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Rounding floats to integers on Cortex-M4

Dejan Durdenic

I've encountered an odd problem on STM32F4xx CPU with hardware FPU enabled:

When rounding floats to integers, compiler always uses VCVT (round towards zero) instruction instead of VCVTR (round using rounding mode register settings).

There are compiler options for various IEEE compatibility modes, some of them explicitly define "round to nearest" behaviour, but it seems to be ignored.

Even worse, FLT_ROUNDS in float.h  is defined as floats are rounded, but they're truncated.

fpgetround() and fpsetround() do not work right either.

Any ideas?

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  • 0 in reply to Dejan Durdenic

    See the C programming language standard section 6.3.1.4 Real floating and integer in

    http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf

    • When a finite value of real floating type is converted to an integer type other than _Bool, the fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part cannot be represented by the integer type, the behavior is undefined

    The bit you were reading is about rounding of the results of arithmetic operations on floating point values (and it should have referred to round to nearest or even not just round to nearest). For instance when two floating point values are added the exact result might occupy too many bits of precision so it needs to be rounded to a representable value e.g. if we had a machine which had 3 significant places of decimal in its floats 1.37 and 0.356 added would give 1.73 after automatic rounding. However assigning 1.73 to an integer would give 1 as the result.

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  • 0 in reply to Dejan Durdenic

    See the C programming language standard section 6.3.1.4 Real floating and integer in

    http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf

    • When a finite value of real floating type is converted to an integer type other than _Bool, the fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part cannot be represented by the integer type, the behavior is undefined

    The bit you were reading is about rounding of the results of arithmetic operations on floating point values (and it should have referred to round to nearest or even not just round to nearest). For instance when two floating point values are added the exact result might occupy too many bits of precision so it needs to be rounded to a representable value e.g. if we had a machine which had 3 significant places of decimal in its floats 1.37 and 0.356 added would give 1.73 after automatic rounding. However assigning 1.73 to an integer would give 1 as the result.

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