From: Eduardo Lima Mitev <el...@igalia.com>
Currently, we use byte-scattered write messages for storing 16-bit
into an SSBO. This is because untyped surface messages have a fixed
32-bit size.
This patch optimizes these 16-bit writes by combining 2 values (e.g,
two consecutive components) into a 32-bit register, packing the two
16-bit words.
16-bit single component values will continue to use byte-scattered
write messages.
This optimization reduces the number of SEND messages used for storing
16-bit values potentially by half, which cuts down execution time
significantly because byte-scattered writes are an expensive
operation.
---
src/intel/compiler/brw_fs_nir.cpp | 65 +++++++++++++++++++++++++++++++++------
1 file changed, 56 insertions(+), 9 deletions(-)
diff --git a/src/intel/compiler/brw_fs_nir.cpp
b/src/intel/compiler/brw_fs_nir.cpp
index 76f1359b164..63191ef3746 100644
--- a/src/intel/compiler/brw_fs_nir.cpp
+++ b/src/intel/compiler/brw_fs_nir.cpp
@@ -4222,6 +4222,9 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld,
nir_intrinsic_instr *instr
unsigned first_component = ffs(writemask) - 1;
unsigned length = ffs(~(writemask >> first_component)) - 1;
+ fs_reg current_val_reg =
+ offset(val_reg, bld, first_component * type_slots);
+
/* We can't write more than 2 64-bit components at once. Limit the
* length of the write to what we can do and let the next iteration
* handle the rest
@@ -4229,11 +4232,45 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld,
nir_intrinsic_instr *instr
if (type_size > 4) {
length = MIN2(2, length);
} else if (type_size == 2) {
- /* For 16-bit types we are using byte scattered writes, that can
- * only write one component per call. So we limit the length, and
- * let the write happening in several iterations.
+ /* For 16-bit types we pack two consecutive values into a
+ * 32-bit word and use an untyped write message. For single values
+ * we need to use byte-scattered writes because untyped writes work
+ * on multiples of 32 bits.
+ *
+ * For example, if there is a 3-component vector we submit one
+ * untyped-write message of 32-bit (first two components), and one
+ * byte-scattered write message (the last component).
*/
- length = 1;
+ if (length >= 2) {
+ /* pack two consecutive 16-bit words into a 32-bit register,
+ * using the same original source register.
+ */
+ fs_reg dest = retype(current_val_reg, BRW_REGISTER_TYPE_UW);
+ dest.stride = 0;
+
+ fs_reg src = retype(val_reg, BRW_REGISTER_TYPE_UW);
+ src.stride = 2;
+
+ unsigned writemask_tmp = writemask;
+ for (unsigned i = 0; i < 2; i++) {
+ unsigned first_component_tmp = ffs(writemask_tmp) - 1;
+
+ fs_reg tmp = offset(dest, bld, i);
+ tmp.stride = 2;
+
+ bld.MOV(tmp,
+ offset(src, bld, first_component_tmp * type_slots));
+
+ writemask_tmp &= (15 << (first_component_tmp + 1));
+ }
+
+ length = 2;
+ } else {
+ /* For single 16-bit values, we just limit the length to 1 and
+ * use a byte-scattered write message below.
+ */
+ length = 1;
+ }
}
fs_reg offset_reg;
@@ -4248,18 +4285,28 @@ fs_visitor::nir_emit_intrinsic(const fs_builder &bld,
nir_intrinsic_instr *instr
brw_imm_ud(type_size * first_component));
}
- if (type_size == 2) {
+ if (type_size == 2 && length == 1) {
/* Untyped Surface messages have a fixed 32-bit size, so we need
- * to rely on byte scattered in order to write 16-bit elements.
+ * to rely on byte scattered in order to write a single 16-bit
+ * element.
*/
emit_byte_scattered_write(bld, surf_index, offset_reg,
- offset(val_reg, bld, first_component *
type_slots),
+ current_val_reg,
1 /* dims */, length * type_slots,
BRW_PREDICATE_NONE);
} else {
+ unsigned write_size = length * type_slots;
+
+ /* We need to half the write size of the untyped write message when
+ * submitting two packed 16-bit values, because length is 2 but the
+ * type size is 16-bit. So we are effectively writing just 32 bits
+ * (size = 1).
+ */
+ if (type_size == 2)
+ write_size /= 2;
emit_untyped_write(bld, surf_index, offset_reg,
- offset(val_reg, bld, first_component *
type_slots),
- 1 /* dims */, length * type_slots,
+ current_val_reg,
+ 1 /* dims */, write_size,
BRW_PREDICATE_NONE);
}
--
2.11.0
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