wiki:GcnSdwaDpp

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GCN 1.2 VOP_SDWA and VOP_DPP encodings

The GCN 1.2 architecture provides two new VOP encoding enhancements which adds new features. The VOP_SDWA adds possibility to choosing and interesting bytes, words in operands. The VOP_DPP adds new parallel features to manipulating wavefronts (moving data between threads).

VOP_SDWA

The VOP_SDWA encoding is enabled by setting 0xf9 in VSRC0 field in VOP1/VOP2/VOPC encoding. List of fields:

Bits Name Description
0-7 SRC0 First source vector operand
8-10 DST_SEL Destination selection
11-12 DST_UNUSED Format for unused bits
13 CLAMP CLAMP modifier
16-18 SRC0_SEL Source data selection for SRC0
19 SRC0_SEXT Signed extension for SRC0
20 SRC0_NEG Negation modifier for SRC0
21 SRC0_ABS Absolute value for SRC0
24-26 SRC0_SEL Source data selectio for SRC1
27 SRC1_SEXT Signed extension for SRC1
28 SRC1_NEG Negation modifier for SRC1
29 SRC1_ABS Absolute value for SRC1

The DST_SEL modifier determines which first part of destination dword will be placed to choosen part with same length of this dword. This make operation (RESULT & PARTMASK) << PARTSHIFT. The SRC0_SEL and SRC1_SEL determines what part of source dword will be placed to first part of this source dword. This make operation (SOURCE>>PARTSHIFT) & PARTMASK. Possible part selection for DST_SEL, SRC0_SEL and SRC1_SEL:

Value Name CLRX names Description
0 SDWA_BYTE_0 BYTE0, BYTE_0, B0 Byte 0 of dword
1 SDWA_BYTE_1 BYTE1, BYTE_1, B1 Byte 1 of dword
2 SDWA_BYTE_2 BYTE2, BYTE_2, B2 Byte 2 of dword
3 SDWA_BYTE_3 BYTE3, BYTE_3, B3 Byte 3 of dword
4 SDWA_WORD_0 WORD0, WORD_0, W0 Word 0 of dword
5 SDWA_WORD_1 WORD1, WORD_1, W1 Word 1 of dword
6 SDWA_DWORD DWORD, DW Whole dword

The DST_UNUSED modifier specify how to fill bits that do not hold by choosen part. Following options:

Value Name CLRX name Descrption
0 SDWA_UNUSED_PAD PAD Fill by zeroes
1 SDWA_UNUSED_SEXT SEXT Fill by last bit of parts (sign), zero pad lower bits
2 SDWA_UNUSED_PRESERVE PRESERVE Preserve unused bits

The modifier SEXT (applied by using sext(operand)) apply sign extension (fill bits after part) to source operand while for source operand was not selected whole dword (SDWA_DWORD not choosen).