[FFmpeg-devel] [PATCH] Fix apply_welch_window_sse2 compilation on Mac OS X/x86

[Trent Piepho]

On Thu, 18 Oct 2007, Pierre d'Herbemont wrote:
> On Oct 17, 2007, at 10:05 PM, Loren Merritt wrote:
>
> > But I don't think the fix is correct: It's not valid to jmp from
> > one asm block to another. If you split the asm block, you also have
> > to write the loop part in C. And the reason I put it in asm is that
> > gcc doesn't generate as efficient a loop.
>
> There is no problem in jumping from one asm block to an other.
> However there can be problem with labels.

Suppose gcc _did_ spill a register to the stack?

You also have no guarantee that gcc won't mode code around so that what was
before or after an asm is now on the other side.  And no, making the asm
volatile does not work!  Suppose you code this:

for(i=0;i<100;i++) {
   asm volatile("1: ..." : : "=r"(i));
   asm volatile("... je 1b" : :);
}

gcc might decide the perfect place to put the i++ is between the two asm
blocks, which will completely mess up the for loop, since now i is
incremented inside the asm loop.