Re: [Qemu-devel] [PATCH 2/9] tcg/optimizer: check types in copy propagation

[Richard Henderson]

On 09/19/2012 01:00 PM, Aurelien Jarno wrote:
> The copy propagation doesn't check the types of the temps during copy
> propagation. However TCG is using the mov_i32 for the i64 to i32
> conversion and thus the two are not equivalent.
> 
> With this patch tcg_opt_gen_mov() doesn't consider two temps with
> different types as copies anymore.
> 
> So far it seems the optimization was not aggressive enough to trigger
> this bug, but it will be triggered later in this series once the copy
> propagation is improved.

Exactly where/how does this manifest as problematic?

We do this mov_i32 trick from i64->i32 when we're truncating.
Given that we're not (yet) targeting mips64 and having to
maintain 32-bit sign-extended quantities, I can't see how
that would matter.

We do the i32->i64 trick immediately before a proper extension.

In either case I can't see how plain copy propagation should
matter until some other operation is involved.  So, do we have
some other data propagation bug that is being masked here?


r~