Loss of precision errors in FP conversions

[bearophile]

In Bugzilla I have just added an enhancement request that asks for a little change in D, I don't know if it was already discussed or if it's already present in Bugzilla:
http://d.puremagic.com/issues/show_bug.cgi?id=5864

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.


Partially related note: currently std.math doesn't seem to use the cosf, sinf C functions, but it uses sqrtf:

import std.math: sqrt, sin, cos;
void main() {
    float x = 1.0f;
    static assert(is(typeof(  sqrt(x)  ) == float)); // OK
    static assert(is(typeof(  sin(x)   ) == float)); // ERR
    static assert(is(typeof(  cos(x)   ) == float)); // ERR
}

Bye,
bearophile