> On Jul 30, 2024, at 6:17 AM, Richard Biener <rguent...@suse.de> wrote:
>
> The following adds a target hook to specify whether regs of MODE can be
> used to transfer bits. The hook is supposed to be used for value-numbering
> to decide whether a value loaded in such mode can be punned to another
> mode instead of re-loading the value in the other mode and for SRA to
> decide whether MODE is suitable as container holding a value to be
> used in different modes.
>
> ...
>
> +@deftypefn {Target Hook} bool TARGET_MODE_CAN_TRANSFER_BITS (machine_mode
> @var{mode})
> +Define this to return false if the mode @var{mode} cannot be used
> +for memory copying. The default is to assume modes with the same
> +precision as size are fine to be used.
> +@end deftypefn
> +
I'm a bit confused about the meaning of this hook; the summary at the top
speaks of type punning while the documentation talks about memory copying.
Those seem rather different.
I'm also wondering about this being tied to a mode rather than a register
class. To given an example: on the PDP11 there are two main register classes,
"general" and "float". General registers handle any bit pattern and support
arithmetic operations on integer modes; float registers do not transparently
transfer every bit pattern and support float modes. So only general registers
are suitable for memory copies (though on a PDP-11 you don't need registers to
do memory copy). And for type punning, you could load an SF mode value into
general registers (a pair) and type-pun them to SImode without reloading.
So what does that mean for this hook on that target?
paul
