Re: [Mesa3d-dev] dynamic code generation with gcc

[Keith W. <ke...@va...>]

Gareth Hughes wrote:
> 
> Josh Vanderhoof wrote:
> >
> > I'm getting sidetracked here but:
> >
> > 1. Before you start doing crazy optimizations why wouldn't you rewrite
> >    it like this (get rid of the "dma.space" variable):
> >
> >         void foo_Vertex3fv( const GLfloat *v )
> >         {
> >            GET_FOO_CONTEXT(fmesa);
> >            COPY_3V( fmesa->current.o3n3t2.obj, v );
> >            if ( fmesa->dma.head + 8 <= fmesa->dma.end_of_space ) {
> >               COPY_DWORDS( fmesa->dma.head, fmesa->current.o3n3tc2, 8 );
> >               fmesa->dma.head += 8;
> >            } else {
> >               fmesa->get_dma( fmesa, fmesa->current.o3n3tc2, 8 );
> >            }
> >         }
> 
> Sure -- it was just a cut and paste of some old code Keith sent me.
> Minor point.
> 
> > 2. Hard-coding the address would help you, but not by very much.  I
> >    would expect it to save you one MOV instruction.  On Intel cpu's,
> >    the "reg+offset" addressing mode is "free".  (You probably knew
> >    that already.)
> 
> Yep :-)  GET_FOO_CONTEXT() is the big one, and perhaps I should have
> stressed that a little more.  This may involve a function call to
> determine the current context, due to thread-safety issues.
> 
> > 3. If you want to go all out on this code, you could probably use
> >    mprotect() to avoid the buffer overflow test entirely.  That would
> >    only be a good idea if buffer overflows are rare though.
> 
> You need buffer overflows as they end up flushing the DMA buffer.  In
> this case, get_dma() would flush the current buffer and acquire a new
> one.

But of course, we should never be getting rid of or aquiring new dma buffers;
we should be just aging regions within a buffer we always hold, and perhaps in
rare cases growing/shrinking it.

However, mprotect doesn't help you here either, as the regions being aged are
pretty dynamic.

Keith