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25 * \file lower_instructions.cpp
27 * Many GPUs lack native instructions for certain expression operations, and
28 * must replace them with some other expression tree. This pass lowers some
29 * of the most common cases, allowing the lowering code to be implemented once
30 * rather than in each driver backend.
32 * Currently supported transformations:
35 * - INT_DIV_TO_MUL_RCP
43 * Breaks an ir_binop_sub expression down to add(op0, neg(op1))
45 * This simplifies expression reassociation, and for many backends
46 * there is no subtract operation separate from adding the negation.
47 * For backends with native subtract operations, they will probably
48 * want to recognize add(op0, neg(op1)) or the other way around to
49 * produce a subtract anyway.
51 * DIV_TO_MUL_RCP and INT_DIV_TO_MUL_RCP:
52 * --------------------------------------
53 * Breaks an ir_unop_div expression down to op0 * (rcp(op1)).
55 * Many GPUs don't have a divide instruction (945 and 965 included),
56 * but they do have an RCP instruction to compute an approximate
57 * reciprocal. By breaking the operation down, constant reciprocals
58 * can get constant folded.
60 * DIV_TO_MUL_RCP only lowers floating point division; INT_DIV_TO_MUL_RCP
61 * handles the integer case, converting to and from floating point so that
64 * EXP_TO_EXP2 and LOG_TO_LOG2:
65 * ----------------------------
66 * Many GPUs don't have a base e log or exponent instruction, but they
67 * do have base 2 versions, so this pass converts exp and log to exp2
68 * and log2 operations.
72 * Many older GPUs don't have an x**y instruction. For these GPUs, convert
73 * x**y to 2**(y * log2(x)).
77 * Breaks an ir_unop_mod expression down to (op1 * fract(op0 / op1))
79 * Many GPUs don't have a MOD instruction (945 and 965 included), and
80 * if we have to break it down like this anyway, it gives an
81 * opportunity to do things like constant fold the (1.0 / op1) easily.
84 #include "main/core.h" /* for M_LOG2E */
85 #include "glsl_types.h"
87 #include "ir_optimization.h"
89 class lower_instructions_visitor : public ir_hierarchical_visitor {
91 lower_instructions_visitor(unsigned lower)
92 : progress(false), lower(lower) { }
94 ir_visitor_status visit_leave(ir_expression *);
99 unsigned lower; /** Bitfield of which operations to lower */
101 void sub_to_add_neg(ir_expression *);
102 void div_to_mul_rcp(ir_expression *);
103 void int_div_to_mul_rcp(ir_expression *);
104 void mod_to_fract(ir_expression *);
105 void exp_to_exp2(ir_expression *);
106 void pow_to_exp2(ir_expression *);
107 void log_to_log2(ir_expression *);
111 * Determine if a particular type of lowering should occur
113 #define lowering(x) (this->lower & x)
116 lower_instructions(exec_list *instructions, unsigned what_to_lower)
118 lower_instructions_visitor v(what_to_lower);
120 visit_list_elements(&v, instructions);
125 lower_instructions_visitor::sub_to_add_neg(ir_expression *ir)
127 ir->operation = ir_binop_add;
128 ir->operands[1] = new(ir) ir_expression(ir_unop_neg, ir->operands[1]->type,
129 ir->operands[1], NULL);
130 this->progress = true;
134 lower_instructions_visitor::div_to_mul_rcp(ir_expression *ir)
136 assert(ir->operands[1]->type->is_float());
138 /* New expression for the 1.0 / op1 */
140 expr = new(ir) ir_expression(ir_unop_rcp,
141 ir->operands[1]->type,
144 /* op0 / op1 -> op0 * (1.0 / op1) */
145 ir->operation = ir_binop_mul;
146 ir->operands[1] = expr;
148 this->progress = true;
152 lower_instructions_visitor::int_div_to_mul_rcp(ir_expression *ir)
154 assert(ir->operands[1]->type->is_integer());
156 /* Be careful with integer division -- we need to do it as a
