Re: [PATCH 2/4] riscv: uaccess: use input constraints for ptr of __put_user

Jisheng Zhang
2024-06-26T13:26:40Z

On Wed, Jun 26, 2024 at 03:12:50PM +0200, Andreas Schwab wrote:
> On Jun 25 2024, Jisheng Zhang wrote:
>
> > I believe the output constraints "=m" is not necessary, because
> > the instruction itself is "write", we don't need the compiler
> > to "write" for us.
>
> No, this is backwards. Being an output operand means that the *asm* is
> writing to it, and the compiler can read the value from there afterwards
> (and the previous value is dead before the asm).

Hi Andreas,

I compared tens of __put_user() caller's generated code between orig
version and patched version, they are the same. Sure maybe this is
not enough.

But your explanation can be applied to x86 and arm64 __put_user()
implementations, asm is also writing, then why there's no output
constraints there?(see the other two emails)? Could you please help
me to understand the tricky points?

Thanks in advance