USMMLA (vector)

Unsigned and signed 8-bit integer matrix multiply-accumulate (vector)

This instruction multiplies the 2x8 matrix of unsigned 8-bit integer values in the first source vector by the 8x2 matrix of signed 8-bit integer values in the second source vector. The resulting 2x2 32-bit integer matrix product is destructively added to the 32-bit integer matrix accumulator in the destination vector. This is equivalent to performing an 8-way dot product per destination element.

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	1	0	0	1	1	1	0	1	0	0						1	0	1	0	1	1
	Q	U						size			Rm									B		Rn					Rd

USMMLA <Vd>.4S, <Vn>.16B, <Vm>.16B

Decoding algorithm

if !IsFeatureImplemented(FEAT_I8MM) then EndOfDecode(Decode_UNDEF);
constant integer d = UInt(Rd);
constant integer n = UInt(Rn);
constant integer m = UInt(Rm);
constant boolean op1_unsigned = TRUE;
constant boolean op2_unsigned = FALSE;

Operation

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

V[d, 128] = MatMulAdd(addend, operand1, operand2, op1_unsigned, op2_unsigned);

Explanations

<Vd>: Is the name of the SIMD&FP third source and destination register, encoded in the "Rd" field.
<Vn>: Is the name of the first SIMD&FP source register, encoded in the "Rn" field.
<Vm>: Is the name of the second SIMD&FP source register, encoded in the "Rm" field.

Operational Notes

Arm expects that the USMMLA (vector) instruction will deliver a peak integer multiply throughput that is at least as high as can be achieved using two USDOT (vector) instructions, with a goal that it should have significantly higher throughput. 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.