On Dec 26, 2015 3:04 PM, "Connor Abbott" <<a href="mailto:cwabbott0@gmail.com">cwabbott0@gmail.com</a>> wrote: > > On Sat, Dec 26, 2015 at 5:57 PM, Jason Ekstrand <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> wrote: > > > > On Dec 26, 2015 2:22 PM, "Connor Abbott" <<a href="mailto:cwabbott0@gmail.com">cwabbott0@gmail.com</a>> wrote: > >> > >> On Sat, Dec 26, 2015 at 2:09 PM, Jason Ekstrand <<a href="mailto:jason@jlekstrand.net">jason@jlekstrand.net</a>> > >> wrote: > >> > This commit adds a NIR pass for lowering away returns in functions. If > >> > the > >> > return is in a loop, it is lowered to a break. If it is not in a loop, > >> > it's lowered away by moving/deleting code as needed. > >> > --- > >> > > >> > Unfortunately, the loop-handling portion hasn't had proper testing. I'm > >> > pretty sure the approach is good, but my SPIR-V branch doesn't have > >> > working > >> > loops at the moment so I can't properly test. > >> > > >> > src/glsl/Makefile.sources | 1 + > >> > src/glsl/nir/nir.h | 3 + > >> > src/glsl/nir/nir_lower_returns.c | 243 > >> > +++++++++++++++++++++++++++++++++++++++ > >> > 3 files changed, 247 insertions(+) > >> > create mode 100644 src/glsl/nir/nir_lower_returns.c > >> > > >> > diff --git a/src/glsl/Makefile.sources b/src/glsl/Makefile.sources > >> > index fc10f14..fa3868c 100644 > >> > --- a/src/glsl/Makefile.sources > >> > +++ b/src/glsl/Makefile.sources > >> > @@ -43,6 +43,7 @@ NIR_FILES = \ > >> > nir/nir_lower_alu_to_scalar.c \ > >> > nir/nir_lower_atomics.c \ > >> > nir/nir_lower_clip.c \ > >> > + nir/nir_lower_returns.c \ > >> > nir/nir_lower_global_vars_to_local.c \ > >> > nir/nir_lower_gs_intrinsics.c \ > >> > nir/nir_lower_load_const_to_scalar.c \ > >> > diff --git a/src/glsl/nir/nir.h b/src/glsl/nir/nir.h > >> > index b6c8ffe..2e29617 100644 > >> > --- a/src/glsl/nir/nir.h > >> > +++ b/src/glsl/nir/nir.h > >> > @@ -1935,6 +1935,9 @@ int nir_gs_count_vertices(const nir_shader > >> > *shader); > >> > > >> > bool nir_split_var_copies(nir_shader *shader); > >> > > >> > +bool nir_lower_returns_impl(nir_function_impl *impl); > >> > +bool nir_lower_returns(nir_shader *shader); > >> > + > >> > void nir_lower_var_copy_instr(nir_intrinsic_instr *copy, void > >> > *mem_ctx); > >> > void nir_lower_var_copies(nir_shader *shader); > >> > > >> > diff --git a/src/glsl/nir/nir_lower_returns.c > >> > b/src/glsl/nir/nir_lower_returns.c > >> > new file mode 100644 > >> > index 0000000..ec77dde > >> > --- /dev/null > >> > +++ b/src/glsl/nir/nir_lower_returns.c > >> > @@ -0,0 +1,243 @@ > >> > +/* > >> > + * Copyright © 2015 Intel Corporation > >> > + * > >> > + * Permission is hereby granted, free of charge, to any person > >> > obtaining a > >> > + * copy of this software and associated documentation files (the > >> > "Software"), > >> > + * to deal in the Software without restriction, including without > >> > limitation > >> > + * the rights to use, copy, modify, merge, publish, distribute, > >> > sublicense, > >> > + * and/or sell copies of the Software, and to permit persons to whom > >> > the > >> > + * Software is furnished to do so, subject to the following conditions: > >> > + * > >> > + * The above copyright notice and this permission notice (including the > >> > next > >> > + * paragraph) shall be included in all copies or substantial portions > >> > of the > >> > + * Software. > >> > + * > >> > + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, > >> > EXPRESS OR > >> > + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF > >> > MERCHANTABILITY, > >> > + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT > >> > SHALL > >> > + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR > >> > OTHER > >> > + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, > >> > ARISING > >> > + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER > >> > DEALINGS > >> > + * IN THE SOFTWARE. > >> > + */ > >> > + > >> > +#include "nir.h" > >> > +#include "nir_builder.h" > >> > +#include "nir_control_flow.h" > >> > + > >> > +struct lower_returns_state { > >> > + nir_builder builder; > >> > + struct exec_list *parent_cf_list; > >> > + struct exec_list *cf_list; > >> > + nir_loop *loop; > >> > + nir_if *if_stmt; > >> > + nir_variable *return_flag; > >> > +}; > >> > + > >> > +static bool lower_returns_in_cf_list(struct exec_list *cf_list, > >> > + struct lower_returns_state > >> > *state); > >> > + > >> > +static bool > >> > +lower_returns_in_loop(nir_loop *loop, struct lower_returns_state > >> > *state) > >> > +{ > >> > + nir_loop *parent = state->loop; > >> > + state->loop = loop; > >> > + bool progress = lower_returns_in_cf_list(&loop->body, state); > >> > + state->loop = parent; > >> > + > >> > + /* Nothing interesting */ > >> > + if (!progress) > >> > + return false; > >> > + > >> > + /* In this case, there was a return somewhere inside of the loop. > >> > That > >> > + * return would have been turned into a write to the return_flag > >> > + * variable and a break. We need to insert a predicated return > >> > right > >> > + * after the loop ends. > >> > + */ > >> > + > >> > + assert(state->return_flag); > >> > + > >> > + nir_intrinsic_instr *load = > >> > + nir_intrinsic_instr_create(state->builder.shader, > >> > nir_intrinsic_load_var); > >> > + load->num_components = 1; > >> > + load->variables[0] = nir_deref_var_create(load, state->return_flag); > >> > + nir_ssa_dest_init(&load->instr, &load->dest, 1, "return"); > >> > + nir_instr_insert(nir_after_cf_node(&loop->cf_node), &load->instr); > >> > + > >> > + nir_if *if_stmt = nir_if_create(state->builder.shader); > >> > + if_stmt->condition = nir_src_for_ssa(&load->dest.ssa); > >> > + nir_cf_node_insert(nir_after_instr(&load->instr), > >> > &if_stmt->cf_node); > >> > + > >> > + nir_jump_instr *ret = > >> > + nir_jump_instr_create(state->builder.shader, nir_jump_return); > >> > + nir_instr_insert(nir_before_cf_list(&if_stmt->then_list), > >> > &ret->instr); > >> > + > >> > + return true; > >> > +} > >> > + > >> > +static bool > >> > +lower_returns_in_if(nir_if *if_stmt, struct lower_returns_state *state) > >> > +{ > >> > + bool progress; > >> > + > >> > + nir_if *parent = state->if_stmt; > >> > + state->if_stmt = if_stmt; > >> > + progress = lower_returns_in_cf_list(&if_stmt->then_list, state); > >> > + progress = lower_returns_in_cf_list(&if_stmt->else_list, state) || > >> > progress; > >> > + state->if_stmt = parent; > >> > + > >> > + return progress; > >> > +} > >> > + > >> > +static bool > >> > +lower_returns_in_block(nir_block *block, struct lower_returns_state > >> > *state) > >> > +{ > >> > + if (block->predecessors->entries == 0 && > >> > + block != nir_start_block(state->builder.impl)) { > >> > + /* This block is unreachable. Delete it and everything after it. > >> > */ > >> > + nir_cf_list list; > >> > + nir_cf_extract(&list, nir_before_cf_node(&block->cf_node), > >> > + nir_after_cf_list(state->cf_list)); > >> > + > >> > + if (exec_list_is_empty(&list.list)) { > >> > + /* There's nothing here, which also means there's nothing in > >> > this > >> > + * block so we have nothing to do. > >> > + */ > >> > + return false; > >> > + } else { > >> > + nir_cf_delete(&list); > >> > + return true; > >> > + } > >> > + } > >> > + > >> > + nir_instr *last_instr = nir_block_last_instr(block); > >> > + if (last_instr == NULL) > >> > + return false; > >> > + > >> > + if (last_instr->type != nir_instr_type_jump) > >> > + return false; > >> > + > >> > + nir_jump_instr *jump = nir_instr_as_jump(last_instr); > >> > + if (jump->type != nir_jump_return) > >> > + return