Power ISA™ Version 2.03 - Power.org

Version 2.03
E.3 Floating-Point Selection [Category: Floating-Point]
This section gives examples of how the Floating Select
instruction can be used to implement floating-point minimum
and maximum functions, and certain simple
forms of if-then-else constructions, without branching.
The examples show program fragments in an imaginary,
C-like, high-level programming language, and the
corresponding program fragment using fsel and other
Power ISA instructions. In the examples, a, b, x, y, and
z are floating-point variables, which are assumed to be
in FPRs fa, fb, fx, fy, and fz. FPR fs is assumed to be
available for scratch space.
Additional examples can be found in Section E.2,
“Floating-Point Conversions [Category: Floating-Point]”
on page 328.
Warning: Care must be taken in using fsel if IEEE
compatibility is required, or if the values being tested
can be NaNs or infinities; see Section E.3.4.
E.3.1
E.3.2
Comparison to Zero
High-level language: Power ISA: Notes
if a ≥ 0.0 then x y
else x z
if a > 0.0 then x y
else x z
if a = 0.0 then x y
else x z
fsel fx,fa,fy,fz (1)
fneg fs,fa
fsel fx,fs,fz,fy
fsel fx,fa,fy,fz
fneg fs,fa
fsel fx,fs,fx,fz
Minimum and Maximum
E.3.3 Simple if-then-else
Constructions
(1,2)
(1)
High-level language: Power ISA: Notes
x min(a,b)
x max(a,b)
fsub fs,fa,fb
fsel fx,fs,fb,fa
fsub fs,fa,fb
fsel fx,fs,fa,fb
(3,4,5)
(3,4,5)
High-level language: Power ISA: Notes
if a ≥ b then x y
else x z
if a > b then x y
else x z
if a = b then x y
else x z
fsub fs,fa,fb
fsel fx,fs,fy,fz
fsub fs,fb,fa
fsel fx,fs,fz,fy
fsub fs,fa,fb
fsel fx,fs,fy,fz
fneg fs,fs
fsel fx,fs,fx,fz
(4,5)
(3,4,5)
(4,5)
E.3.4
Notes
The following Notes apply to the preceding examples
and to the corresponding cases using the other three
arithmetic relations (