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

[d-bugmail at puremagic.com]

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 at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at 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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