[LLVMdev] Loop elimination with floating point counter.

[Daniel Berlin]

FWIW, I believe icc -O3 turns on the equivalent of -ffast-math by default.
I could be misremembering which compilers do this though :)
This flag allows you to make all kinds of nice simplfiying assumptions
about floating point.

On Thu, Jan 8, 2009 at 7:45 PM, Owen Anderson <resistor at mac.com> wrote:
> I assume it checks that the end condition and the increment can both
> be represented precisely with integer types.
>
> --Owen
>
> On Jan 8, 2009, at 9:49 AM, Martin Geisse wrote:
>
>> Isn't "simplifying" the loop index to an integer also dependent on
>> precision issues? The following loop is infinite:
>>
>> for (float i=0.0; i<...; i+=1.0) {}
>>
>> where ... is a large "integral" float literal, e.g. 2^40. At some
>> point, adding 1.0 to the loop index would not cause any changes
>> because of precision issues. However, if the float type is replaced
>> by some sufficiently large integer type, the loop terminates.
>>
>> The necessary reasoning may be simpler though.
>>
>> Greetings,
>> Martin Geisse
>>
>> On Jan 8, 2009, at 6:34 PM, Owen Anderson wrote:
>>
>>> It's because with 1.0f, the loop index is simplified into an integer.
>>> With 1.2f, it isn't.  The loop deletion pass is dependent on the loop
>>> analyses being able to determine that the loop is finite, which they
>>> don't attempt to do for loops with floating point indices.
>>> Attempting
>>> to do so would require additional reasoning about floating point
>>> precision issues.
>>>
>>> --Owen
>>>
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