sign wraping

[ramshankar r]

Hi,

I am wondering about the no sign wrap feature in llvm and how it will be applied to Fortran. My question is when can we promote 32-bit loop induction integers used in array indices to 64-bit. This has a major impact in llvm's optimization, including Indvar substitution, loop strength reduction among others (including vectorization). I have heard about gcc's fno-range-check, which allows the compiler to expect wrapping when integers overflow. Will flang need a flag like that to be able add the nsw flag?

Thanks,

Ram

[Hal Finkel]

Hi Ram,

This is a very good question. I'm not an expert in interpreting the Fortran standard, but perhaps we're okay here:

[begin snippets from the spec (http://www.j3-fortran.org/doc/year/10/10-007.pdf)]

4.1.2 Set of values

For each type, there is a set of valid values. The set of valid values for logical is completely determined by this
part of ISO/IEC 1539. The sets of valid values for integer, character, and real are processor dependent...

4.4.2.2 Integer type

The set of values for the integer type is a subset of the mathematical integers. The processor shall provide one or
more representation methods that define sets of values for data of type integer... The integer type includes a zero value, which is
considered to be neither negative nor positive. The value of a signed integer zero is the same as the value of an
unsigned integer zero.

7.1.5.2.4 Evaluation of numeric intrinsic operations

The execution of any numeric operation whose result is not defined by the arithmetic used by the processor is
prohibited. Raising a negative-valued primary of type real to a real power is prohibited.

[end snippets from the spec]

So, as far as I can tell, we get to define the processor's arithmetic however we want, and so we're free to define it to not have well-defined wrapping semantics.

-Hal

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--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory