USDOT (by element)

Dot product with unsigned and signed integers (vector, by element)

This instruction performs the dot product of the four unsigned 8-bit integer values in each 32-bit element of the first source register with the four signed 8-bit integer values in an indexed 32-bit element of the second source register, accumulating the result into the corresponding 32-bit element of the destination register.

From Armv8.2 to Armv8.5, this is an OPTIONAL instruction. From Armv8.6 it is mandatory for implementations that include Advanced SIMD to support it. ID_AA64ISAR1_EL1.I8MM indicates whether this instruction is supported.

Encoding: Vector

Variants: FEAT_I8MM (PROFILE_A)

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0		0	0	1	1	1	1	1	0							1	1	1	1		0
	Q	U						US		L	M	Rm				opcode				H		Rn					Rd

USDOT <Vd>.<Ta>, <Vn>.<Tb>, <Vm>.4B[<index>]

Decoding algorithm

if !IsFeatureImplemented(FEAT_I8MM) then EndOfDecode(Decode_UNDEF);
constant integer n = UInt(Rn);
constant integer m = UInt(M:Rm);
constant integer d = UInt(Rd);
constant integer i = UInt(H:L);
constant integer datasize = 64 << UInt(Q);
constant integer elements = datasize DIV 32;

Operation

CheckFPAdvSIMDEnabled64();
constant bits(datasize) operand1 = V[n, datasize];
constant bits(128)      operand2 = V[m, 128];
constant bits(datasize) operand3 = V[d, datasize];
bits(datasize) result;

for e = 0 to elements-1
    bits(32) res = Elem[operand3, e, 32];
    for b = 0 to 3
        constant integer element1 = UInt(Elem[operand1, 4 * e + b, 8]);
        constant integer element2 = SInt(Elem[operand2, 4 * i + b, 8]);
        res = res + element1 * element2;
    Elem[result, e, 32] = res;
V[d, datasize] = result;

Explanations

<Vd>: Is the name of the SIMD&FP third source and destination register, encoded in the "Rd" field.
<Ta>: <Vn>: Is the name of the first SIMD&FP source register, encoded in the "Rn" field.
<Tb>: <Vm>: Is the name of the second SIMD&FP source register, encoded in the "M:Rm" fields.
<index>: Is the immediate index of a 32-bit group of four 8-bit values, in the range 0 to 3, encoded in the "H:L" fields.

Operational Notes

If PSTATE.DIT is 1:

• The execution time of this instruction is independent of:
  • The values of the data supplied in any of its registers.
  • The values of the NZCV flags.
• The response of this instruction to asynchronous exceptions does not vary based on:
  • The values of the data supplied in any of its registers.
  • The values of the NZCV flags.