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Re: [PATCH] Simplify perturb_byte logic.

On Wed, Dec 11, 2013 at 07:04:28PM -0800, Roland McGrath wrote:
> > Should be always_inline, for macros converted to function otherwise we
> > could degrade performance.
> Or we could improve performance because code size actually matters more and
> the second-guessing of the compiler hasn't been helping in years (if ever)...

Yes if gcc could actually improve size by that. Size without always_inline is

-rwxr-x--- 1 ondra stu 10710437 Dec 13 09:01 

And adding always_inline improves this to
-rwxr-x--- 1 ondra stu 10710167 Dec 13 09:07 

And if compiler could accurately compute impact of inlining, not just
function size * estimated path probability.

For function in question a code is

  if (__glibc_unlikely (perturb_byte))
    memset (p, perturb_byte ^ 0xff, n);

Which is around 5 bytes (compare instruction and jump) in fast path.
Always adding call which is already 5 bytes and much more by additional
register spills on fast path is is simply bad idea (and  gcc design does
not allow estimate how many registers will get spilled.)

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