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. 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>