[Dri-devel] Re: [Mesa3d-dev] Re-occurrence of macros arguments within its definition in Mesa headers

[F. <j_r...@ya...>]

On Mon, Jul 01, 2002 at 07:43:40AM +0100, Keith Whitwell wrote:
>José Fonseca wrote:
[...]
>>
>>I think this is a bug in Mesa. First because a macro should generally 
>>behave as an
>>ordinary function, unless there is a very special reason not to do so.
>
>Hmm, not really, this is something people are supposed to understand about 
>macros generally - it's one of the classic C programming mistakes to assume 
>that macros evaluate their arguments exactly once.

I was aware of that common pitfall, but I though that it was common [and
good] practice to define the macros to avoid exactly this pitfall. Imagine 
if in the linux kernel, for example, macros such as cpu_to_le32 didn't 
followed that convention.

Of course that many times the solution is to rewrite the macro as a inline function.

>
>>Second because this relies on compiler optimization to remove the
>>redundancy of calculating OUT twice. Although I haven't check what is
>>the exact assembler output of gcc, the call of INTERP_F with 
>>calculations in
>>its arguments is used on the default vertex template, hence used across
>>all drivers. See src/tnl_dd/t_dd_vbtmp.h:669:
>
>You should check the output.  Gcc does lots of 
>common-subexpression-elimination (cse).  I'd be suprised if it didn't find 
>this one.
>

You're right in respect with gcc. The following code in t_dd_vbtmp.h

  INTERP_F( t, dst->v.u0, out->v.u0 * qout, in->v.u0 * qin );

compiles into

	fld     %st(1)
	fld     %st(1)
	fxch    %st(1)
	fmuls   24(%ebx)
	fxch    %st(1)
	fmuls   24(%edx)
	fsub    %st(1), %st
	fmul    %st(4), %st
	faddp   %st, %st(1)
	movl    -36(%ebp), %eax
	fstps   24(%eax)

that is, three fp multiplications in total, so OUT isn't recalculated (as
that would result in four).

José Fonseca