src/compiler/nir/nir_inline_functions.c

   1 /*
   2  * Copyright © 2015 Intel Corporation
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining a
   5  * copy of this software and associated documentation files (the "Software"),
   6  * to deal in the Software without restriction, including without limitation
   7  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
   8  * and/or sell copies of the Software, and to permit persons to whom the
   9  * Software is furnished to do so, subject to the following conditions:
  10  *
  11  * The above copyright notice and this permission notice (including the next
  12  * paragraph) shall be included in all copies or substantial portions of the
  13  * Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  16  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
  18  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  19  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  20  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  21  * IN THE SOFTWARE.
  22  */
  23
  24 #include "nir.h"
  25 #include "nir_builder.h"
  26 #include "nir_control_flow.h"
  27 #include "nir_vla.h"
  28
  29 void nir_inline_function_impl(struct nir_builder *b,
  30                               const nir_function_impl *impl,
  31                               nir_ssa_def **params)
  32 {
  33    nir_function_impl *copy = nir_function_impl_clone(b->shader, impl);
  34
  35    /* Insert a nop at the cursor so we can keep track of where things are as
  36     * we add/remove stuff from the CFG.
  37     */
  38    nir_intrinsic_instr *nop =
  39       nir_intrinsic_instr_create(b->shader, nir_intrinsic_nop);
  40    nir_builder_instr_insert(b, &nop->instr);
  41
  42    exec_list_append(&b->impl->locals, &copy->locals);
  43    exec_list_append(&b->impl->registers, &copy->registers);
  44
  45    nir_foreach_block(block, copy) {
  46       nir_foreach_instr_safe(instr, block) {
  47          /* Returns have to be lowered for this to work */
  48          assert(instr->type != nir_instr_type_jump ||
  49                 nir_instr_as_jump(instr)->type != nir_jump_return);
  50
  51          if (instr->type != nir_instr_type_intrinsic)
  52             continue;
  53
  54          nir_intrinsic_instr *load = nir_instr_as_intrinsic(instr);
  55          if (load->intrinsic != nir_intrinsic_load_param)
  56             continue;
  57
  58          unsigned param_idx = nir_intrinsic_param_idx(load);
  59          assert(param_idx < impl->function->num_params);
  60          assert(load->dest.is_ssa);
  61          nir_ssa_def_rewrite_uses(&load->dest.ssa,
  62                                   nir_src_for_ssa(params[param_idx]));
  63
  64          /* Remove any left-over load_param intrinsics because they're soon
  65           * to be in another function and therefore no longer valid.
  66           */
  67          nir_instr_remove(&load->instr);
  68       }
  69    }
  70
  71    /* Pluck the body out of the function and place it here */
  72    nir_cf_list body;
  73    nir_cf_list_extract(&body, &copy->body);
  74    nir_cf_reinsert(&body, nir_before_instr(&nop->instr));
  75
  76    b->cursor = nir_instr_remove(&nop->instr);
  77 }
  78
  79 static bool inline_function_impl(nir_function_impl *impl, struct set *inlined);
  80
  81 static bool
  82 inline_functions_block(nir_block *block, nir_builder *b,
  83                        struct set *inlined)
  84 {
  85    bool progress = false;
  86    /* This is tricky.  We're iterating over instructions in a block but, as
  87     * we go, the block and its instruction list are being split into
  88     * pieces.  However, this *should* be safe since foreach_safe always
  89     * stashes the next thing in the iteration.  That next thing will
  90     * properly get moved to the next block when it gets split, and we
  91     * continue iterating there.
  92     */
  93    nir_foreach_instr_safe(instr, block) {
  94       if (instr->type != nir_instr_type_call)
  95          continue;
  96
  97       progress = true;
  98
  99       nir_call_instr *call = nir_instr_as_call(instr);
 100       assert(call->callee->impl);
 101
 102       /* Make sure that the function we're calling is already inlined */
 103       inline_function_impl(call->callee->impl, inlined);
 104
 105       b->cursor = nir_instr_remove(&call->instr);
 106
 107       /* Rewrite all of the uses of the callee's parameters to use the call
 108        * instructions sources.  In order to ensure that the "load" happens
 109        * here and not later (for register sources), we make sure to convert it
 110        * to an SSA value first.
