Re: Stack parameter - pass by value - frame usage

[Hariharan Sandanagobalane <hariharans at picochip dot com>]

Ian Lance Taylor wrote:
> Hariharan Sandanagobalane <hariharans@picochip.com> writes:
>
> > I looked at an inefficient code sequence for a simple program using
> > GCC's picochip port (not yet submitted to mainline).
>
> Are you working with mainline sources?

I was not. I tried the same with gcc 4.3 branch and it does fix most of 
the problems. There are still corner cases where it produces unoptimal 
code. I will try to figure out whats wrong in those and get back to you.

Regards
Hari

> > Note that the parameter is being written to the frame in the last 2
> > instructions above. This, i am guessing is the reason for the
> > get_frame_size() returning 4 later on, though the actual save of the
> > struct parameter value on the stack is being eliminated at later
> > optimization phases (CSE and DCE, i believe).
> >
> > Why does the compiler do this? I vaguely remember x86 storing all
> > parameter values on stack. Is that the reason for this behaviour? Is
> > there anything i can do in the port to get around this problem?
>
> At a guess, it's because the frontend decided that the struct was
> addressable and needed to be pushed on the stack.  I thought this got
> cleaned up recently, though.
>
> Ian
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