Re: [PATCH v1 5/7] fpu/softfloat: avoid undefined behaviour when normalising empty sigs

[Richard Henderson <richard.henderson@linaro.org>]

On 3/27/20 3:09 AM, Peter Maydell wrote:
> On Fri, 27 Mar 2020 at 09:49, Alex Bennée <alex.bennee@linaro.org> wrote:
>>
>> The undefined behaviour checker pointed out that a shift of 64 would
>> lead to undefined behaviour.
>>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> ---
>>  fpu/softfloat.c | 11 ++++++++---
>>  1 file changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
>> index 301ce3b537b..444d35920dd 100644
>> --- a/fpu/softfloat.c
>> +++ b/fpu/softfloat.c
>> @@ -3834,9 +3834,14 @@ void normalizeFloatx80Subnormal(uint64_t aSig, int32_t *zExpPtr,
>>  {
>>      int8_t shiftCount;
>>
>> -    shiftCount = clz64(aSig);
>> -    *zSigPtr = aSig<<shiftCount;
>> -    *zExpPtr = 1 - shiftCount;
>> +    if (aSig) {
>> +        shiftCount = clz64(aSig);
>> +        *zSigPtr = aSig << shiftCount;
>> +        *zExpPtr = 1 - shiftCount;
>> +    } else {
>> +        *zSigPtr = 0;
>> +        *zExpPtr = 1 - 64;
>> +    }
>>  }
> 
> Ah yes, I saw this one in Coverity: CID 1421991.
> 
> RTH marked the Coverity issue as a false positive with the rationale
> "We assume an out-of-range shift count is merely IMPLEMENTATION DEFINED
>  and not UNDEFINED (in the Arm ARM sense), and so cannot turn a 0 value
>  into a non-zero value."
> but I think I disagree with that. We can assume that for the TCG IR
> where we get to define shift semantics because we're doing the codegen,
> but we can't assume it in C code, because it's not included in the set
> of extended guarantees provided by -fwrapv as far as I know.

Perhaps.  Of course we also know from our broad knowledge of architectures,
that a compiler would really have to go out of its way for this to happen.

I really hate C in this way, sometimes.

I wonder if I have the energy to petition the committee to drop, for C202? all
of the "undefined" nonsense that only applies to sign-magnitute and
ones-compliment computers, which haven't been seen since the 70's...

> That said, is it valid for this function to be called with a zero
> aSig value ? I think all these normalizeFloat*Subnormal() functions
> assume non-zero sig input, and the only callsite where it's not clearly
> obvious that this is obvious that the sig input is non-zero is the call to
> normalizeFloatx80Subnormal() from addFloatx80Sigs(). So perhaps we
> just need to check and fix that callsite ??

You're right -- addFloatx80Sigs is the only use out of 26 that doesn't have a
preceding check for 0.


r~