Re: [Qemu-devel] [Bug 1464611] [NEW] 4 * redundant conditions

[Peter Maydell]

On 12 June 2015 at 14:03, Eric Blake <address@hidden> wrote:
> On 06/12/2015 05:01 AM, Peter Maydell wrote:
>
>>> 4.
>>>
>>> [qemu/target-arm/translate-a64.c:5729]: (style) Redundant condition:
>>> size<3. 'A && (!A || B)' is equivalent to 'A || B'
>>>
>>>       if (size > 3
>>>             || (size < 3 && is_q)
>>>             || (size == 3 && !is_q)) {
>>
>> ...but I'm less sure about this one. I'm not even sure
>> what it's trying to suggest this should simplify to:
>> just dropping "size < 3" is obviously wrong, and the
>> condition format isn't "A && (!A || B)" either.
>
> Let's break it down into the 6 possibilities based on the binary *
> ternary conditions being checked:
>
>> 3, is_q   => accept
>> 3, !is_q  => accept
> == 3, is_q  => reject
> == 3, !is_q => accept
> < 3, is_q   => accept
> < 3, !is_q  => reject
>
> Here's a shorter conditional with the same properties, but it's gross:
>
> if (size > 3 || (is_q != (size == 3))) {
>
> Too much mental thought to prove it accepts the same set of conditions.

Yeah, I think this is the kind of thing where I say "the compiler
should do this simplification if it cares enough" :-)

-- PMM