RBIT

Reverse bits (predicated)

Reverse bits in each active element of the source vector, and place the results in the corresponding elements of the destination vector. Inactive elements in the destination vector register remain unmodified or are set to zero, depending on whether merging or zeroing predication is selected.

Encoding: Merging

Variants: FEAT_SVE || FEAT_SME (FEAT_SVE || FEAT_SME)

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	0	0	1	0	1			1	0	0	1	1	1	1	0	0
								size						opc				Z	Pg			Zn					Zd

RBIT <Zd>.<T>, <Pg>/M, <Zn>.<T>

Decoding algorithm

if !IsFeatureImplemented(FEAT_SVE) && !IsFeatureImplemented(FEAT_SME) then
    EndOfDecode(Decode_UNDEF);
constant integer esize = 8 << UInt(size);
constant integer g = UInt(Pg);
constant integer n = UInt(Zn);
constant integer d = UInt(Zd);
constant boolean merging = TRUE;

Encoding: Zeroing

Variants: FEAT_SVE2p2 || FEAT_SME2p2 (FEAT_SVE2p2 || FEAT_SME2p2)

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	0	0	1	0	1			1	0	0	1	1	1	1	0	1
								size						opc				Z	Pg			Zn					Zd

RBIT <Zd>.<T>, <Pg>/Z, <Zn>.<T>

Decoding algorithm

if !IsFeatureImplemented(FEAT_SVE2p2) && !IsFeatureImplemented(FEAT_SME2p2) then
    EndOfDecode(Decode_UNDEF);
constant integer esize = 8 << UInt(size);
constant integer g = UInt(Pg);
constant integer n = UInt(Zn);
constant integer d = UInt(Zd);
constant boolean merging = FALSE;

Operation

CheckSVEEnabled();
constant integer VL = CurrentVL;
constant integer PL = VL DIV 8;
constant integer elements = VL DIV esize;
constant bits(PL) mask = P[g, PL];
constant bits(VL) operand = if AnyActiveElement(mask, esize) then Z[n, VL] else Zeros(VL);
bits(VL) result = if merging then Z[d, VL] else Zeros(VL);

for e = 0 to elements-1
    if ActivePredicateElement(mask, e, esize) then
        constant bits(esize) element = Elem[operand, e, esize];
        Elem[result, e, esize] = BitReverse(element);

Z[d, VL] = result;

Explanations

<Zd>: Is the name of the destination scalable vector register, encoded in the "Zd" field.
<T>: <Pg>: Is the name of the governing scalable predicate register P0-P7, encoded in the "Pg" field.
<Zn>: Is the name of the source scalable vector register, encoded in the "Zn" field.

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 operand registers when its governing predicate register contains the same value for each execution.
  • 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 operand registers when its governing predicate register contains the same value for each execution.
  • The values of the NZCV flags.

The merging variant of this instruction might be immediately preceded in program order by a MOVPRFX instruction. The MOVPRFX must conform to all of the following requirements, otherwise the behavior of the MOVPRFX and the merging variant of this instruction is CONSTRAINED UNPREDICTABLE:

• The MOVPRFX can be predicated or unpredicated.
• A predicated MOVPRFX must use the same governing predicate register as the merging variant this instruction.
• A predicated MOVPRFX must use the larger of the destination element size and first source element size in the preferred disassembly of the merging variant of this instruction.
• The MOVPRFX must specify the same destination register as the merging variant of this instruction.
• The destination register must not refer to architectural register state referenced by any other source operand register of the merging variant of this instruction.