Re: Optimize glyph row clearing and copying routines

[John Yates <john@yates-sheets.org>]

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It is true that the standard's definition of memcpy is in terms of copying
a sequence of bytes.  It is also true that memcpy is one of the most
important and most heavily optimized library functions.

These days any credible compiler has a means of determining that an
invocation of a function named 'memcpy' is actually an invocation of the
standard's memcpy.  E.g. gcc's exposed memcpy is an inline whose body
simply calls __builtin_memcpy.

With such knowledge a compiler can bring to bear all kinds of
optimizations.  Dmitry's measurements seem to bear this out.

/john


On Tue, Sep 24, 2013 at 2:35 AM, Eli Zaretskii <eliz@gnu.org> wrote:

> Does this change really speed up the code?  AFAIU, previously the
> struct assignment could use word-size copies (because a struct is
> always aligned), but now you cast the arguments to 'char *' and use
> memcpy, which could fall back on copying single bytes or shorter
> words.
>
> Did you measure the impact?  Was it significant enough to justify this
> change?
>
>

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