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digitalmars.D.bugs - [Issue 5866] New: std.math.sin(float), std.math.cos(float) to return float

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5866

           Summary: std.math.sin(float), std.math.cos(float) to return
                    float
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: wrong-code
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc


--- Comment #0 from bearophile_hugs eml.cc 2011-04-20 04:08:20 PDT ---
Currently (DMD 2.052) std.math doesn't seem to use the cosf, sinf C functions,
but it seems to use sqrtf (or something similar):


import std.math: sqrt, sin, cos;
import core.stdc.math: sqrtf, sinf, cosf;
void main() {
     float x = 1.0f;
     // C functions
     static assert(is(typeof( sqrtf(x) ) == float)); // OK
     static assert(is(typeof( sinf(x)  ) == float)); // OK
     static assert(is(typeof( cosf(x)  ) == float)); // OK
     // D functions
     static assert(is(typeof( sqrt(x)  ) == float)); // OK
     static assert(is(typeof( sin(x)   ) == float)); // ERR
     static assert(is(typeof( cos(x)   ) == float)); // ERR
}


I'd like std.math.sin/cos to return a float value when the input argument is a
float.

This is useful if you want to perform operations on floats and keep
intermediate expressions (that call sin/cos) as floats:

enum float PI_FLOAT = 3.14159265358979323846264f; // not present in std.math
float x = 0.3f;
float y = 2.0f * PI_FLOAT * sin(x); // contains no double->float conversions

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next sibling parent d-bugmail puremagic.com writes:
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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug yahoo.com.au


--- Comment #1 from Don <clugdbug yahoo.com.au> 2011-04-20 05:03:20 PDT ---
Bearophile, I don't think that would achieve what you think.

float  = float * float + float

contains a conversion from double to float.

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--- Comment #2 from bearophile_hugs eml.cc 2011-04-20 09:44:40 PDT ---
(In reply to comment #1)

 float  = float * float + float
 
 contains a conversion from double to float.


Thank you for your comment. In programming I'm always finding new and new ways
to be wrong or just to show ignorance :-) But this program, gives no errors
with DMD 2.052:


float f1() { return 1.0f; }
float f2() { return 2.0f; }
float f3() { return 3.0f; }
void main() {
    static assert(is(typeof(f1() * f2()) == float));
    static assert(is(typeof(f1() * f2() + f3()) == float));
    float result;
    static assert(is(typeof(result = f1() * f2() + f3()) == float));
}


And a good C lint gives no double->error warnings in this C program:

float f1() { return 1.0f; }
float f2() { return 2.0f; }
float f3() { return 3.0f; }
int main() {
    float result = f1() * f2() + f3();
    return 0;
}


So where's the double->float conversion?

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--- Comment #3 from Don <clugdbug yahoo.com.au> 2011-04-20 12:34:16 PDT ---
(In reply to comment #2)

 (In reply to comment #1)

 float  = float * float + float
 
 contains a conversion from double to float.


 
 Thank you for your comment. In programming I'm always finding new and new ways
 to be wrong or just to show ignorance :-) But this program, gives no errors
 with DMD 2.052:
 
 
 float f1() { return 1.0f; }
 float f2() { return 2.0f; }
 float f3() { return 3.0f; }
 void main() {
     static assert(is(typeof(f1() * f2()) == float));
     static assert(is(typeof(f1() * f2() + f3()) == float));
     float result;
     static assert(is(typeof(result = f1() * f2() + f3()) == float));
 }
 
 
 And a good C lint gives no double->error warnings in this C program:
 
 float f1() { return 1.0f; }
 float f2() { return 2.0f; }
 float f3() { return 3.0f; }
 int main() {
     float result = f1() * f2() + f3();
     return 0;
 }
 
 
 So where's the double->float conversion?


It's not exactly a double->float conversion. But it's part of the same issue:
to what extent is the compiler allowed to perform floating-point promotions,
and use extra precision?

It shows up when f3 is negative, and slightly less than f1*f2. If the compiler
is using doubles for intermediate results (which it is allowed to do), the
results are not the same as if floats were used throughout. 
An obvious case is when f1 = float.max, f2 = 1.5, f3 = -float.max.
If it stays as float, the result will be +infinity. If intermediate doubles are
used, the result is 0.5 * float.max. (It affects precision as well as range,
but the range examples are more obvious).
Floating point multiply-accumulate (FMA) can do the same thing.

As long as you support x87, or allow FMA, this stuff is inevitable.

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--- Comment #4 from bearophile_hugs eml.cc 2011-04-20 14:39:26 PDT ---
(In reply to comment #3)


 As long as you support x87, or allow FMA, this stuff is inevitable.


Lot of complexities, I see.

Going back to the point of this enhancement request, if cos(float) and
sin(float) return a double, there is zero chance that the intermediate
computations get performed among floats. So I think this enhancement request
stands still.

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SomeDude <lovelydear mailmetrash.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lovelydear mailmetrash.com
           Severity|normal                      |enhancement


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--- Comment #5 from bearophile_hugs eml.cc 2012-04-24 19:03:51 PDT ---
Now converted to enhancement by SomeDude.

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