[Mesa-dev] [PATCH 1/2] nir: update nir_lower_returns to only predicate instructions when needed

[Timothy Arceri]

On Tue, 2016-12-20 at 09:44 -0800, Jason Ekstrand wrote:
> On Mon, Dec 19, 2016 at 8:18 PM, Timothy Arceri <timothy.arceri at colla
> bora.com> wrote:
> > Unless an if statement contains nested returns we can simply add
> > any following instructions to the branch without the return.
> > 
> > V2: fix handling if_nested_return value when there is a sibling
> > if/loop
> > that doesn't contain a return. (Spotted by Ken)
> > ---
> >  src/compiler/nir/nir_lower_returns.c | 37
> > ++++++++++++++++++++++++++++++------
> >  1 file changed, 31 insertions(+), 6 deletions(-)
> > 
> > diff --git a/src/compiler/nir/nir_lower_returns.c
> > b/src/compiler/nir/nir_lower_returns.c
> > index cf49d5b..5eec984 100644
> > --- a/src/compiler/nir/nir_lower_returns.c
> > +++ b/src/compiler/nir/nir_lower_returns.c
> > @@ -30,6 +30,8 @@ struct lower_returns_state {
> >     struct exec_list *cf_list;
> >     nir_loop *loop;
> >     nir_variable *return_flag;
> > +   /* Are there other return statments nested in the current if */
> > +   bool if_nested_return;
> >  };
> > 
> >  static bool lower_returns_in_cf_list(struct exec_list *cf_list,
> > @@ -82,8 +84,10 @@ lower_returns_in_loop(nir_loop *loop, struct
> > lower_returns_state *state)
> >      * flag set to true.  We need to predicate everything following
> > the loop
> >      * on the return flag.
> >      */
> > -   if (progress)
> > +   if (progress) {
> >        predicate_following(&loop->cf_node, state);
> > +      state->if_nested_return = true;
> > +   }
> > 
> >     return progress;
> >  }
> > @@ -91,10 +95,13 @@ lower_returns_in_loop(nir_loop *loop, struct
> > lower_returns_state *state)
> >  static bool
> >  lower_returns_in_if(nir_if *if_stmt, struct lower_returns_state
> > *state)
> >  {
> > -   bool progress;
> > +   bool progress, then_progress;
> > 
> > -   progress = lower_returns_in_cf_list(&if_stmt->then_list,
> > state);
> > -   progress = lower_returns_in_cf_list(&if_stmt->else_list, state)
> > || progress;
> > +   bool if_nested_return = state->if_nested_return;
> > +   state->if_nested_return = false;
> > +
> > +   then_progress = lower_returns_in_cf_list(&if_stmt->then_list,
> > state);
> > +   progress = lower_returns_in_cf_list(&if_stmt->else_list, state)
> > || then_progress;
> 
> I don't really get why we need this if_nested_return thing.  Why
> can't we just have two progress booleans called then_progress and
> else_progress and just do
> 
> if (then_progress && else_progress) {
>    predicate_following
> } else if (!then_progress && !else_progress) {
>    return false;
> } else {
>    /* Put it in one side or the other based on progress */
> }
> 
> That seems way simpler.

Way simpler yes but it doesn't do what we need it to :) Ken had the
same suggestion yesterday. The problem is it won't handle a case like
this:

if () {
  if () {
     return;
  } else {
    // If we exit from here we need to predicate the code following
    // the outer if, we cant just stick it in the else block.
  }
} else {

}

... code following outer if ...


>  
> >     /* If either of the recursive calls made progress, then there
> > were
> >      * returns inside of the body of the if.  If we're in a loop,
> > then these
> > @@ -106,8 +113,25 @@ lower_returns_in_if(nir_if *if_stmt, struct
> > lower_returns_state *state)
> >      * after a return, we need to predicate everything following on
> > the
> >      * return flag.
> >      */
> > -   if (progress && !state->loop)
> > -      predicate_following(&if_stmt->cf_node, state);
> > +   if (progress && !state->loop) {
> > +      if (state->if_nested_return) {
> > +         predicate_following(&if_stmt->cf_node, state);
> > +      } else {
> > +         /* If there are no nested returns we can just add the
> > instructions to
> > +          * the end of the branch that doesn't have the return.
> > +          */
> > +         nir_cf_list list;
> > +         nir_cf_extract(&list, nir_after_cf_node(&if_stmt-
> > >cf_node),
> > +                        nir_after_cf_list(state->cf_list));
> > +
> > +         if (then_progress)
> > +            nir_cf_reinsert(&list, nir_after_cf_list(&if_stmt-
> > >else_list));
> > +         else
> > +            nir_cf_reinsert(&list, nir_after_cf_list(&if_stmt-
> > >then_list));
> > +      }
> > +   }
> > +
> > +   state->if_nested_return = progress || if_nested_return;
> > 
> >     return progress;
> >  }
> > @@ -221,6 +245,7 @@ nir_lower_returns_impl(nir_function_impl *impl)
> >     state.cf_list = &impl->body;
> >     state.loop = NULL;
> >     state.return_flag = NULL;
> > +   state.if_nested_return = false;
> >     nir_builder_init(&state.builder, impl);
> > 
> >     bool progress = lower_returns_in_cf_list(&impl->body, &state);
> > --
> > 2.9.3
> > 
> > _______________________________________________
> > mesa-dev mailing list
> > mesa-dev at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/mesa-dev
> > 
> 
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