FAIL: gdb.base/store.exp: F_1.i
> FAIL: gdb.base/store.exp: F_1.j
> FAIL: gdb.base/store.exp: F_1.k
> FAIL: gdb.base/store.exp: f_2.j
> FAIL: gdb.base/store.exp: f_2.k
> FAIL: gdb.base/store.exp: F_2.i
> FAIL: gdb.base/store.exp: F_2.j
> FAIL: gdb.base/store.exp: F_2.k
> FAIL: gdb.base/store.exp: f_3.j
> FAIL: gdb.base/store.exp: f_3.k
> FAIL: gdb.base/store.exp: F_3.i
> FAIL: gdb.base/store.exp: F_3.j
> FAIL: gdb.base/store.exp: F_3.k
> FAIL: gdb.base/store.exp: f_4.j
> FAIL: gdb.base/store.exp: f_4.k
> FAIL: gdb.base/store.exp: F_4.i
> FAIL: gdb.base/store.exp: F_4.j
> FAIL: gdb.base/store.exp: F_4.k
>
>                  === gdb Summary ===
>
> # of expected passes            220
> # of unexpected failures        20
>
> Now, GDB knows how to do bit field assignment properly, but MIPS is one
> of those architectures that uses a hook for the register-to-value
> conversion. Although we can properly tell when the type being passed is
> a structure or union, we cannot tell when it is a bit field, because the
> bit field data lives in a value structure. Such data only lives in a
> "type" structure when the parent structure is being referenced, thus you
> can collect them from the flds_bnds members.
>
> A bit field type structure looks pretty much the same as any other
> primitive type like int or char, so we can't distinguish them. Forcing
> more fields into the type structure wouldn't help much, because the type
> structs are shared.
>
> It feels to me GDB's type system is a bit dated and needs to be more
> precise about what it is describing, but for now i just want to fix a
> pending bug.
>
> The most elegant solution i could find without having to touch a number
> of other type-related data structures is making the
> gdbarch_convert_register_p predicate accept a value structure instead of
> a type structure. That way we can properly tell when a bit field is
> being manipulated in registers.
>
> There is still a little problem though. We don't always have a
> meaningful value struct to pass to this predicate, like both ocurrences
> of it in findvar.c. In those cases i went for a dummy value.
>
> In the end, it is functional but a bit ugly. Unless folks have a better
> suggestion, is this ok?
>
> I did tests with x86, mips32 be/le and mips64 be/le. No regressions found.
>
> The lack of bit field data in the type struct also affects other
> functions that rely on type descriptions, though there may not be
> explicit failures due to those yet.
>
> Luis

Ping! Any thoughts on the proposed solution?