[Bug target/85593] GCC on ARM allocates R3 for local variable when calling naked function with O2 optimizations enabled

[austinpmorton at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85593

--- Comment #4 from Austin Morton <austinpmorton at gmail dot com> ---
In my particular case I was able to work around the issue by removing the naked
attribute and using extended assembly with a clobbers list.

The resulting code is nearly identical (allowing GCC to generate the correct
pro/epilog instead of hand writing it), and gcc correctly allocates R4 instead
of R3.

This still feels like a bug in GCC.  In the example I gave, if you compiled the
naked function in a separate C file and linked them it would generate the
correct code.  The issue is that GCC is able to "see" the naked function and is
performing optimizations that it shouldn't as a result.

I believe that GCC should treat naked functions as opaque as far as
optimizations are concerned.
At the very least, there should be a note about this kind of issue included in
the documentation of the naked attribute.