v_fract_f64 GPU Native ISA AMD Vector

Vector Arithmetic

v_fract_f64

Compute the fractional portion of a double-precision float input and store the result in floating point format into a vector register.

Encoding

Binary Layout (AMD CDNA 5)
0111111
31:25
VDST
24:17
unassigned
16
0111110
15:9
SRC0
8:0
 

The ENC_VOP1 layout from AMD's machine-readable ISA specification (AMD CDNA 5). Opcode 62 in OP rebuilds 0x7E007C00, an identifier AMD lists for this encoding. Bits marked unassigned have no field in the specification. Not the same in every generation: AMD CDNA 4 (opcode 50, field layout), AMD CDNA 3 (opcode 50, field layout), AMD CDNA 2 (opcode 50, field layout), AMD CDNA 1 (opcode 50, field layout), AMD RDNA 3.5 (field layout), AMD RDNA 3 (field layout), AMD RDNA 2 (field layout), AMD RDNA 1 (field layout). The same in AMD RDNA 4.

Format VOP1
Width 32 bits
Opcode 62
Identifier 0x7E007C00

Operands

  • VDST
    Written. Operand must be a vector GPR. Uses an 8-bit operand field. Data format: 64-bit double-precision floating point value. (OPR_VGPR, FMT_NUM_F64)
  • SRC0
    Read. All operands. Covers all operands that are allowed as scalar or vector sources. Data format: 64-bit double-precision floating point value. (OPR_SRC, FMT_NUM_F64)

In AMD's order, as its machine-readable ISA specification lists them for the ENC_VOP1 encoding (AMD CDNA 5).

GFX Target Compatibility

TargetSupport
gfx1100✅ Supported

0.5ULP accuracy, denormals are accepted. This is intended to comply with the DX specification of fract where the function behaves like an extension of integer modulus; be aware this may differ from how fract() is defined in other domains. For example: fract(- 1.2) = 0.8 in DX. Obey round mode, result clamped to 0x3fefffffffffffff.

Related

More in Vector Arithmetic

Reference

AMDGPU / GFX ISA

Semantics

D0.f64 = S0.f64 + -floor(S0.f64)

Example

v_fract_f64 v[5:6], -1

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