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i965/nir/opt_peephole_ffma: Bypass fusion if any operand of fadd and fmul is a const

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When both fadd and fmul instructions have at least one operand that is a
constant and it is only used once, the total number of instructions can
be reduced from 3 (1 ffma + 2 load_const) to 2 (1 fmul + 1 fadd); because
the constants will be progagated as immediate operands of fmul and fadd.

This patch detects these situations and prevents fusing fmul+fadd into ffma.

Shader-db results on i965 Haswell:

total instructions in shared programs: 6235835 -> 6225895 (-0.16%)
instructions in affected programs:     1124094 -> 1114154 (-0.88%)
total loops in shared programs:        1979 -> 1979 (0.00%)
helped:                                7612
HURT:                                  843
GAINED:                                4
LOST:                                  0

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
This commit is contained in:
Eduardo Lima Mitev
2015-10-22 15:32:13 +02:00
parent fb3b5669ce
commit de51676b41
1 changed files with 31 additions and 0 deletions
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src/mesa/drivers/dri/i965/brw_nir_opt_peephole_ffma.c
+31
View File
@@ -133,6 +133,28 @@ get_mul_for_src(nir_alu_src *src, int num_components,
return alu;
}
/**
* Given a list of (at least two) nir_alu_src's, tells if any of them is a
* constant value and is used only once.
*/
static bool
any_alu_src_is_a_constant(nir_alu_src srcs[])
{
for (unsigned i = 0; i < 2; i++) {
if (srcs[i].src.ssa->parent_instr->type == nir_instr_type_load_const) {
nir_load_const_instr *load_const =
nir_instr_as_load_const (srcs[i].src.ssa->parent_instr);
if (list_is_singular(&load_const->def.uses) &&
list_empty(&load_const->def.if_uses)) {
return true;
}
}
}
return false;
}
static bool
brw_nir_opt_peephole_ffma_block(nir_block *block, void *void_state)
{
@@ -183,6 +205,15 @@ brw_nir_opt_peephole_ffma_block(nir_block *block, void *void_state)
mul_src[0] = mul->src[0].src.ssa;
mul_src[1] = mul->src[1].src.ssa;
/* If any of the operands of the fmul and any of the fadd is a constant,
* we bypass because it will be more efficient as the constants will be
* propagated as operands, potentially saving two load_const instructions.
*/
if (any_alu_src_is_a_constant(mul->src) &&
any_alu_src_is_a_constant(add->src)) {
continue;
}
if (abs) {
for (unsigned i = 0; i < 2; i++) {
nir_alu_instr *abs = nir_alu_instr_create(state->mem_ctx,