• Python

if sys.maxsize > 2**32:
    f64()
else:
    f32()

• Pascal

begin
  {$ifdef cpu64}
  f64;
  {$endif}
  {$ifdef cpu32}
  f32;
  {$endif}
end.

• C++

if constexpr(sizeof(nullptr) == 8) {
  f64();
} else {
  f32();
}

• C++

#ifdef __x86_64
f64();
#else
#ifdef _M_AMD64
f64();
#else
f32();
#endif
#endif

• C#

if(Environment.Is64BitOperatingSystem)
{
    f64();
}
else
{
    f32();
}

• D

version(X86)
    f32();
version(X86_64)
    f64();

• D

static if (size_t.sizeof == 4)
    f32();
static if (size_t.sizeof == 8)
    f64();

• Fortran

program main
  use iso_c_binding
  implicit none
  type(c_ptr) :: x
  logical, parameter :: is_64 = c_sizeof(x) == 8
  if (is_64) then
     call f64
  else
     call f32
  end if
end program main

• Go

if strconv.IntSize==32 {
	f32()
}
if strconv.IntSize==64 {
	f64()
}

• Haskell

detectArch :: IO ()
detectArch = do
    case arch == "x86_64" of
        True -> do f64
        False -> case arch == "x86" of
                    True -> do f32

• JS

const is64Bit = os.arch() === 'x64' || process.env.hasOwnProperty('PROCESSOR_ARCHITEW6432');
is64Bit ? _f64() : _f32();

• Java

switch(System.getProperty("sun.arch.data.model")) {
  case "64": 
    f64(); 
    break;
  case "32":
    f32();
    break;
}

• Lisp

(defun detect-architecture ()
  (let ((arch (machine-type)))
    (cond ((equalp arch "X86-64") (f64))
          ((equalp arch "X86") (f32))
          (t "unknown"))))

• PHP

<?php

if (PHP_INT_SIZE === 4)
    f32();
else if (PHP_INT_SIZE === 8)
    f64();

• Perl

if ($Config{archname} =~ '64') {
    f64;
} else {
    f32;
}

• Ruby

case 1.size
  when 8 then f64
  when 4 then f32
end

• Rust

#[cfg(target_pointer_width = "64")]
f64();

#[cfg(target_pointer_width = "32")]
f32();	

• Rust

match std::mem::size_of::<&char>() {
    4 => f32(),
    8 => f64(),
    _ => {}
}