[Mesa-dev] [PATCH 3/5] i965/fs: New peephole optimization to generate SEL.
Paul Berry
stereotype441 at gmail.com
Fri Nov 1 23:02:05 CET 2013
On 31 October 2013 18:57, Matt Turner <mattst88 at gmail.com> wrote:
> fs_visitor::try_replace_with_sel optimizes only if statements whose
> "then" and "else" bodies contain a single MOV instruction. It also
> could not handle constant arguments, since they cause an extra MOV
> immediate to be generated (since we haven't run constant propagation,
> there are more than the single MOV).
>
> This peephole fixes both of these and operates as a normal optimization
> pass.
>
> fs_visitor::try_replace_with_sel is still arguably necessary, since it
> runs before pull constant loads are lowered.
>
> total instructions in shared programs: 1417596 -> 1405551 (-0.85%)
> instructions in affected programs: 168590 -> 156545 (-7.14%)
> GAINED: 10
> LOST: 4
> ---
> src/mesa/drivers/dri/i965/Makefile.sources | 1 +
> src/mesa/drivers/dri/i965/brw_fs.cpp | 1 +
> src/mesa/drivers/dri/i965/brw_fs.h | 1 +
> src/mesa/drivers/dri/i965/brw_fs_sel_peephole.cpp | 217
> ++++++++++++++++++++++
> 4 files changed, 220 insertions(+)
> create mode 100644 src/mesa/drivers/dri/i965/brw_fs_sel_peephole.cpp
>
> diff --git a/src/mesa/drivers/dri/i965/Makefile.sources
> b/src/mesa/drivers/dri/i965/Makefile.sources
> index 06ac843..79ed3dd 100644
> --- a/src/mesa/drivers/dri/i965/Makefile.sources
> +++ b/src/mesa/drivers/dri/i965/Makefile.sources
> @@ -60,6 +60,7 @@ i965_FILES = \
> brw_fs_fp.cpp \
> brw_fs_generator.cpp \
> brw_fs_live_variables.cpp \
> + brw_fs_sel_peephole.cpp \
> brw_fs_reg_allocate.cpp \
> brw_fs_vector_splitting.cpp \
> brw_fs_visitor.cpp \
> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp
> b/src/mesa/drivers/dri/i965/brw_fs.cpp
> index df17684..345dc19 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
> @@ -3107,6 +3107,7 @@ fs_visitor::run()
> progress = opt_algebraic() || progress;
> progress = opt_cse() || progress;
> progress = opt_copy_propagate() || progress;
> + progress = opt_peephole_sel() || progress;
> progress = dead_code_eliminate() || progress;
> progress = dead_code_eliminate_local() || progress;
> progress = dead_control_flow_eliminate(this) || progress;
> diff --git a/src/mesa/drivers/dri/i965/brw_fs.h
> b/src/mesa/drivers/dri/i965/brw_fs.h
> index 849e49f..a592dda 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs.h
> +++ b/src/mesa/drivers/dri/i965/brw_fs.h
> @@ -363,6 +363,7 @@ public:
> bool try_emit_saturate(ir_expression *ir);
> bool try_emit_mad(ir_expression *ir, int mul_arg);
> void try_replace_with_sel();
> + bool opt_peephole_sel();
> void emit_bool_to_cond_code(ir_rvalue *condition);
> void emit_if_gen6(ir_if *ir);
> void emit_unspill(fs_inst *inst, fs_reg reg, uint32_t spill_offset,
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_sel_peephole.cpp
> b/src/mesa/drivers/dri/i965/brw_fs_sel_peephole.cpp
> new file mode 100644
> index 0000000..9626751
> --- /dev/null
> +++ b/src/mesa/drivers/dri/i965/brw_fs_sel_peephole.cpp
> @@ -0,0 +1,217 @@
> +/*
> + * Copyright © 2013 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 "brw_fs.h"
> +#include "brw_cfg.h"
> +
> +/** @file brw_fs_sel_peephole.cpp
> + *
> + * This file contains the opt_peephole_sel() optimization pass that
> replaces
> + * MOV instructions to the same destination in the "then" and "else"
> bodies of
> + * an if statement with SEL instructions.
> + */
> +
> +#define MAX_MOVS 8 /**< The maximum number of MOVs to attempt to match. */
>
In the last round of review, you gave me some nice clarification on why
this was chosen to be 8. Would you mind documenting that in the comment?
Other than that, the patch is:
Reviewed-by: Paul Berry <stereotype441 at gmail.com>
> +
> +/**
> + * Scans forwards from an IF counting consecutive MOV instructions in the
> + * "then" and "else" blocks of the if statement.
> + *
> + * A pointer to the fs_inst* for IF is passed as the <if_inst> argument.
> The
> + * function stores pointers to the MOV instructions in the <then_mov> and
> + * <else_mov> arrays.
> + *
> + * \return the minimum number of MOVs found in the two branches or zero if
> + * an error occurred.
> + *
> + * E.g.:
> + * IF ...
> + * then_mov[0] = MOV g4, ...
> + * then_mov[1] = MOV g5, ...
> + * then_mov[2] = MOV g6, ...
> + * ELSE ...
> + * else_mov[0] = MOV g4, ...
> + * else_mov[1] = MOV g5, ...
> + * else_mov[2] = MOV g7, ...
> + * ENDIF
> + * returns 3.
