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Floating-point base 2 logarithm as integer.

This instruction returns the signed integer base 2 logarithm of each floating-point input element |x| after normalization.

This is the unbiased exponent of x used in the representation of the floating-point value, such that, for positive x, x = significand × 2exponent.

The integer results are placed in elements of the destination vector which have the same width (esize) as the floating-point input elements:

* If x is normal, the result is the base 2 logarithm of x.

* If x is subnormal, the result corresponds to the normalized representation.

* If x is infinite, the result is 2(esize-1)-1.

* If x is ±0.0 or NaN, the result is -2(esize-1).

Inactive elements in the destination vector register remain unmodified.



FLOGB <Zd>.<T>, <Pg>/M, <Zn>.<T>

if !HaveSVE2() then UNDEFINED;
if size == '00' then UNDEFINED;
integer esize = 8 << UInt(size);
integer g = UInt(Pg);
integer n = UInt(Zn);
integer d = UInt(Zd);

Assembler Symbols


Is the name of the destination scalable vector register, encoded in the "Zd" field.

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

Is the name of the governing scalable predicate register P0-P7, encoded in the "Pg" field.


Is the name of the source scalable vector register, encoded in the "Zn" field.


integer elements = VL DIV esize;
bits(PL) mask = P[g];
bits(VL) operand = Z[n];
bits(VL) result = Z[d];

for e = 0 to elements-1
    bits(esize) element = Elem[operand, e, esize];
    if ElemP[mask, e, esize] == '1' then
        Elem[result, e, esize] = FPLogB(element, FPCR);

Z[d] = result;

Operational information

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.
  • A predicated MOVPRFX instruction using the same governing predicate register and source element size as this instruction.