[Mesa-dev] [PATCH] nir: move ALU instruction before the jump instruction

[Juan A. Suarez Romero]

On Tue, 2019-02-12 at 16:22 -0800, Ian Romanick wrote:
> On 2/12/19 12:58 AM, Juan A. Suarez Romero wrote:
> > opt_split_alu_of_phi moves ALU instruction to the end of continue block.
> > 
> > But if the continue block ends with a jump instruction (an explicit
> > "continue" instruction) then the ALU must be inserted before the jump,
> > as it is illegal to add instructions after the jump.
> 
> I'm assuming you found this by inspection?  Since this pass only
> operates when the first block of the loop only has two predecessors (the
> block before the loop and the implicit continue at the end of the loop),
> this shouldn't be a a problem in practice... or were you able to trigger
> it somehow?
> 

Found when dealing with the SPIR-V code that I've sent in 
https://lists.freedesktop.org/archives/mesa-dev/2019-February/214906.html


The obtained NIR code has an explicit continue at the end of the loop (see 
http://paste.debian.net/1067619/, in particular the loop with header block_2).


	J.A.



> > CC: Ian Romanick <ian.d.romanick at intel.com>
> > Fixes: 0881e90c099 ("nir: Split ALU instructions in loops that read phis")
> > ---
> >  src/compiler/nir/nir_opt_if.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/src/compiler/nir/nir_opt_if.c b/src/compiler/nir/nir_opt_if.c
> > index 9afb901be14..932af9e37ab 100644
> > --- a/src/compiler/nir/nir_opt_if.c
> > +++ b/src/compiler/nir/nir_opt_if.c
> > @@ -488,7 +488,7 @@ opt_split_alu_of_phi(nir_builder *b, nir_loop *loop)
> >            *
> >            * Insert the new instruction at the end of the continue block.
> >            */
> > -         b->cursor = nir_after_block(continue_block);
> > +         b->cursor = nir_after_block_before_jump(continue_block);
> >  
> >           nir_ssa_def *const alu_copy =
> >              clone_alu_and_replace_src_defs(b, alu, continue_srcs);