[jira] [Created] (MYNEWT-746) Sim - Floating point calculations sometimes incorrect

["Christopher Collins (JIRA)" <ji...@apache.org>]

Christopher Collins created MYNEWT-746:
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             Summary: Sim - Floating point calculations sometimes incorrect
                 Key: MYNEWT-746
                 URL: https://issues.apache.org/jira/browse/MYNEWT-746
             Project: Mynewt
          Issue Type: Bug
            Reporter: Christopher Collins
             Fix For: v1_1_0_rel


*Note: this problem does not occur in OS X; it has only been seen in Linux.*

In sim, some floating point calculations yield wildly incorrect results (usually 0 or NaN).  The cause appears to be that the floating point registers are not restored when longjmp() is called.  If subsequent code tries to use an intermediate value stored in a floating point register, the result is indeterminate.

There is a gcc / clang compiler option which mostly solves this problem: *-ffloat-store*:

{quote}
-ffloat-store
    Do not store floating point variables in registers, and inhibit other options that might change whether a floating point value is taken from a register or memory.

    This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables. 
{quote}

The problems disappeared when I started using this option.  However, it seems there is still the occasional miscalculation, as a test failure just occurred due to an incorrect floating point value.  In this case, assigning {{18.0}} to an int yielded the value {{-2147483648}}.



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