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Re: Patch to add __builtin_printf -> fputs transformations


On Wed, 20 Sep 2000, Kaveh R. Ghazi wrote:

> You can actually combine arg-full printfs by concatenating the format
> specifiers and chaining the remaining arguments together.  However
> analyzing multiple calls simultaneously wasn't even possible until
> function-at-a-time mode was enabled in the C front end.  It only just
> happened a week ago and I certainly haven't had time to study it yet
> in the context of these opts.
> 
> Here's an article from Jan '99 which was my first draft "master plan".
> (It does include concatenating similar *put* calls into one.  A
> similar algorithm would work for printf.)
> 
> http://gcc.gnu.org/ml/gcc-patches/1999-01/msg00187.html

Some warnings on the more general optimisations:

Concatenating format specifiers won't work if $ operand numbers are
used.  Setting pedantic prevents printf optimisations from looking at such
formats for now, but it's something to be aware of in case a future C
standard adds $ formats.

%n formats also prevent concatenation.

All the usual cross-compilation issues of getting the right output for the
target system apply.

Floating point conversions should follow the runtime rounding mode - at
least within #pragma STDC FENV_ACCESS ON.

NaNs and infinities should be passed to the library for it to format in
its implementation-defined style.  Likewise, %p should also be passed to
the library at runtime.

%lc and %ls formats depend on the runtime locale.

-- 
Joseph S. Myers
jsm28@cam.ac.uk


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