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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<31:0>);

Z[d] = result;

Operational information

This instruction might be immediately preceded in program order by a MOVPRFX instruction. The MOVPRFX instruction must conform to all of the following requirements, otherwise the behavior of the MOVPRFX and this instruction is unpredictable:

  • The MOVPRFX instruction must be unpredicated, or be predicated using the same governing predicate register and source element size as this instruction.
  • 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.