Re: [PATCH] Fix another wrong-code bug with -fstrict-volatile-bitfields

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, Mar 4, 2015 at 3:13 PM, Bernd Edlinger
<bernd.edlinger@hotmail.de> wrote:
> bounced... again, without html.
>
>
> Hi Richard,
>
> while working on another bug in the area of   -fstrict-volatile-bitfields
> I became aware of another example where   -fstrict-volatile-bitfields may generate
> wrong code.   This is reproducible on a !STRICT_ALIGNMENT target like x86_64.
>
> The problem is   that strict_volatile_bitfield_p tries to allow more than necessary
> if !STRICT_ALIGNMENT.    Everything works OK on ARM for instance.
>
> If this function returns true, we may later call narrow_bit_field_mem, and
> the check in strict_volatile_bitfield_p should mirror the logic there:
> narrow_bit_field_mem just uses GET_MODE_BITSIZE (mode) and does not
> care about STRICT_ALIGNMENT, and in the end  *new_bitnum + bitsize may
> reach beyond the end of the region. This causes store_fixed_bit_field_1
> to silently fail to generate correct code.

Hmm, but the comment sounds like if using GET_MODE_ALIGNMENT is
more correct (even for !strict-alignment) - if mode is SImode and mode
alignment is 16 (HImode aligned) then we don't need to split the load
if bitnum is 16 and bitsize is 32.

So maybe narrow_bit_field_mem needs to be fixed as well?

Thanks,
Richard.

> The attached patch was   boot-strapped and
> regression-tested on x86_64-linux-gnu.
>
> OK for trunk and 4.9?
>
>
> Thanks
> Bernd.
>