F.8.1 Global transformations

1

Floating-point arithmetic operations and external function calls may entail side effects which optimization shall honor, at least where the state of the FENV_ACCESS pragma is “on”. The flags and modes in the floating-point environment may be regarded as global variables; floating-point operations (+, *, etc.) implicitly read the modes and write the flags.

2

Concern about side effects may inhibit code motion and removal of seemingly useless code. For example, in

#include <fenv.h>
#pragma STDC FENV_ACCESS ON
void f(double x)
{
     /* ... */
     for (i = 0; i < n; i++) x + 1;
     /* ... */
}

x + 1 might raise floating-point exceptions, so cannot be removed. And since the loop body might not execute (maybe 0 >= n), x + 1 cannot be moved out of the loop. (Of course these optimizations are valid if the implementation can rule out the nettlesome cases.)

3

This specification does not require support for trap handlers that maintain information about the order or count of floating-point exceptions. Therefore, between function calls, floating-point exceptions need not be precise: the actual order and number of occurrences of floating-point exceptions (> 1) may vary from what the source code expresses. Thus, the preceding loop could be treated as

if (0 < n) x + 1;