CODEWITHSUNDEEP
cgccfloating-pointc89
user1464603
user1464603

Reputation: 457

C float literal translation

We have two embedded projects: One of them is using the cosmic compiler and the other one is using GCC. Both abide by ISO/IEC 9899:1990.

When we initialize a float with the literal 14.8f, it gets translated to the binary representation of 0x416CCCCC on the cosmic compiler and 0x416CCCCD by GCC.

The IEC standard at chapter 6.3.1.4, item 2, Floating types states:

If the value being converted is in the range of values that can be represented but cannot be represented exactly, the result is either the nearest higher or nearest lower value, chosen in an implementation-defined manner.

as we are using these numbers as threshold, this obviously makes a difference.

The cosmic compiler states that it uses a round down implementation.
As GCC is quite more complex I was wondering if it has a compiler flag that allows choosing of the behavior at compile time. So far I have only found that you can choose FE_DOWNWARD, but that is related to run-time rather than compile-time.

Does anyone have a clue of such a flag for compile-time conversion?

Upvotes: 6

Views: 410

Answers (2)

supercat
supercat

Reputation: 81197

If one wants to write the code in a manner that will yield matching behavior on C89 compilers, writing floating-point constants as a precisely-representable whole number divided by a power of two e.g. (15518925.0f/1048576.0f) should yield unambiguous results on all common floating-point implementations, regardless of rounding modes. There isn't generally any way to control how compile-time-constant expressions are rounded, but if one writes such expressions using the above form one can render such questions moot.

Upvotes: 1

unwind
unwind

Reputation: 399949

Just for reference's sake, the relevant chapter in GCC's manual states:

How the nearest representable value or the larger or smaller representable value immediately adjacent to the nearest representable value is chosen for certain floating constants (C90 6.1.3.1, C99 and C11 6.4.4.2).

C99 Annex F is followed.

And in my draft C99 standard, Annex F says:

F.7.2 Translation

During translation the IEC 60559 default modes are in effect:

— The rounding direction mode is rounding to nearest.
— The rounding precision mode (if supported) is set so that results are not shortened.
— Trapping or stopping (if supported) is disabled on all floating-point exceptions

So that seem to clearly state that

  • GCC uses rounding to nearest.
  • You can't change it.

Using the hexadecimal syntax to get the exact desired float seems like the proper solution here, and (I guess) the reason that syntax exists.

Upvotes: 4

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