Re: [PATCH v2] target/ppc: Fix 64-bit decrementer

[Peter Maydell]

On Tue, 14 Sept 2021 at 09:56, Cédric Le Goater <clg@kaod.org> wrote:
>
> The current way the mask is built can overflow with a 64-bit decrementer.
> Use sextract64() instead.
>
> Cc: Luis Fernando Fujita Pires <luis.pires@eldorado.org.br>
> Fixes: a8dafa525181 ("target/ppc: Implement large decrementer support for 
> TCG")
> Signed-off-by: Cédric Le Goater <clg@kaod.org>
> ---
>
>  v2: replaced MAKE_64BIT_MASK by sextract64
>
>  hw/ppc/ppc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c
> index 7375bf4fa910..4f14464c9220 100644
> --- a/hw/ppc/ppc.c
> +++ b/hw/ppc/ppc.c
> @@ -876,7 +876,7 @@ static void __cpu_ppc_store_decr(PowerPCCPU *cpu, 
> uint64_t *nextp,
>      bool negative;
>
>      /* Truncate value to decr_width and sign extend for simplicity */
> -    value &= ((1ULL << nr_bits) - 1);
> +    value = sextract64(value, 0, nr_bits);
>      negative = !!(value & (1ULL << (nr_bits - 1)));
>      if (negative) {
>          value |= (0xFFFFFFFFULL << nr_bits);

I think these lines that say "if negative then force all the
high bits to one" are also no longer required. That is, this
entire section of code:
    value &= ((1ULL << nr_bits) - 1);
    negative = !!(value & (1ULL << (nr_bits - 1)));
    if (negative) {
        value |= (0xFFFFFFFFULL << nr_bits);
    }

is an open-coded sign-extension, which can all be replaced with
the single line
    value = sextract64(value, 0, nr_bits);

(and also: if nr_bits is 64 then the "<< nr_bits"
is undefined behaviour.)

thanks
-- PMM