PING [Patch][Middle-end]Add -fzero-call-used-regs=[skip|used-gpr|all-gpr|used|all]

[Alexandre Oliva]

On Jul 28, 2020, Qing Zhao via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:

>> 2. The main code generation part is moved from i386 backend to middle-end;
>> 3. Add 4 target-hooks;
>> 4. Implement these 4 target-hooks on i386 backend. 
>> 5. On a target that does not implement the target hook, issue error

I wonder...  How important is it that the registers be zeroed, rather
than just avoid leaking internal state from the function?

It occurred to me that we could implement this in an entirely
machine-independent way by just arranging for the option to change the
calling conventions for all registers that are not used by return to be
regarded as call-saved.  Then the prologue logic would save the incoming
value of the registers, and the epilogue would restore them, and we're
all set.  It might even cover propagation of exceptions out of the
function.


Even if zeroing registers is desirable, it might still be possible to
build upon the above to do that in a machine-independent fashion, using
the annotations used to output call frame info to identify the slots in
which the to-be-zeroed registers were saved, and store zeros there,
either by modifying the save insns, or by adding extra stores to the end
of the prologue, at least as a default implementation for a target hook,
that could be overridden with something that does the job in more
efficient but target-specific ways.


-- 
Alexandre Oliva, happy hacker
https://FSFLA.org/blogs/lxo/
Free Software Activist
GNU Toolchain Engineer