157 * float and re-truncate, since rcp(n > 1) of an integer would
160 ir_rvalue *op0, *op1;
161 const struct glsl_type *vec_type;
163 vec_type = glsl_type::get_instance(GLSL_TYPE_FLOAT,
164 ir->operands[1]->type->vector_elements,
165 ir->operands[1]->type->matrix_columns);
167 if (ir->operands[1]->type->base_type == GLSL_TYPE_INT)
168 op1 = new(ir) ir_expression(ir_unop_i2f, vec_type, ir->operands[1], NULL);
170 op1 = new(ir) ir_expression(ir_unop_u2f, vec_type, ir->operands[1], NULL);
172 op1 = new(ir) ir_expression(ir_unop_rcp, op1->type, op1, NULL);
174 vec_type = glsl_type::get_instance(GLSL_TYPE_FLOAT,
175 ir->operands[0]->type->vector_elements,
176 ir->operands[0]->type->matrix_columns);
178 if (ir->operands[0]->type->base_type == GLSL_TYPE_INT)
179 op0 = new(ir) ir_expression(ir_unop_i2f, vec_type, ir->operands[0], NULL);
181 op0 = new(ir) ir_expression(ir_unop_u2f, vec_type, ir->operands[0], NULL);
183 vec_type = glsl_type::get_instance(GLSL_TYPE_FLOAT,
184 ir->type->vector_elements,
185 ir->type->matrix_columns);
187 op0 = new(ir) ir_expression(ir_binop_mul, vec_type, op0, op1);
189 if (ir->operands[1]->type->base_type == GLSL_TYPE_INT) {
190 ir->operation = ir_unop_f2i;
191 ir->operands[0] = op0;
193 ir->operation = ir_unop_i2u;
194 ir->operands[0] = new(ir) ir_expression(ir_unop_f2i, op0);
196 ir->operands[1] = NULL;
198 this->progress = true;
202 lower_instructions_visitor::exp_to_exp2(ir_expression *ir)
204 ir_constant *log2_e = new(ir) ir_constant(float(M_LOG2E));
206 ir->operation = ir_unop_exp2;
207 ir->operands[0] = new(ir) ir_expression(ir_binop_mul, ir->operands[0]->type,
208 ir->operands[0], log2_e);
209 this->progress = true;
213 lower_instructions_visitor::pow_to_exp2(ir_expression *ir)
215 ir_expression *const log2_x =
216 new(ir) ir_expression(ir_unop_log2, ir->operands[0]->type,
219 ir->operation = ir_unop_exp2;
220 ir->operands[0] = new(ir) ir_expression(ir_binop_mul, ir->operands[1]->type,
221 ir->operands[1], log2_x);
222 ir->operands[1] = NULL;
223 this->progress = true;
227 lower_instructions_visitor::log_to_log2(ir_expression *ir)
229 ir->operation = ir_binop_mul;
230 ir->operands[0] = new(ir) ir_expression(ir_unop_log2, ir->operands[0]->type,
231 ir->operands[0], NULL);
232 ir->operands[1] = new(ir) ir_constant(float(1.0 / M_LOG2E));
233 this->progress = true;
237 lower_instructions_visitor::mod_to_fract(ir_expression *ir)
239 ir_variable *temp = new(ir) ir_variable(ir->operands[1]->type, "mod_b",
241 this->base_ir->insert_before(temp);
243 ir_assignment *const assign =
244 new(ir) ir_assignment(new(ir) ir_dereference_variable(temp),
245 ir->operands[1], NULL);
247 this->base_ir->insert_before(assign);
249 ir_expression *const div_expr =
250 new(ir) ir_expression(ir_binop_div, ir->operands[0]->type,
252 new(ir) ir_dereference_variable(temp));
254 /* Don't generate new IR that would need to be lowered in an additional
257 if (lowering(DIV_TO_MUL_RCP))
258 div_to_mul_rcp(div_expr);
260 ir_rvalue *expr = new(ir) ir_expression(ir_unop_fract,
261 ir->operands[0]->type,
265 ir->operation = ir_binop_mul;
266 ir->operands[0] = new(ir) ir_dereference_variable(temp);
267 ir->operands[1] = expr;
268 this->progress = true;
272 lower_instructions_visitor::visit_leave(ir_expression *ir)
274 switch (ir->operation) {
276 if (lowering(SUB_TO_ADD_NEG))
281 if (ir->operands[1]->type->is_integer() && lowering(INT_DIV_TO_MUL_RCP))
282 int_div_to_mul_rcp(ir);
283 else if (ir->operands[1]->type->is_float() && lowering(DIV_TO_MUL_RCP))
288 if (lowering(EXP_TO_EXP2))
293 if (lowering(LOG_TO_LOG2))
298 if (lowering(MOD_TO_FRACT) && ir->type->is_float())
303 if (lowering(POW_TO_EXP2))
308 return visit_continue;
311 return visit_continue;