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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

313029282726252423222120191817161514131211109876543210
00100101size1010111000100PgRdn

32-bit

UQDECP <Wdn>, <Pg>.<T>

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

313029282726252423222120191817161514131211109876543210
00100101size1010111000110PgRdn

64-bit

UQDECP <Xdn>, <Pg>.<T>

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

<Wdn>

Is the 32-bit name of the source and destination general-purpose register, encoded in the "Rdn" field.

<Xdn>

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

<Pg>

Is the name of the governing scalable predicate register, encoded in the "Pg" 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(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);
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