https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79469
felix <felix.von.s at posteo dot de> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |felix.von.s at posteo dot de
--- Comment #1 from felix <felix.von.s at posteo dot de> ---
> Unfortunately the macro trick is arcane and has one potential issue: the
> compiler may actually generate code to check `cond` at run-time if it cannot
> deduce that it's a pure function.
Oh, don't worry; the Other Compiler’s solution isn’t perfect either :)
__builtin_assume(({ for (;;) {}; 1; }));
This emits code for the infinite loop. For a less silly example:
__attribute__((__const__,__always_inline__))
inline static int is_pow2(unsigned j) {
__auto_type i = j;
for (int c = 0; c < CHAR_BIT * sizeof(i); ++c) {
i = (i >> 1) | ((i & 1) << (CHAR_BIT * sizeof(i) - 1));
if (i == 1)
break;
}
return i == 1;
}
int foo(void) {
extern unsigned bar(void);
__auto_type x = bar();
__builtin_assume(is_pow2(x));
return __builtin_popcount(x);
}
This will also emit code for the loop, even though the result is not used at
runtime. The code is not very idiomatic, but I don’t believe it’s wrong either:
the __attribute__((const)) on is_pow2 I consider correct, because the side
effects do not escape the function.
If anyone takes this up, it would be nice if GCC at least did not outright
pessimise such code.
