[Bug stdio/21127] printf functions can take much memory with %g when the format precision is large

["carlos at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=21127

Carlos O'Donell <carlos at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-05-06
                 CC|                            |carlos at redhat dot com
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Carlos O'Donell <carlos at redhat dot com> ---
For modern glibc (last 2 years) the digit printing goes through
__vfprintf_internal, and uses the __printf_buffer code that Florian Weimer
added in 2022.

The __printf_buffer code manages to print the test case without using more than
a ~295MiB of heap at peak usage to compute the internal result.

The test is still quite slow because we need to use multi-precision digits to
compute the result e.g. __printf_fp_buffer_1() -> hack_digit().

As we cross from 10.5 -> 9.5 things change. Internally scalesize drops to zero
and the fracsize goes up to 2, this is the cut-point in the internal
algorithm's optimization.

I can confirm that this is the reason you see the transition in performance,
and I can confirm that it takes a lot of memory to compute a correct answer to
the number of requested digits.

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