The key is the
>
> XEXP (x, 1) == convert_memory_address_addr_space
> (to_mode, XEXP (x, 1), as)
>
> test. It ensures basically that the constant has 31-bit precision, because
> otherwise the constant would change from e.g. (const_int -0x7ffffffc) to
> (const_int 0x80000004) when zero-extending it from SImode to DImode.
>
> But I'm not sure it's safe. You have,
>
> (zero_extend:DI (plus:SI FOO:SI) (const_int Y))
>
> and you want to convert it to
>
> (plus:DI FOO:DI (zero_extend:DI (const_int Y)))
>
> (where the zero_extend is folded). Ignore that FOO is a SYMBOL_REF (this
> piece of code does not assume anything about its shape); if FOO ==
> 0xfffffffc and Y = 8, the result will be respectively 0x4 (valid) and
> 0x100000004 (invalid).
This example contradicts what you said above "It ensures basically that the
constant has 31-bit precision".
> What happens if you just return NULL instead of the assertion (good idea
> adding it!)?
>
> Of course then you need to:
>
> 1) check the return values of convert_memory_address_addr_space_1, and
> propagate NULL up to simplify_unary_operation;
>
> 2) check in simplify-rtx.c whether the return value of
> convert_memory_address_1 is NULL, and only return if the return value is not
> NULL. This is not yet necessary (convert_memory_address is the last
> transformation for both SIGN_EXTEND and ZERO_EXTEND) but it is better to
> keep code clean.
I will give it a try.