optimizing bitfield compilation

[Xingzhi Pan]

On Mon, Apr 11, 2011 at 1:46 PM, Ian Lance Taylor <iant@google.com> wrote:
> Xingzhi Pan <vengeance.storm@gmail.com> writes:
>
>> 1. Does gcc compile C bitfields into some intermediate representation (IR)?
>
> Yes.
>
>> 2. If so, does the optimization happen BEFORE or AFTER the IR?  Or both?
>
> What optimization in particular?  BEFORE or AFTER what?  In general, the
> answer is going to be that optimizations are applied to the IR, so they
> are applied after the IR is created and before it is converted to
> assembly code.  gcc has bitfield optimizations which apply to both the
> GIMPLE and the RTL IRs.

Thanks.  This is the answer I want.  I didn't have any particular
optimization in mind when asking this question.

>
>> 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.

>
>> 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?

>
> You said you wanted "to learn how to optimize bitfield compilation" but
> I don't know what that means to you.

Sorry about the confusion.  I'm doing a research on optimizing
bitfield compilation (probably to be implemented in some dialect of
Haskell which deals with low-level system programming).

Thank you very much for your help.

>
> Ian
>