[Mesa-dev] [PATCH] nir: allow stitching of non-empty block

[Juan A. Suarez Romero]

On Fri, 2019-02-08 at 15:39 -0600, Jason Ekstrand wrote:
> I had a chat with Caio about this and I'm skeptical.  In general, users of the CF manipulation code shouldn't be stitching two blocks together where the first contains a jump and the second is non-empty.  If the caller knows that this case is ok, then they can check for it and empty out the one block before stitching.  Also, I'm not really seeing how peel_initial_if would hit this case from your example.
> 

I'll re-check again, as with the last changes in Mesa the problem moved to a different place.
	J.A.
> --Jason
> 
> 
> On Fri, Jan 25, 2019 at 11:37 AM Juan A. Suarez Romero <jasuarez at igalia.com> wrote:
> > When stitching two blocks A and B, where A's last instruction is a jump,
> > 
> > it is not required that B is empty; it can be plainly removed.
> > 
> > 
> > 
> > This can happen in a situation like this:
> > 
> > 
> > 
> > vec1 1 ssa_1 = load_const (true)
> > 
> > vec1 1 ssa_2 = load_const (false)
> > 
> > block block_1:
> > 
> > [...]
> > 
> > loop {
> > 
> >   vec1 ssa_3 = phi block_1: ssa_2, block_4: ssa_1
> > 
> >   if ssa_3 {
> > 
> >     block block_2:
> > 
> >     [...]
> > 
> >     break
> > 
> >   } else {
> > 
> >     block block_3:
> > 
> >   }
> > 
> >   vec1 ssa_4 = <alu operation>
> > 
> >   if ssa_4 {
> > 
> >     block block_4:
> > 
> >     continue
> > 
> >   } else {
> > 
> >     block block_5:
> > 
> >   }
> > 
> >   block block_6:
> > 
> >   [...]
> > 
> > }
> > 
> > 
> > 
> > And opt_peel_loop_initial_if is applied. In this case, we would be
> > 
> > ending up stitching block_2 (which finalizes with a jump) with
> > 
> > block_4, which is not empty.
> > 
> > 
> > 
> > CC: Jason Ekstrand <jason at jlekstrand.net>
> > 
> > ---
> > 
> >  src/compiler/nir/nir_control_flow.c | 1 -
> > 
> >  1 file changed, 1 deletion(-)
> > 
> > 
> > 
> > diff --git a/src/compiler/nir/nir_control_flow.c b/src/compiler/nir/nir_control_flow.c
> > 
> > index ddba2e55b45..27508f230d6 100644
> > 
> > --- a/src/compiler/nir/nir_control_flow.c
> > 
> > +++ b/src/compiler/nir/nir_control_flow.c
> > 
> > @@ -550,7 +550,6 @@ stitch_blocks(nir_block *before, nir_block *after)
> > 
> >      */
> > 
> > 
> > 
> >     if (nir_block_ends_in_jump(before)) {
> > 
> > -      assert(exec_list_is_empty(&after->instr_list));
> > 
> >        if (after->successors[0])
> > 
> >           remove_phi_src(after->successors[0], after);
> > 
> >        if (after->successors[1])
> > 
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