Re: [Qemu-devel] [PATCH v5 0/3] tcg: enhance code generation quality for qemu_ld/st IRs

[Paolo Bonzini]

Il 09/10/2012 18:19, Aurelien Jarno ha scritto:
>>> > > 
>> > 
>> > Instead of calling the MMU helper with an additional argument (7), and
>> > then jump back (8) to the next code (4), what about pushing the address
>> > of the next code (4) on the stack and use a jmp instead of the call. In
>> > that case you don't need the extra argument to the helpers.
>> > 
> Maybe it wasn't very clear. This is based on the fact that call is
> basically push %rip + jmp. Therefore we can fake the return address by
> putting the value we want, here the address of the next code. This mean
> that we don't need to pass the extra argument to the helper for the 
> return address, as GET_PC() would work correctly (it basically reads the
> return address on the stack).
> 
> For other architectures, it might not be a push, but rather a move to
> link register, basically put the return address where the calling
> convention asks for.
> 
> OTOH I just realized it only works if the end of the slow path (moving
> the value from the return address to the correct register). It might be
> something doable.

Branch predictors will not oldschool tricks like this one. :)

Paolo