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

["ramana at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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

Ramana Radhakrishnan <ramana at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ramana at gcc dot gnu.org

--- Comment #3 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> ---
Changing the testcase to indicate the clobbered register r3 makes the test pass
but that is wrong in a naked function. Extended inline assembler *cannot* be
used within a naked function because you inadvertently could end up requiring a
stack slot and therefore cause the function to require a prologue and epilogue
which is exactly contrary to what we want with the naked function ! 

I *think* the problem really is the fact that you have ipa-ra coming along and
deciding that r3 isn't used at all in the naked function . You can see the
problem disappear with -fno-ipa-ra but that is not a workaround I would
recommend using in your general C flags , because you are using a hammer
disabling a nice optimization to make something like the example "work". 

Thus I think what you want is to get rid of naked functions in general and
write the whole thing in assembler and stop faffing about with naked functions
in general. IIRC there is a hook for ipa-ra that says what registers can be
clobbered : can't find it immediately. I suppose for naked functions it is
*all* registers.



regards
Ramana