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

[Gareth H. <ga...@va...>]

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.

-- Gareth