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<!---======================= begin_copyright_notice ============================
Copyright (C) 2020-2022 Intel Corporation
SPDX-License-Identifier: MIT
============================= end_copyright_notice ==========================-->
## Opcode
SVM = 0x4e
GATHER = 0x03
## Format
| | | | | | | |
| --- | --- | --- | --- | --- | --- | --- |
| 0x4e(SVM) | 0x03(GATHER) | Exec_size | Pred | Block_size | Num_blocks | Addresses | Dst |
## Semantics
```
for (i = 0; i < exec_size; ++i) {
if (ChEn[i]) {
if (block_size == 4 || block_size == 8) {
for (j = 0; j < num_blocks; ++j) {
dst[j*exec_size+i] = *(Addresses[i] + j*block_size); //4 or 8 byte
}
}
else { // block_size == 1
UD min_block_size = num_blocks < 4 ? 4 : num_blocks;
for (j = 0; j < num_blocks; ++j) {
dst[i*min_block_size+j] = *(Addresses[i] + j); //1 byte
}
}
}
}
```
## Description
```
Performs 8 element scattered read from virtual addresses and stores the
result into <dst>. Each element may have multiple blocks.
```
- **Exec_size(ub):** Execution size
- Bit[2..0]: size of the region for source and destination operands
- 0b000: 1 element (scalar)
- 0b001: 2 elements
- 0b010: 4 elements
- 0b011: 8 elements
- 0b100: 16 elements
- Bit[7..4]: execution mask (explicit control over the enabled channels)
- 0b0000: M1
- 0b0001: M2
- 0b0010: M3
- 0b0011: M4
- 0b0100: M5
- 0b0101: M6
- 0b0110: M7
- 0b0111: M8
- 0b1000: M1_NM
- 0b1001: M2_NM
- 0b1010: M3_NM
- 0b1011: M4_NM
- 0b1100: M5_NM
- 0b1101: M6_NM
- 0b1110: M7_NM
- 0b1111: M8_NM
- **Pred(uw):** Predication control
- **Block_size(ub):**
- Bit[1..0]: encodes the byte size of each block
- 0b00: 1 byte
- 0b01: 4 byte
- 0b11: 8 byte
- **Num_blocks(ub):**
- Bit[1..0]: encodes the number of blocks to read for each address
- 0b00: 1 block
- 0b01: 2 blocks
- 0b10: 4 blocks
- 0b11: 8 blocks. It is only valid for 4 byte blocks and execution size 8
- **Addresses(raw_operand):** The general variable storing the addresses to read. The first 8 elements of the operand will be used as the virtual addresses to read from, and they are in the unit of bytes. Each address must be aligned to the <block_size>. Must have type UQ
- **Dst(raw_operand):** The general variable storing the results of the read. The format of the return value depends on <block_size> and <num_blocks>
#### Properties
## Text
```
[(<P>)] SVM_GATHER.<block_size>.<num_blocks> (<exec_size>) <addresses> <dst>
```
## Notes
```
4 byte gather:
+-----------------+-----------------+----+----+-----+----+----+-----------------+
| A0[0] | A1[0] | | | ... | | | A7[0] |
+-----------------+-----------------+----+----+-----+----+----+-----------------+
| ... |
+-----------------+-----------------+----+----+-----+----+----+-----------------+
| A0[#blocks-1] | A1[#blocks-1] | | | ... | | | A7[#blocks-1] |
+-----------------+-----------------+----+----+-----+----+----+-----------------+
8 byte gather:
+-----------------+-----------------+-----------------+-----------------+
| A0[0] | A1[0] | A2[0] | A3[0] |
+-----------------+-----------------+-----------------+-----------------+
| A4[0] | A5[0] | A6[0] | A7[0] |
+-----------------+-----------------+-----------------+-----------------+
| ... |
+-----------------+-----------------+-----------------+-----------------+
| A0[#blocks-1] | A1[#blocks-1] | A2[#blocks-1] | A3[#blocks-1] |
+-----------------+-----------------+-----------------+-----------------+
| A4[#blocks-1] | A5[#blocks-1] | A6[#blocks-1] | A7[#blocks-1] |
+-----------------+-----------------+-----------------+-----------------+
1 byte gather, 1/2/4 block (Ai[N] has undefined value if N>=num_blocks)
+-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
| A0[0:3] | A1[0:3] | A2[0:3] | A3[0:3] | A4[0:3] | A5[0:3] | A6[0:3] | A7[0:3] |
+-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
1 byte gather, 8 block
+-----------+-----------+-----------+-----------+
| A0[0:7] | A1[0:7] | A2[0:7] | A3[0:7] |
+-----------+-----------+-----------+-----------+
| A4[0:7] | A5[0:7] | A6[0:7] | A7[0:7] |
+-----------+-----------+-----------+-----------+
Each table row represents one GRF, and A0[0] corresponds to the
first element in <dst> in all of the above tables. The operand's
type must be the same size as <block_size>.
Note that: When num_blocks > 1, exec_size must >= 8.
```
|
