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UQDECP (vector)

Unsigned saturating decrement vector by count of true predicate elements.

Counts the number of true elements in the source predicate and then uses the result to decrement all destination vector elements. The results are saturated to the element unsigned integer range.

The predicate size specifier may be omitted in assembler source code, but this is deprecated and will be prohibited in a future release of the architecture.



UQDECP <Zdn>.<T>, <Pm>.<T>

if !HaveSVE() then UNDEFINED;
if size == '00' then UNDEFINED;
integer esize = 8 << UInt(size);
integer m = UInt(Pm);
integer dn = UInt(Zdn);
boolean unsigned = TRUE;

Assembler Symbols


Is the name of the source and destination scalable vector register, encoded in the "Zdn" field.

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

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


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

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

for e = 0 to elements-1
    integer element = Int(Elem[operand1, e, esize], unsigned);
    (Elem[result, e, esize], -) = SatQ(element - count, esize, unsigned);

Z[dn] = result;

Operational information


  • 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.

This instruction might be immediately preceded in program order by a MOVPRFX instruction that conforms to all of the following requirements, otherwise the behavior of either or both instructions is unpredictable:

  • The MOVPRFX instruction must specify the same destination register as this instruction.
  • The destination register must not refer to architectural register state referenced by any other source operand register of this instruction.
The MOVPRFX instructions that can be used with this instruction are as follows:
  • An unpredicated MOVPRFX instruction.