On 10/09/17 15:02, Richard Earnshaw (lists) wrote:
> On 06/09/17 14:17, Bernd Edlinger wrote:
>> Index: gcc/doc/rtl.texi
>> ===================================================================
>> --- gcc/doc/rtl.texi (revision 251752)
>> +++ gcc/doc/rtl.texi (working copy)
>> @@ -2252,6 +2252,13 @@
>> If one of the operands is a constant, it should be placed in the
>> second operand and the comparison code adjusted as appropriate.
>>
>> +There may be exceptions to this rule if the mode @var{m} does not
>> +carry enough information for the swapped comparison operator, or
>> +if we try to detect overflow from the subtraction. That means, while
>> +0-X may overfow X-0 can never overflow. Under these conditions
s/overfow/overflow/
>> +a compare may have the constant expression at the first operand.
>> +Examples are the ARM negdi2_compare pattern and similar.
>> +
>> A @code{compare} specifying two @code{VOIDmode} constants is not valid
>> since there is no way to know in what mode the comparison is to be
>> performed; the comparison must either be folded during the compilation
>>
>
>
> Er, hold on. Comparisons don't 'overflow'. They compare two values and
> tell you something about their relationship. If A cmp B doesn't tell me
> the same basic things as B swapped(cmp) A, then the world will fall
> apart big time. So your documentation patch can't be right.
>
Hi Richard,
I think a CC-mode like CC_NCV is inherently unsymmetrical
which means that it does in general not provide enough information for
the swapped(cmp), and I think the truth is that the N-flag
is the sign of A-B, and V-flag is 1 if A-B overflows otherwise 0.
And if you ask for Branch if less-than that is Branch if N ^ V.
But in the moment I have no idea how to proceed.
Thanks
Bernd.
