Re: Somewhat Puzzline Error

On 15/05/2025 02:22, Cy Schubert wrote:
> On 32-bit platforms, given this definition,
> 
> typedef uint128_t fiat_25519_uint128;

I think that 128-bit integer types are a thing only on 64-bit platforms (and 
perhaps not all of them).
I am not sure why compilation does not bail out earlier on 32-bit platforms.

The types are provided by a compiler.
E.g., https://gcc.gnu.org/onlinedocs/gcc/_005f_005fint128.html
Also, https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60846

I imagine that clang is being compatible with gcc.

> The following should be ok.
> 
>    fiat_25519_uint128 x26 = (x25 + (x10 + (x9 + (x7 + x4))));
>    uint64_t x27 = (uint64_t)(x26 >> 51);
> 
> But on 32-bit platforms only this results in the following error.
> 
> In file included from /home/cy/src-krb5/crypto/krb5/src/plugins/preauth/spake/ed
> wards25519.c:193:
> /home/cy/src-krb5/crypto/krb5/src/plugins/preauth/spake/edwards25519_fiat.h:101:
> 33: error: shift count >= width of type [-Werror,-Wshift-count-overflow]
>    101 |   uint64_t x27 = (uint64_t)(x26 >> 51);
>        |                                 ^  ~~
> x26 should be 128 bits. Or is uint128_t not 128 bits on 32-bit platforms?
> 
> Of course there is no error on 64-bit platforms.

-- 
Andriy Gapon