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DECP (scalar)

Decrement scalar by count of true predicate elements.

Counts the number of true elements in the source predicate and then uses the result to decrement the scalar destination.

313029282726252423222120191817161514131211109876543210
00100101size1011011000100PmRdn

SVE

DECP <Xdn>, <Pm>.<T>

if !HaveSVE() then UNDEFINED;
integer esize = 8 << UInt(size);
integer m = UInt(Pm);
integer dn = UInt(Rdn);

Assembler Symbols

<Xdn>

Is the 64-bit name of the source and destination general-purpose register, encoded in the "Rdn" field.

<Pm>

Is the name of the source scalable predicate register, encoded in the "Pm" field.

<T> Is the size specifier, encoded in size:
size <T>
00 B
01 H
10 S
11 D

Operation

CheckSVEEnabled();
integer elements = VL DIV esize;
bits(64) operand1 = X[dn];
bits(PL) operand2 = P[m];
integer count = 0;

for e = 0 to elements-1
    if ElemP[operand2, e, esize] == '1' then
        count = count + 1;

X[dn] = operand1 - count;

Operational information

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.