Re: using gcc built-ins for bitops?

[Gabriel Paubert]

On Thu, Jun 24, 2004 at 09:09:36AM +0200, Arjan van de Ven wrote:
> Hi,
> 
> gcc 3.4 gained support for several typical bitops as builtin directives.
> Using these over inline asm has a few advantages:
> * gcc can optimize constants into these better
> * gcc can reorder and schedule the code better
> * gcc can allocate registers etc better for the code
> 
> The question is if we consider it desirable to go down this road or not. In
> order to help that discussion I've attached a patch below that switches the
> i386 ffz() function to the gcc builtin version, conditional on gcc having
> support for this. Before I go down the road of converting more functions
> and/or architectures.... is this worth doing?
> 
> Greetings,
>    Arjan van de Ven
> 
> 
> --- linux-2.6.7/include/asm-i386/bitops.h~	2004-06-23 23:45:06.048614387 +0200
> +++ linux-2.6.7/include/asm-i386/bitops.h	2004-06-23 23:45:06.048614387 +0200
> @@ -344,6 +344,8 @@
>   *
>   * Undefined if no zero exists, so code should check against ~0UL first.
>   */
> + 
> +#ifndef HAVE_BUILTIN_CTZL
>  static inline unsigned long ffz(unsigned long word)
>  {
>  	__asm__("bsfl %1,%0"
> @@ -351,6 +353,12 @@
>  		:"r" (~word));
>  	return word;
>  }
> +#else
> +static inline unsigned long ffz (unsigned long word) 
> +{ 
> +	return __builtin_ctzl (~word); 
> +}
> +#endif
>  
>  /**
>   * __ffs - find first bit in word.
> --- linux-2.6.7/include/linux/compiler-gcc3.h~	2004-06-24 09:26:04.123455290 +0200
> +++ linux-2.6.7/include/linux/compiler-gcc3.h	2004-06-24 09:26:04.123455290 +0200
> @@ -19,6 +19,11 @@
>  # define __attribute_used__	__attribute__((__unused__))
>  #endif
>  
> +#if __GNUC_MINOR__ >= 4

Please do no test only on minor. People have been arguing
whether the next major release of GCC should be called 3.5
or 4.0 since tree-ssa has been merged.

This said, I'd rather use GCC intrinsics when they can
perform the task, especially the ones that expand
to more than one machine instruction.

> +#define HAVE_BUILTIN_CTZL
> +#endif

	Gabriel
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