Re: [RFC PATCH 1/8] qemu/int128: avoid undefined behavior in int128_lshift

[Richard Henderson]

On 3/30/22 11:59, matheus.ferst@eldorado.org.br wrote:
> From: Matheus Ferst <matheus.ferst@eldorado.org.br>
>
> Avoid the left shift of negative values in int128_lshift by casting
> a/a.hi to unsigned.
>
> Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>

Eh, maybe.  We do this all over qemu, and I think any undefinedness you're thinking of in 
the base C standard is removed by the -fwrapv with which all files are compiled.


r~

> ---
>   include/qemu/int128.h | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/qemu/int128.h b/include/qemu/int128.h
> index 2c4064256c..2a19558ac6 100644
> --- a/include/qemu/int128.h
> +++ b/include/qemu/int128.h
> @@ -85,7 +85,7 @@ static inline Int128 int128_rshift(Int128 a, int n)
>   
>   static inline Int128 int128_lshift(Int128 a, int n)
>   {
> -    return a << n;
> +    return (__uint128_t)a << n;
>   }
>   
>   static inline Int128 int128_add(Int128 a, Int128 b)
> @@ -305,7 +305,7 @@ static inline Int128 int128_lshift(Int128 a, int n)
>       if (n >= 64) {
>           return int128_make128(0, l);
>       } else if (n > 0) {
> -        return int128_make128(l, (a.hi << n) | (a.lo >> (64 - n)));
> +        return int128_make128(l, ((uint64_t)a.hi << n) | (a.lo >> (64 - n)));
>       }
>       return a;
>   }