Arm® A64 Instruction Set Architecture: Future Architecture Technologies in the A architecture profile

UQDECP (scalar)

Unsigned saturating decrement scalar by active predicate element count.

Counts the number of active elements in the source predicate and then uses the result to decrement the scalar destination. The result is saturated to the general-purpose register's unsigned integer range.

It has encodings from 2 classes: 32-bit and 64-bit

32-bit

if !HaveSVE() then UNDEFINED;
integer esize = 8 << UInt(size);
integer g = UInt(Pg);
integer dn = UInt(Rdn);
boolean unsigned = TRUE;
integer ssize = 32;

64-bit

if !HaveSVE() then UNDEFINED;
integer esize = 8 << UInt(size);
integer g = UInt(Pg);
integer dn = UInt(Rdn);
boolean unsigned = TRUE;
integer ssize = 64;

Assembler Symbols

CheckSVEEnabled();
integer elements = VL DIV esize;
bits(PL) mask = P[g];
bits(ssize) operand = X[dn];
bits(ssize) result;
integer count = 0;

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

integer element = Int(operand, unsigned);
(result, -) = SatQ(element - count, ssize, unsigned);
X[dn] = Extend(result, 64, unsigned);