[Issue 5864] New: Loss of precision errors in FP conversions

April 19, 2011
Posted by bearophile_hugs@eml.cc
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bearophile_hugs@eml.cc
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http://d.puremagic.com/issues/show_bug.cgi?id=5864

           Summary: Loss of precision errors in FP conversions
           Product: D
           Version: D2
          Platform: Other
        OS/Version: Windows
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2011-04-19 16:27:30 PDT ---
In a program like this:

void main() {
    uint x = 10_000;
    ubyte b = x;
}


DMD 2.052 raises a compilation error like this, because the b=x assignment may lose some information, some bits of x:

test.d(3): Error: cannot implicitly convert expression (x) of type uint to
ubyte

I think that a safe and good system language has to help avoid unwanted (implicit) loss of information during data conversions.

This is a case of loss of precision where D generates no compile errors:


import std.stdio;
void main() {
    real f1 = 1.0000111222222222333;
    writefln("%.19f", f1);
    double f2 = f1; // loss of FP precision
    writefln("%.19f", f2);
    float f3 = f2; // loss of FP precision
    writefln("%.19f", f3);
}

Despite some information is lost, see the output:
1.0000111222222222332
1.0000111222222223261
1.0000110864639282226

So one possible way to face this situation is to statically disallow double=>float, real=>float, and real=>double conversions (on some computers real=>double conversions don't cause loss of information, but I suggest to ignore this, to increase code portability), and introduce compile-time errors like:

test.d(5): Error: cannot implicitly convert expression (f1) of type real to
double
test.d(7): Error: cannot implicitly convert expression (f2) of type double to
float


Today float values seem less useful, because with serial CPU instructions the performance difference between operations on float and double is often not important, and often you want the precision of doubles. But modern CPUs (and current GPUs) have vector operations too. They are currently able to perform operations on 4 float values or 2 double values (or 8 float or 4 doubles) at the same time for each instruction. Such vector instructions are sometimes used directly in C-GCC code using SSE intrinsics, or come out of auto-vectorization optimization of loops done by GCC on normal serial C code. In this situation the usage of float instead of double gives almost a twofold performance increase. There are programs (like certain ray-tracing code) where the precision of a float is enough. So a compile-time error that catches currently implicit double->float conversions may help the programmer avoid unwanted usages of doubles that allow the compiler to pack 4/8 floats in a vector register during loop vectorizations.

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