Re: [PATCH v2 02/50] accel/tcg: Add getpc helper

On 7/29/26 20:09, Anton Johansson via qemu development wrote:
> Introduces a helper function to return the current pc (returnaddress of
> the helper), this allows helper-to-tcg to correctly translate nested
> helper functions that checks for faulting memory operations.
> 
> This is useful as an optimization where a helper functions has a
> commonly taken fast path that doesn't fault e.g.
> 
>    void HELPER(outer)(...)
>    {
>        ... // non faulting operations
>        if (some_uncommon_condition) {
>            helper_inner(..., GETPC())
>        }
>    }
> 
> the outer helper along with the condition can then be emitted as TCG,
> and helper_inner() will be emitted as a gen_helper_inner(, ra), where ra
> is the result of gen_helper_getpc().
> 
> NOTE: This is not ideal since we're introducing extra helper calls,
>        and the solution doesn't deal with the "inner" function not being
>        a helper.  A better solution and what we'll probably do in the
>        next version of the patchset is to instead emit a helper
>        definition for the "inner" function that uses GETPC().

This is both confusing and wrong.

If you're transforming outer to hot tcg + cold helper, the cold helper would *still* use 
GETPC(), which would still produce an address within the jit code block for the guest 
instruction.


r~