s_cselect_b64 GPU Native ISA AMD Scalar

Scalar Arithmetic

s_cselect_b64

Select the first input if SCC is true otherwise select the second input, then store the selected input into a scalar register.

Encoding

Binary Layout (AMD CDNA 5)
10
31:30
0110001
29:23
SDST
22:16
SSRC1
15:8
SSRC0
7:0
 

The ENC_SOP2 layout from AMD's machine-readable ISA specification (AMD CDNA 5). Opcode 49 in OP rebuilds 0x98800000, an identifier AMD lists for this encoding. Not the same in every generation: AMD CDNA 4 (opcode 11), AMD CDNA 3 (opcode 11), AMD CDNA 2 (opcode 11), AMD CDNA 1 (opcode 11), AMD RDNA 2 (opcode 11), AMD RDNA 1 (opcode 11). The same in AMD RDNA 4, AMD RDNA 3.5, AMD RDNA 3.

Format SOP2
Width 32 bits
Opcode 49
Identifier 0x98800000

Operands

  • SDST
    Written. All scalar destination operands. Data format: 64 bits of arbitrary data. (OPR_SDST, FMT_NUM_B64)
  • SSRC0
    Read. All scalar operands. Covers all operands that are allowed as scalar sources. Data format: 64 bits of arbitrary data. (OPR_SSRC, FMT_NUM_B64)
  • SSRC1
    Read. All scalar operands. Covers all operands that are allowed as scalar sources. Data format: 64 bits of arbitrary data. (OPR_SSRC, FMT_NUM_B64)

In AMD's order, as its machine-readable ISA specification lists them for the ENC_SOP2 encoding (AMD CDNA 5).

GFX Target Compatibility

TargetSupport
gfx1100✅ Supported

Related

More in Scalar Arithmetic

Reference

AMDGPU / GFX ISA

Semantics

D0.u64 = SCC ? S0.u64 : S1.u64

Example

s_cselect_b64 s[0:1], 0, s[4:5]

A real instruction accepted by the LLVM assembler, taken verbatim from LLVM's own AMDGPU MC test suite. Not from an AMD document, and not authored here.

Sources