>
+ */
> +static int
> +count_movs_from_if(fs_inst *then_mov[MAX_MOVS], fs_inst
> *else_mov[MAX_MOVS],
> + fs_inst *if_inst, fs_inst *else_inst)
> +{
> + fs_inst *m = if_inst;
> +
> + assert(m->opcode == BRW_OPCODE_IF);
> + m = (fs_inst *) m->next;
> +
> + int then_movs = 0;
> + while (then_movs < MAX_MOVS && m->opcode == BRW_OPCODE_MOV) {
> + then_mov[then_movs] = m;
> + m = (fs_inst *) m->next;
> + then_movs++;
> + }
> +
> + m = (fs_inst *) else_inst->next;
> +
> + int else_movs = 0;
> + while (else_movs < MAX_MOVS && m->opcode == BRW_OPCODE_MOV) {
> + else_mov[else_movs] = m;
> + m = (fs_inst *) m->next;
> + else_movs++;
> + }
> +
> + return MIN2(then_movs, else_movs);
> +}
> +
> +/**
> + * Try to replace IF/MOV+/ELSE/MOV+/ENDIF with SEL.
> + *
> + * Many GLSL shaders contain the following pattern:
> + *
> + * x = condition ? foo : bar
> + *
> + * or
> + *
> + * if (...) a.xyzw = foo.xyzw;
> + * else a.xyzw = bar.xyzw;
> + *
> + * The compiler emits an ir_if tree for this, since each subexpression
> might be
> + * a complex tree that could have side-effects or short-circuit logic.
> + *
> + * However, the common case is to simply select one of two constants or
> + * variable values---which is exactly what SEL is for. In this case, the
> + * assembly looks like:
> + *
> + * (+f0) IF
> + * MOV dst src0
> + * ...
> + * ELSE
> + * MOV dst src1
> + * ...
> + * ENDIF
> + *
> + * where each pair of MOVs to a common destination and can be easily
> translated
> + * into
> + *
> + * (+f0) SEL dst src0 src1
> + *
> + * If src0 is an immediate value, we promote it to a temporary GRF.
> + */
> +bool
> +fs_visitor::opt_peephole_sel()
> +{
> + bool progress = false;
> +
> + cfg_t cfg(this);
> +
> + for (int b = 0; b < cfg.num_blocks; b++) {
> + bblock_t *block = cfg.blocks[b];
> +
> + /* IF instructions, by definition, can only be found at the ends of
> + * basic blocks.
> + */
> + fs_inst *if_inst = (fs_inst *) block->end;
> + if (if_inst->opcode != BRW_OPCODE_IF)
> + continue;
> +
> + if (!block->else_inst)
> + continue;
> +
> + fs_inst *else_inst = (fs_inst *) block->else_inst;
> +
> + fs_inst *else_mov[MAX_MOVS] = { NULL };
> + fs_inst *then_mov[MAX_MOVS] = { NULL };
> + bool malformed_mov_found = false;
> +
> + int movs = count_movs_from_if(then_mov, else_mov, if_inst,
> else_inst);
> +
> + if (movs == 0)
> + continue;
> +
> + fs_inst *sel_inst[MAX_MOVS] = { NULL };
> + fs_inst *mov_imm_inst[MAX_MOVS] = { NULL };
> +
> + /* Generate SEL instructions for pairs of MOVs to a common
> destination. */
> + for (int i = 0; i < movs; i++) {
> + if (!then_mov[i] || !else_mov[i])
> + break;
> +
> + /* Check that the MOVs are the right form. */
> + if (!then_mov[i]->dst.equals(else_mov[i]->dst) ||
> + then_mov[i]->is_partial_write() ||
> + else_mov[i]->is_partial_write()) {
> + malformed_mov_found = true;
> + break;
> + }
> +
> + /* Only the last source register can be a constant, so if the
> MOV in
> + * the "then" clause uses a constant, we need to put it in a
> + * temporary.
> + */
> + fs_reg src0(then_mov[i]->src[0]);
> + if (src0.file == IMM) {
> + src0 = fs_reg(this, glsl_type::float_type);
> + src0.type = then_mov[i]->src[0].type;
> + mov_imm_inst[i] = MOV(src0, then_mov[i]->src[0]);
> + }
> +
> + sel_inst[i] = SEL(then_mov[i]->dst, src0, else_mov[i]->src[0]);
> +
> + if (brw->gen == 6 && if_inst->conditional_mod) {
> + /* For Sandybridge with IF with embedded comparison */
> + sel_inst[i]->predicate = BRW_PREDICATE_NORMAL;
> + } else {
> + /* Separate CMP and IF instructions */
> + sel_inst[i]->predicate = if_inst->predicate;
> + sel_inst[i]->predicate_inverse = if_inst->predicate_inverse;
> + }
> + }
> +
> + if (malformed_mov_found)
> + continue;
> +
> + /* Emit a CMP if our IF used the embedded comparison */
> + if (brw->gen == 6 && if_inst->conditional_mod) {
> + fs_inst *cmp_inst = CMP(reg_null_d, if_inst->src[0],
> if_inst->src[1],
> + if_inst->conditional_mod);
> + if_inst->insert_before(cmp_inst);
> + }
> +
> + for (int i = 0; i < movs; i++) {
> + if (mov_imm_inst[i])
> + if_inst->insert_before(mov_imm_inst[i]);
> + if_inst->insert_before(sel_inst[i]);
> +
> + then_mov[i]->remove();
> + else_mov[i]->remove();
> + }
> +
> + progress = true;
> + }
> +
> + if (progress)
> + invalidate_live_intervals();
> +
> + return progress;
> +}
> --
> 1.8.3.2
>
>
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