 111        */
 112       const unsigned num_params = call->num_params;
 113       NIR_VLA(nir_ssa_def *, params, num_params);
 114       for (unsigned i = 0; i < num_params; i++) {
 115          params[i] = nir_ssa_for_src(b, call->params[i],
 116                                      call->callee->params[i].num_components);
 117       }
 118
 119       nir_inline_function_impl(b, call->callee->impl, params);
 120    }
 121
 122    return progress;
 123 }
 124
 125 static bool
 126 inline_function_impl(nir_function_impl *impl, struct set *inlined)
 127 {
 128    if (_mesa_set_search(inlined, impl))
 129       return false; /* Already inlined */
 130
 131    nir_builder b;
 132    nir_builder_init(&b, impl);
 133
 134    bool progress = false;
 135    nir_foreach_block_safe(block, impl) {
 136       progress |= inline_functions_block(block, &b, inlined);
 137    }
 138
 139    if (progress) {
 140       /* SSA and register indices are completely messed up now */
 141       nir_index_ssa_defs(impl);
 142       nir_index_local_regs(impl);
 143
 144       nir_metadata_preserve(impl, nir_metadata_none);
 145    } else {
 146       nir_metadata_preserve(impl, nir_metadata_all);
 147    }
 148
 149    _mesa_set_add(inlined, impl);
 150
 151    return progress;
 152 }
 153
 154 /** A pass to inline all functions in a shader into their callers
 155  *
 156  * For most use-cases, function inlining is a multi-step process.  The general
 157  * pattern employed by SPIR-V consumers and others is as follows:
 158  *
 159  *  1. nir_lower_variable_initializers(shader, nir_var_function_temp)
 160  *
 161  *     This is needed because local variables from the callee are simply added
 162  *     to the locals list for the caller and the information about where the
 163  *     constant initializer logically happens is lost.  If the callee is
 164  *     called in a loop, this can cause the variable to go from being
 165  *     initialized once per loop iteration to being initialized once at the
 166  *     top of the caller and values to persist from one invocation of the
 167  *     callee to the next.  The simple solution to this problem is to get rid
 168  *     of constant initializers before function inlining.
 169  *
 170  *  2. nir_lower_returns(shader)
 171  *
 172  *     nir_inline_functions assumes that all functions end "naturally" by
 173  *     execution reaching the end of the function without any return
 174  *     instructions causing instant jumps to the end.  Thanks to NIR being
 175  *     structured, we can't represent arbitrary jumps to various points in the
 176  *     program which is what an early return in the callee would have to turn
 177  *     into when we inline it into the caller.  Instead, we require returns to
 178  *     be lowered which lets us just copy+paste the callee directly into the
 179  *     caller.
 180  *
 181  *  3. nir_inline_functions(shader)
 182  *
 183  *     This does the actual function inlining and the resulting shader will
 184  *     contain no call instructions.
 185  *
 186  *  4. nir_opt_deref(shader)
 187  *
 188  *     Most functions contain pointer parameters where the result of a deref
 189  *     instruction is passed in as a parameter, loaded via a load_param
 190  *     intrinsic, and then turned back into a deref via a cast.  Function
 191  *     inlining will get rid of the load_param but we are still left with a
 192  *     cast.  Running nir_opt_deref gets rid of the intermediate cast and
 193  *     results in a whole deref chain again.  This is currently required by a
 194  *     number of optimizations and lowering passes at least for certain
 195  *     variable modes.
 196  *
 197  *  5. Loop over the functions and delete all but the main entrypoint.
 198  *
 199  *     In the Intel Vulkan driver this looks like this:
 200  *
 201  *        foreach_list_typed_safe(nir_function, func, node, &nir->functions) {
 202  *           if (func != entry_point)
 203  *              exec_node_remove(&func->node);
 204  *        }
 205  *        assert(exec_list_length(&nir->functions) == 1);
 206  *
 207  *    While nir_inline_functions does get rid of all call instructions, it
 208  *    doesn't get rid of any functions because it doesn't know what the "root
 209  *    function" is.  Instead, it's up to the individual driver to know how to
 210  *    decide on a root function and delete the rest.  With SPIR-V,
 211  *    spirv_to_nir returns the root function and so we can just use == whereas
 212  *    with GL, you may have to look for a function named "main".
 213  *
 214  *  6. nir_lower_variable_initializers(shader, ~nir_var_function_temp)
 215  *
 216  *     Lowering constant initializers on inputs, outputs, global variables,
 217  *     etc. requires that we know the main entrypoint so that we know where to
 218  *     initialize them.  Otherwise, we would have to assume that anything
 219  *     could be a main entrypoint and initialize them at the start of every
 220  *     function but that would clearly be wrong if any of those functions were
 221  *     ever called within another function.  Simply requiring a single-
 222  *     entrypoint function shader is the best way to make it well-defined.
 223  */
 224 bool
 225 nir_inline_functions(nir_shader *shader)
 226 {
 227    struct set *inlined = _mesa_pointer_set_create(NULL);
 228    bool progress = false;
 229
 230    nir_foreach_function(function, shader) {
 231       if (function->impl)
 232          progress = inline_function_impl(function->impl, inlined) || progress;
 233    }
 234
 235    _mesa_set_destroy(inlined, NULL);
 236
 237    return progress;
 238 }