optimizing bitfield compilation

[Ian Lance Taylor]

Xingzhi Pan <vengeance.storm@gmail.com> writes:

>>> 3. Where can I find relevant source code?  I already did a find+grep
>>> in gcc-4.4.3/gcc directory but I lack the general knowledge of the
>>> overall structure of gcc so the result didn't help much.
>>
>> I'm not clear what you looking for.
>
> After seeing your answer above, I would like to rephrase my questions
> as: which source files should I look at to learn how the optimizations
> of bitfield are done?  I guess that means I need to look at the IR
> optimization relevant to bitfield.  Any further narrowing-down is
> appreciated.

GCC has various optimizations which apply to all structure field
references.  It does not have a lot of optimizations which apply
specifically to bitfield references.  However, there is at least one:
look for optimize_bit_field_compare in gcc/fold-const.c.

It may also be interesting to look at how GIMPLE bitfield references are
converted to RTL, in store_bit_field and and extract_bit_field in
gcc/expmed.c.

>>> 4. Is there any special flags for this feature?
>>
>> Not clear here either, sorry.
>
> I mean is there any gcc command line switch to turn on/off these
> bitfield optimizations?

Not independently of other optimizations, no.

Ian