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DECD, DECH, DECW (vector)

Decrement vector by 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 uses the result to decrement all destination vector elements.

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.

It has encodings from 3 classes: Doubleword , Halfword and Word

Doubleword

313029282726252423222120191817161514131211109876543210
000001001111imm4110001patternZdn

Doubleword

DECD <Zdn>.D{, <pattern>{, MUL #<imm>}}

if !HaveSVE() then UNDEFINED;
integer esize = 64;
integer dn = UInt(Zdn);
bits(5) pat = pattern;
integer imm = UInt(imm4) + 1;

Halfword

313029282726252423222120191817161514131211109876543210
000001000111imm4110001patternZdn

Halfword

DECH <Zdn>.H{, <pattern>{, MUL #<imm>}}

if !HaveSVE() then UNDEFINED;
integer esize = 16;
integer dn = UInt(Zdn);
bits(5) pat = pattern;
integer imm = UInt(imm4) + 1;

Word

313029282726252423222120191817161514131211109876543210
000001001011imm4110001patternZdn

Word

DECW <Zdn>.S{, <pattern>{, MUL #<imm>}}

if !HaveSVE() then UNDEFINED;
integer esize = 32;
integer dn = UInt(Zdn);
bits(5) pat = pattern;
integer imm = UInt(imm4) + 1;

Assembler Symbols

<Zdn>

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

<pattern> Is the optional pattern specifier, defaulting to ALL, encoded in pattern:
pattern <pattern>
00000 POW2
00001 VL1
00010 VL2
00011 VL3
00100 VL4
00101 VL5
00110 VL6
00111 VL7
01000 VL8
01001 VL16
01010 VL32
01011 VL64
01100 VL128
01101 VL256
0111x #uimm5
101x1 #uimm5
10110 #uimm5
1x0x1 #uimm5
1x010 #uimm5
1xx00 #uimm5
11101 MUL4
11110 MUL3
11111 ALL
<imm>

Is the immediate multiplier, in the range 1 to 16, defaulting to 1, encoded in the "imm4" field.

Operation

CheckSVEEnabled();
integer elements = VL DIV esize;
integer count = DecodePredCount(pat, esize);
bits(VL) operand1 = Z[dn];
bits(VL) result;

for e = 0 to elements-1
    Elem[result, e, esize] = Elem[operand1, e, esize] - (count * imm);

Z[dn] = result;
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