Re: [Qemu-devel] [PATCH for-3.1] target/arm: Remove can't-happen if() from handle_vec_simd_shli()

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 30/10/18 17:25, Peter Maydell wrote:
> In handle_vec_simd_shli() we have a check:
>       if (size > 3 && !is_q) {
>           unallocated_encoding(s);
>           return;
>       }
> However this can never be true, because we calculate
>      int size = 32 - clz32(immh) - 1;
> where immh is a 4 bit field which we know cannot be all-zeroes.
> So the clz32() return must be in {28,29,30,31} and the resulting
> size is in {0,1,2,3}, and "size > 3" is never true.
> 
> This unnecessary code confuses Coverity's analysis:
> in CID 1396476 it thinks we might later index off the
> end of an array because the condition implies that we
> might have a size > 3.
> 
> Remove the code, and instead assert that the size is in [0..3],
> since the decode that enforces that is somewhat distant from
> this function.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

> ---
> Alex, if you could run this through the risu testset just as
> a sanity check that would be very helpful.
> 
>   target/arm/translate-a64.c | 8 +++-----
>   1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/target/arm/translate-a64.c b/target/arm/translate-a64.c
> index 88195ab9490..fd36425f1ae 100644
> --- a/target/arm/translate-a64.c
> +++ b/target/arm/translate-a64.c
> @@ -9483,12 +9483,10 @@ static void handle_vec_simd_shli(DisasContext *s, bool is_q, bool insert,
>       int immhb = immh << 3 | immb;
>       int shift = immhb - (8 << size);
>   
> -    if (extract32(immh, 3, 1) && !is_q) {
> -        unallocated_encoding(s);
> -        return;
> -    }
> +    /* Range of size is limited by decode: immh is a non-zero 4 bit field */
> +    assert(size >= 0 && size <= 3);
>   
> -    if (size > 3 && !is_q) {
> +    if (extract32(immh, 3, 1) && !is_q) {
>           unallocated_encoding(s);
>           return;
>       }
>