On Mon, Jul 07, 2025 at 11:59:48AM -0400, Jason Merrill wrote:
> > Another case are targets with implicit -fno-delete-null-pointer-checks or
> > when users use that explicitly. In that case one variable or function can
> > actually end up being at address 0, so am not sure if it is correct to
> > pretend at constant evaluation time something that is not true at runtime.
> > Though on the other side, pointers with NULL value in the standard mean that
> > there is no object at that location, so any kind of placing variables or
> > functions at that location will cause problems everywhere anyway.
>
> A simple thing might be to adjust Patrick's patch to only set the flag
>
> if (!targetm.addr_space.zero_address_valid (ADDR_SPACE_GENERIC))
That could work too, we'd probably just need to update msp430 so that
it overrides the zero_address_valid hook:
grep flag_delete_null_pointer_checks config/*/*.cc
config/avr/avr.cc: /* Disable flag_delete_null_pointer_checks if zero is a
valid address. */
config/avr/avr.cc: flag_delete_null_pointer_checks = 0;
config/msp430/msp430.cc: flag_delete_null_pointer_checks = 0;
grep zero_address_valid config/*/*.cc
config/avr/avr.cc: if (targetm.addr_space.zero_address_valid
(ADDR_SPACE_GENERIC))
config/avr/avr.cc:avr_addr_space_zero_address_valid (addr_space_t)
config/avr/avr.cc:#define TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID
avr_addr_space_zero_address_valid
config/i386/i386.cc:ix86_addr_space_zero_address_valid (addr_space_t as)
config/i386/i386.cc:#define TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID
ix86_addr_space_zero_address_valid
(i386 has it for non-generic as).
Though, that would only help avr and msp430, when one uses say arm-elf or
some other bare metal target and explicit -fno-delete-null-pointer-checks
with vars at zero address.
The other possibility is to add some flag (Optimization like
flag_delete_null_pointer_checks) that would simply always be false unless
/* When instrumenting the pointers, we don't want to remove
the null pointer checks. */
if (opts->x_flag_sanitize & (SANITIZE_NULL | SANITIZE_NONNULL_ATTRIBUTE
| SANITIZE_RETURNS_NONNULL_ATTRIBUTE))
opts->x_flag_delete_null_pointer_checks = 0;
when it was previously non-zero. And use that flag in the decision whether
to temporarilz change flag_delete_null_pointer_checks or not.
Jakub
