On Tue, 14 Sept 2021 at 09:56, Cédric Le Goater <c...@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.pi...@eldorado.org.br>
> Fixes: a8dafa525181 ("target/ppc: Implement large decrementer support for
> TCG")
> Signed-off-by: Cédric Le Goater <c...@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
