[PATCH] [compiler-rt] Avoid undefined behaviour in __floatsisf and __floatsidf

[Matthew Fernandez via llvm-commits]

Sorry, my mistake. I was under the impression such a thing was defined. I'll redo this using your original suggestion 
and submit a new patch.

On 02/11/15 23:05, Stephen Canon wrote:
> This doesn’t actually avoid UB, it just moves it from the negation to a conversion from unsigned to signed.  With the patch, if a is INT_MIN, we have:
>
> 	1. convert to unsigned (fully defined, results in 2**N + a for some N)
> 	2. negate (fully defined, results in -a)
> 	3. implicit convert back to signed in (UB because -INT_MIN cannot be represented as int).
>
> The approach that I suggested before avoids this problem by having the result type of the absolute value operation be unsigned.
>
> – Steve
>
>> On Nov 2, 2015, at 6:32 AM, Matthew Fernandez <matthew.fernandez at gmail.com> wrote:
>>
>> These two functions for soft floating point support negate their signed int argument if it is negative. In the case where the argument is INT_MIN, this negation is undefined behaviour with respect to the C standard. This change performs the negation on an unsigned value, avoiding the just-described situation. This change does not alter the intended semantics of these functions.
>>
>> Signed-off-by: Matthew Fernandez <matthew.fernandez at gmail.com>
>>
>> Index: lib/builtins/floatsidf.c
>> ===================================================================
>> --- lib/builtins/floatsidf.c	(revision 251670)
>> +++ lib/builtins/floatsidf.c	(working copy)
>> @@ -33,7 +33,7 @@
>>      rep_t sign = 0;
>>      if (a < 0) {
>>          sign = signBit;
>> -        a = -a;
>> +        a = -(unsigned int)a;
>>      }
>>
>>      // Exponent of (fp_t)a is the width of abs(a).
>> Index: lib/builtins/floatsisf.c
>> ===================================================================
>> --- lib/builtins/floatsisf.c	(revision 251670)
>> +++ lib/builtins/floatsisf.c	(working copy)
>> @@ -33,7 +33,7 @@
>>      rep_t sign = 0;
>>      if (a < 0) {
>>          sign = signBit;
>> -        a = -a;
>> +        a = -(unsigned int)a;
>>      }
>>
>>      // Exponent of (fp_t)a is the width of abs(a).
>