Hi,
On Thu, 5 Dec 2013 21:12:54, Jeff Law wrote:
>
> On 12/04/13 00:49, Bernd Edlinger wrote:
>>
>> Regarding the consistency of bitregion_start/end ,
>>
>> they should either both be zero, or
>>
>> bitregion_start <= bitpos && bitpos+bitsize-1 <= bitregion_end
> Presumably to satisfy the consecutive bitfields are a single memory
> location stuff from C++11. Thus the bitregion representation (C++11)
> must totally encompass the original COMPONENT_REF. I can certainly see
> how that holds immediately after the call to get_bit_range.
>
> When we adjust the address, we're just accounting for the bit position
> (bitpos). ie, if bitpos was 16 and TO_RTX was (plus (reg) (32)). We
> change TO_RTX to (plus (reg) (48)). This is ultimately meant to
> simplify the resulting insns we generate by folding the bitpos
> adjustment into the address computation. In effect, this results in
> accessing a subobject rather than the full object. Presumably this is
> kosher for C++11's memory model (I really don't know).
>
> *If* it is OK to access the subobject like this in C++11's memory model,
> then is is necessary for bitregion_{start,end} to encompass the original
> object, or should it be made consistent with the bitpos/bitsize? And
> that seems to me to be the fundamental question. What is really the
> purpose of the bitregion_{start,end} representation? If it must
> represent the C++ object, then ISTM we don't want to mess with it. If
> it doesn't, then why did we bother building this alternate
> representation to begin with?
>
Yes, that is correct. let's assume bitpos = 16, and bitsize = 16,
then we have bitregion_start = 16 and bitregion_end = 31.
The to_rtx points to the start of the object.
to_rtx = adjust_address (to_rtx, mode1, bitpos / BITS_PER_UNIT)
makes to_rtx point to two bytes from the beginning.
that is compensated by
bitpos = 0.
So now [to_rtx, bitpos, bitsize] means the same thing than before.
What's wrong is that bitregion_start is still 16 and bitregion_end is still 31
in that example.
The correct values would be bitregion_start = 0 and bitregion_end = 15.
that's what this new statement is meant to do:
bitregion_start = 0;
if (bitregion_end>= (unsigned HOST_WIDE_INT) bitpos)
bitregion_end -= bitpos;
I need the "if" because bitregion_end could be 0, and in that case
I want bitregion_end to stay 0.
Thanks
Bernd.
>
> Jeff
