Messages in this thread | | | Subject | Re: [PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions | From | Joe Perches <> | Date | Wed, 11 May 2022 20:02:28 -0700 |
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On Thu, 2022-05-12 at 10:18 +0900, Vincent Mailhol wrote: > For x86_64, the current ffs() implementation does not produce > optimized code when called with a constant expression. On the > contrary, the __builtin_ffs() function of both GCC and clang is able > to simplify the expression into a single instruction. [] > -static __always_inline int ffs(int x) > +static __always_inline int variable_ffs(int x) > { > int r; > > @@ -310,6 +299,19 @@ static __always_inline int ffs(int x) > return r + 1; > } > > +/** > + * ffs - find first set bit in word > + * @x: the word to search > + * > + * This is defined the same way as the libc and compiler builtin ffs > + * routines, therefore differs in spirit from the other bitops. > + * > + * ffs(value) returns 0 if value is 0 or the position of the first > + * set bit if value is nonzero. The first (least significant) bit > + * is at position 1. > + */ > +#define ffs(x) (__builtin_constant_p(x) ? __builtin_ffs(x) : variable_ffs(x))
How about not defining another function and using parentheses around the function definition to avoid the macro expansion like:
#define ffs(x) (__builtin_constant_p(x) ? __builtin_ffs(x) : ffs(x))
and
static __always_inline int (ffs)(int x) { etc... }
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