CNTB, CNTD, CNTH, CNTW

Set scalar to multiple of predicate constraint element count

Determines the number of active elements implied by the named predicate constraint, multiplies that by an immediate in the range 1 to 16 inclusive, and then places the result in the scalar destination.

The named predicate constraint limits the number of active elements in a single predicate to:

A fixed number (VL1 to VL256)

The largest power of two (POW2)

The largest multiple of three or four (MUL3 or MUL4)

All available, implicitly a multiple of two (ALL).

Unspecified or out of range constraint encodings generate an empty predicate or zero element count rather than Undefined Instruction exception.

Encoding: Byte

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	0	0	0	1	0					1	1	1	0	0	0
								size				imm4									op	pattern					Rd

CNTB <Xd>{, <pattern>{, MUL #<imm>}}

Decoding algorithm

if !IsFeatureImplemented(FEAT_SVE) && !IsFeatureImplemented(FEAT_SME) then
    EndOfDecode(Decode_UNDEF);
constant integer esize = 8;
constant integer d = UInt(Rd);
constant bits(5) pat = pattern;
constant integer imm = UInt(imm4) + 1;

Encoding: Doubleword

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	0	1	1	1	0					1	1	1	0	0	0
								size				imm4									op	pattern					Rd

CNTD <Xd>{, <pattern>{, MUL #<imm>}}

Decoding algorithm

if !IsFeatureImplemented(FEAT_SVE) && !IsFeatureImplemented(FEAT_SME) then
    EndOfDecode(Decode_UNDEF);
constant integer esize = 64;
constant integer d = UInt(Rd);
constant bits(5) pat = pattern;
constant integer imm = UInt(imm4) + 1;

Encoding: Halfword

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	0	0	1	1	0					1	1	1	0	0	0
								size				imm4									op	pattern					Rd

CNTH <Xd>{, <pattern>{, MUL #<imm>}}

Decoding algorithm

if !IsFeatureImplemented(FEAT_SVE) && !IsFeatureImplemented(FEAT_SME) then
    EndOfDecode(Decode_UNDEF);
constant integer esize = 16;
constant integer d = UInt(Rd);
constant bits(5) pat = pattern;
constant integer imm = UInt(imm4) + 1;

Encoding: Word

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	0	1	0	1	0					1	1	1	0	0	0
								size				imm4									op	pattern					Rd

CNTW <Xd>{, <pattern>{, MUL #<imm>}}

Decoding algorithm

if !IsFeatureImplemented(FEAT_SVE) && !IsFeatureImplemented(FEAT_SME) then
    EndOfDecode(Decode_UNDEF);
constant integer esize = 32;
constant integer d = UInt(Rd);
constant bits(5) pat = pattern;
constant integer imm = UInt(imm4) + 1;

Operation

CheckSVEEnabled();
constant integer count = DecodePredCount(pat, esize);
X[d, 64] = (count * imm)<63:0>;

Explanations

<Xd>: Is the 64-bit name of the destination general-purpose register, encoded in the "Rd" field.
<pattern>: <imm>: Is the immediate multiplier, in the range 1 to 16, defaulting to 1, encoded in the "imm4" 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 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.