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Re: [PATCH] Strict volatile bit-fields clean-up


> The trigger is a latent problem in the ada gcc_interface.
> 
> That is we have a bit-field of exactly 8 bit size, which is not
> byte-aligned, but DECL_MODE=QImode, DECL_BIT_FIELD=false which looks quite
> strange, and is totally different from how C structures look like. I should
> mention that there are even some places in the target back-ends, where the
> attribute DECL_BIT_FIELD is used for whatever.

This needs to be investigated because the intent is very clear in gigi, see 
create_field_decl and finish_record_type: we set DECL_BIT_FIELD conservatively
and clear it only when we know that we can.  This code is quite old now.

> However even without that patch, I can arrange for "volatilep &&
> flag_strict_volatile_bitfields> 0" to be true in Ada (even on X86_64, or
> whatever platform you like):
> 
> -- { dg-do run }
> -- { dg-options "-gnatp -fstrict-volatile-bitfields" }
> 
> procedure Misaligned_Volatile is
> 
>    type Byte is mod 2**8;
> 
>    type Block is record
>       B : Boolean;
>       V : Byte;
>    end record;
>    pragma Volatile (Block);
>    pragma Pack (Block);
>    for Block'Alignment use 1;
> 
>    type Pair is array (1 .. 2) of Block;
> 
>    P : Pair;
> begin
>    for K in P'Range loop
>       P(K).V := 237;
>    end loop;
>    for K in P'Range loop
>       if P(K).V /= 237 then
>          raise Program_error;
>       end if;
>    end loop;
> end;
> 
> 
> This Ada test case causes either wrong code generation or an ICE at compile
> time, if the -fstrict-volatile-bitfields option is either given by the
> user, or by the target-specific default as it is on ARM for instance 
> (which is completely pointless on Ada, I know!)...

The test indeed raises Program_Error on x86-64.

-- 
Eric Botcazou


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