false; > >> > + > >> > + if (state->loop) { > >> > + /* We're in a loop. Just set the return flag to true and break. > >> > + * lower_returns_in_loop will do the rest. > >> > + */ > >> > + nir_builder *b = &state->builder; > >> > + b->cursor = nir_before_instr(&jump->instr); > >> > + > >> > + if (state->return_flag == NULL) { > >> > + state->return_flag = > >> > + nir_local_variable_create(b->impl, glsl_bool_type(), > >> > "return"); > >> > + > >> > + /* Set a default value of false */ > >> > + state->return_flag->constant_initializer = > >> > + rzalloc(state->return_flag, nir_constant); > >> > + } > >> > + > >> > + nir_store_var(b, state->return_flag, nir_imm_int(b, NIR_TRUE), > >> > 1); > >> > + jump->type = nir_jump_return; > >> > + } else if (state->if_stmt) { > >> > + /* If we're not in a loop but in an if, just move the rest of the > >> > CF > >> > + * list into the the other case of the if. > >> > + */ > >> > + nir_cf_list list; > >> > + nir_cf_extract(&list, > >> > nir_after_cf_node(&state->if_stmt->cf_node), > >> > + nir_after_cf_list(state->parent_cf_list)); > >> > + > >> > + nir_instr_remove(&jump->instr); > >> > + > >> > + if (state->cf_list == &state->if_stmt->then_list) { > >> > + nir_cf_reinsert(&list, > >> > + > >> > nir_after_cf_list(&state->if_stmt->else_list)); > >> > + } else if (state->cf_list == &state->if_stmt->else_list) { > >> > + nir_cf_reinsert(&list, > >> > + > >> > nir_after_cf_list(&state->if_stmt->then_list)); > >> > + } else { > >> > + unreachable("Invalid CF list"); > >> > + } > >> > + } else { > >> > + nir_instr_remove(&jump->instr); > >> > + > >> > + /* No if, no nothing. Just delete the return and whatever > >> > follows. */ > >> > + nir_cf_list list; > >> > + nir_cf_extract(&list, nir_after_cf_node(&block->cf_node), > >> > + nir_after_cf_list(state->parent_cf_list)); > >> > + nir_cf_delete(&list); > >> > + } > >> > + > >> > + return true; > >> > +} > >> > >> > >> There are a few problems with this approach. > >> > >> First off, imagine something like: > >> > >> if (...) { > >> if (...) return; > >> } > >> ... > >> > >> If I'm reading this correctly, we'd lower this to: > >> > >> if (...) { > >> if (...) ; > >> } > >> ... > > > > Oops... You're right. I meant to add returns. This > > > > if (...) { > > if (...) return; > > // foo > > } > > > > Should be lowered to > > > > if (...) { > > if (...) { > > } else { > > // foo > > } > > return; > > } > > > > Which we can then lower further. > > I don't think that's correct though. What if there's stuff after the > outer if? It'll only get executed if the outer if condition is false, > instead of if either one is false. I think we really need the flag in > this case. If there's stuff in the outer if, it gets moved into the else > > > > That said, I need to think more about the order I iterate over things. > > --Jason > > > >> which isn't right. We want to skip the stuff outside the outer if only > >> if the condition of the inner if is true, that is we want: > >> > >> if (...) { > >> if (...) return_flag = true; > >> } > >> if (!return_flag) { > >> ... > >> } > >> > >> Similarly, this won't work if you have a loop with a return inside an > >> if. I think you need to unconditionally set the return_flag to true, > >> and then in lower_returns_in_if() you need to predicate the rest of > >> the rest of the control flow list with !return_flag if it's not empty > >> and you're not inside a loop. > >> > >> There are also some other things you could do to clean up the result > >> of this pass. Imagine that you have something like: > >> > >> if (...) { > >> while(...) { > >> if (...) return; > >> } > >> } > >> ... > >> > >> this would get lowered to: > >> > >> return_flag = false; > >> if (cond) { > >> while(...) { > >> if(...) { return_flag = true; break; } > >> } > >> } > >> if (!return_flag) { > >> ... > >> } > >> > >> The first thing to notice is that the return_flag will get lowered to > >> a phi node after the first (outer) if which will be true for the then > >> branch and false for the else branch. We can recognize that as being > >> equivalent to the condition of the if: > >> > >> if (cond) { > >> while(...) { > >> if(...) { break; } > >> } > >> } > >> if (!cond) { > >> ... > >> } > >> > >> Now, we can turn if (cond) { ... } if (!cond) { ... } into if (cond) { > >> ... } else { ... }: > >> > >> if (cond) { > >> while(...) { > >> if(...) { break; } > >> } > >> } else { > >> ... > >> } > >> > >> Note that these two optimizations combined make what you do here in > >> the "if (state->if_stmt)" case unnecessary. It also makes it more > >> robust. Imagine that in the earlier example I gave: > >> > >> if (...) { > >> if (...) return; > >> } > >> ... > >> > >> we're able to determine, in NIR, that the outer if's condition is > >> always true. With what we have now, if dead_cf runs first then we get: > >> > >> if (...) { > >> } else { > >> ... > >> } > >> > >> whereas if lower_returns runs first we get: > >> > >> if (...) { return_flag = true; } > >> if (!return_flag) { > >> ... > >> } > >> > >> which we don't want. But with the two optimizations, we always get the > >> right thing. > >> > >> > + > >> > +static bool > >> > +lower_returns_in_cf_list(struct exec_list *cf_list, > >> > + struct lower_returns_state *state) > >> > +{ > >> > + bool progress = false; > >> > + > >> > + struct exec_list *prev_parent_list = state->parent_cf_list; > >> > + state->parent_cf_list = state->cf_list; > >> > + state->cf_list = cf_list; > >> > + > >> > + foreach_list_typed_reverse_safe(nir_cf_node, node, node, cf_list) { > >> > + switch (node->type) { > >> > + case nir_cf_node_block: > >> > + if (lower_returns_in_block(nir_cf_node_as_block(node), state)) > >> > + progress = true; > >> > + break; > >> > + > >> > + case nir_cf_node_if: > >> > + if (lower_returns_in_if(nir_cf_node_as_if(node), state)) > >> > + progress = true; > >> > + break; > >> > + > >> > + case nir_cf_node_loop: > >> > + if (lower_returns_in_loop(nir_cf_node_as_loop(node), state)) > >> > + progress = true; > >> > + break; > >> > + > >> > + default: > >> > + unreachable("Invalid inner CF node type"); > >> > + } > >> > + } > >> > + > >> > + state->cf_list = state->parent_cf_list; > >> > + state->parent_cf_list = prev_parent_list; > >> > + > >> > + return progress; > >> > +} > >> > + > >> > +bool > >> > +nir_lower_returns_impl(nir_function_impl *impl) > >> > +{ > >> > + struct lower_returns_state state; > >> > + > >> > + state.parent_cf_list = NULL; > >> > + state.cf_list = &impl->body; > >> > + state.loop = NULL; > >> > + state.if_stmt = NULL; > >> > + state.return_flag = NULL; > >> > + nir_builder_init(&state.builder, impl); > >> > + > >> > + bool progress = lower_returns_in_cf_list(&impl->body, &state); > >> > + > >> > + if (progress) > >> > + nir_metadata_preserve(impl, nir_metadata_none); > >> > + > >> > + return progress; > >> > +} > >> > + > >> > +bool > >> > +nir_lower_returns(nir_shader *shader) > >> > +{ > >> > + bool progress = false; > >> > + > >> > + nir_foreach_function(shader, function) { > >> > + if (function->impl) > >> > + progress = nir_lower_returns_impl(function->impl) || progress; > >> > + } > >> > + > >> > + return progress; > >> > +} > >> > -- > >> > 2.5.0.400.gff86faf > >> > > >> > _______________________________________________ > >> > mesa-dev mailing list > >> > <a href="mailto:mesa-dev@lists.freedesktop.org">mesa-dev@lists.freedesktop.org</a> > >> > <a href="http://lists.freedesktop.org/mailman/listinfo/mesa-dev">http://lists.freedesktop.org/mailman/listinfo/mesa-dev</a>