Forcing memory mode to some addresses

Forcing memory mode to some addresses

[Aurelien Buhrig]

Hi,

I would like to force accesses to some memory variables in a specific mode.
For example, I would like GCC to access a 16-bit wide hardware
register always in HImode when using a 16bit wide bitfield struct...
Currently, GCC can accesses part of this memory structure using
QImode, which is not always permitted.
Is there a way to do it using an attribute ? Or perhaps another way ?

If a target specific attribute must be defined, what should it
control, and how to recognized it in my backend ?

Thanks,
Aurélien

Re: Forcing memory mode to some addresses

[Ian Lance Taylor]

Aurelien Buhrig <aurelien.buhrig.gcc@gmail.com> writes:

> I would like to force accesses to some memory variables in a specific mode.
> For example, I would like GCC to access a 16-bit wide hardware
> register always in HImode when using a 16bit wide bitfield struct...
> Currently, GCC can accesses part of this memory structure using
> QImode, which is not always permitted.
> Is there a way to do it using an attribute ? Or perhaps another way ?
>
> If a target specific attribute must be defined, what should it
> control, and how to recognized it in my backend ?

You should always use a volatile qualified pointer when accessing a
hardware register.  When you do that, gcc should always use the
appropriate mode.

Ian

Re: Forcing memory mode to some addresses

[Aurelien Buhrig]

1- Do you mean pointer to a volatile struct? I've defined a const
pointer to a volatile bitfield struct
volatile hardware_t * const MY_HARDWARE = (hardware_t *)0Xxxxx


2- What is the appropriate access for a 1bit field of the hardware_t structure ?
For example if I define :
typedef struct {
  unsigned s0:1;
  unsigned s1:1;
  unsigned s2:1;
  ...
} hardware_t;

with MY_HARDWARE->s0, gcc can generate QI or HI access, depending on cases...

Aurelien

2011/11/16 Ian Lance Taylor <iant@google.com>:
> Aurelien Buhrig <aurelien.buhrig.gcc@gmail.com> writes:
>
>> I would like to force accesses to some memory variables in a specific mode.
>> For example, I would like GCC to access a 16-bit wide hardware
>> register always in HImode when using a 16bit wide bitfield struct...
>> Currently, GCC can accesses part of this memory structure using
>> QImode, which is not always permitted.
>> Is there a way to do it using an attribute ? Or perhaps another way ?
>>
>> If a target specific attribute must be defined, what should it
>> control, and how to recognized it in my backend ?
>
> You should always use a volatile qualified pointer when accessing a
> hardware register.  When you do that, gcc should always use the
> appropriate mode.
>
> Ian
>