Re: [PULL 33/44] target/ppc: Fix 64-bit decrementer

[Peter Maydell]

On Thu, 30 Sept 2021 at 06:44, David Gibson <david@gibson.dropbear.id.au> wrote:
>
> From: Cédric Le Goater <clg@kaod.org>
>
> The current way the mask is built can overflow with a 64-bit decrementer.
> Use sextract64() to extract the signed values and remove the logic to
> handle negative values which has become useless.
>
> 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>
> Message-Id: <20210920061203.989563-5-clg@kaod.org>
> Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Hi; Coverity complains about dead code here (CID 1464061):



>       * On MSB edge based DEC implementations the MSB going from 0 -> 1 
> triggers
>       * an edge interrupt, so raise it here too.
>       */
> -    if ((value < 3) ||
> -        ((tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL) && negative) ||
> -        ((tb_env->flags & PPC_DECR_UNDERFLOW_TRIGGERED) && negative
> -          && !(decr & (1ULL << (nr_bits - 1))))) {
> +    if ((signed_value < 3) ||
> +        ((tb_env->flags & PPC_DECR_UNDERFLOW_LEVEL) && signed_value < 0) ||
> +        ((tb_env->flags & PPC_DECR_UNDERFLOW_TRIGGERED) && signed_value < 0
> +          && signed_decr >= 0)) {
>          (*raise_excp)(cpu);
>          return;
>      }

If signed_value < 3 then the first clause of the || evaluates as true,
and so we will definitely take the if() clause. So if we're evaluating
the second operand to the || then we know that signed_value > 3,
which means that 'signed_value < 0' is always false and in turn that
neither of the other two '||' options can be true. The whole expression
is equivalent to just "if (signed_value < 3)".

What was intended here? If this was supposed to be a no-behaviour-change
commit (apart from fixing the 64-bit case) then the condition should
have stayed as "(value < 3)", I think.

thanks
-- PMM