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digitalmars.D.learn - Very strange problem with comparing floating point numbers

reply "Ivan Agafonov" <armadil yandex.ru> writes:
// Tell me about this sutation, may be it is a bug?

import std.math;
import std.stdio;

struct Vector(int size)
{
	union
	{
		float[size] array = 0;
		struct
		{
			static if (size == 2) float x, y;
			static if (size == 3) float x, y, z;
			static if (size == 4) float x, y, z, w;
		}
	}
	
	 property float lengthsqr()
	{
		static if (size == 2) return x*x + y*y;
		static if (size >= 3) return x*x + y*y + z*z;
	}

	 property float length() { return sqrt(lengthsqr()); }
	 property float length2() { float tmp = sqrt(lengthsqr()); 
return tmp; }
}

void main()
{
	auto a = Vector!4([1, 2, 3, 1]);
	auto a3 = Vector!3([1, 2, 3]);
	assert (a.lengthsqr == 14);
	
	auto alen = a.length; auto a3len = a3.length;
	
	// all of this prints the same number: 0x1.deea2p+1
	writefln("%a, %a", alen, a3len);
	writefln("%a, %a", a.length, a3.length);
	writefln("%a, %a", a.length2, a3.length2);
	
	// passes
	assert (alen == a3len);
	assert (a.length2 == a3.length2);
	assert (cast(real)a.length == cast(real)a3.length);
	
	// all of this fails!!!
	assert (a.length == a.length); // This is really shocking
	assert (a.length == a3.length);
}
Sep 29 2012
next sibling parent reply Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
On 9/30/12, Ivan Agafonov <armadil yandex.ru> wrote:
 // Tell me about this sutation, may be it is a bug?

Reduced: import std.stdio; import std.math; property float getFloat() { return sqrt(1.1); } void main() { writeln(getFloat == getFloat); // fail } Dissasembly: __Dmain:; Function begin, communal enter 12, 0 ; 0000 _ C8, 000C, 00 call _D4test8getFloatFNdZf ; 0004 _ E8, 00000000(rel) fstp dword [ebp-0CH] ; 0009 _ D9. 5D, F4 call _D4test8getFloatFNdZf ; 000C _ E8, 00000000(rel) fld dword [ebp-0CH] ; 0011 _ D9. 45, F4 fxch st1 ; 0014 _ D9. C9 fucompp ; 0016 _ DA. E9 fnstsw ax ; 0018 _ DF. E0 sahf ; 001A _ 9E mov eax, 1 ; 001B _ B8, 00000001 jpe ?_033 ; 0020 _ 7A, 02 jz ?_034 ; 0022 _ 74, 02 ?_033: xor eax, eax ; 0024 _ 31. C0 ?_034: call _D3std5stdio14__T7writelnTbZ7writelnFbZv; 0026 _ E8, 00000000(rel) xor eax, eax ; 002B _ 31. C0 leave ; 002D _ C9 ret ; 002E _ C3 ; __Dmain End of function Now compare to this which doesn't fail: void main() { float fx1 = getFloat; float fx2 = getFloat; writeln(fx1 == fx2); // pass } Dissasembly: __Dmain:; Function begin, communal enter 8, 0 ; 0000 _ C8, 0008, 00 call _D4test8getFloatFNdZf ; 0004 _ E8, 00000000(rel) fstp dword [ebp-8H] ; 0009 _ D9. 5D, F8 call _D4test8getFloatFNdZf ; 000C _ E8, 00000000(rel) fstp dword [ebp-4H] ; 0011 _ D9. 5D, FC fld dword [ebp-8H] ; 0014 _ D9. 45, F8 fld dword [ebp-4H] ; 0017 _ D9. 45, FC fucompp ; 001A _ DA. E9 fnstsw ax ; 001C _ DF. E0 sahf ; 001E _ 9E mov eax, 1 ; 001F _ B8, 00000001 jpe ?_033 ; 0024 _ 7A, 02 jz ?_034 ; 0026 _ 74, 02 ?_033: xor eax, eax ; 0028 _ 31. C0 ?_034: call _D3std5stdio14__T7writelnTbZ7writelnFbZv; 002A _ E8, 00000000(rel) xor eax, eax ; 002F _ 31. C0 leave ; 0031 _ C9 ret ; 0032 _ C3 ; __Dmain End of function In the first sample the ASM uses fstp for LHS and fld for RHS (http://en.wikipedia.org/wiki/X86_instruction_listings). The second one uses fstp twice, then fld twice. I don't know, maybe this could be a bug. My ASM is weak but I see some difference here..
Sep 29 2012
next sibling parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 09/29/2012 06:48 PM, Andrej Mitrovic wrote:

 The second one uses fstp twice, then fld twice. I don't know, maybe
 this could be a bug. My ASM is weak but I see some difference here..

I saw similar differences but my asm is even weaker. :) I am writing to confirm that I can reproduce the problem with -m32 on a 64-bit system. The code works as expected without -m32. Ali
Sep 29 2012
prev sibling parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 09/30/2012 04:06 AM, Tommi wrote:
 On Sunday, 30 September 2012 at 01:48:04 UTC, Andrej Mitrovic wrote:
 Dissasembly:
 __Dmain:; Function begin, communal
 enter 12, 0 ; 0000 _ C8, 000C, 00
 call _D4test8getFloatFNdZf ; 0004 _ E8,
 00000000(rel)
 ...

Can I tell DMD to produce the assembly, or what did you do to get that?

For a foo.d, after compiling the program and generating foo.o, the two options on Linux that I know of: 1) obj2asm that comes with dmd: $ obj2asm foo.o > foo.asm 2) objdump that comes with at least my Linux distribution: $ objdump -d foo.o > foo.asm Ali
Sep 30 2012
prev sibling next sibling parent "jerro" <a a.com> writes:
 The second one uses fstp twice, then fld twice. I don't know, 
 maybe
 this could be a bug.

You're right the lack of one fst/fld in the first case is a bug. x87 floating point registers are 80 bit. This: fstp dword [ebp-0CH] Converts the value in ST0 to single precision float and stores it to memory (and pops ST0). When it is later loaded with fld, it is not the same as before storing since some precision is lost (because the D code compares floats and not reals, this is the correct behavior). In the first example, this storing and loading only happens for the first function call. For the second call the value is returned in ST0 and stays in x87 registers until it is compared with fucompp so it is not truncated as the result of the first function call was. That's why the compared values are not equal.
Sep 29 2012
prev sibling next sibling parent "Tommi" <tommitissari hotmail.com> writes:
On Sunday, 30 September 2012 at 01:48:04 UTC, Andrej Mitrovic 
wrote:
 Dissasembly:
 __Dmain:; Function begin, communal
         enter   12, 0                                   ; 0000 
 _ C8, 000C, 00
         call    _D4test8getFloatFNdZf                   ; 0004 
 _ E8,
 00000000(rel)
 ...

Can I tell DMD to produce the assembly, or what did you do to get that?
Sep 30 2012
prev sibling next sibling parent "monarch_dodra" <monarchdodra gmail.com> writes:
On Sunday, 30 September 2012 at 01:29:24 UTC, Ivan Agafonov wrote:
 // Tell me about this sutation, may be it is a bug?

 [SNIP]
 	// all of this fails!!!
 	assert (a.length == a.length); // This is really shocking
 	assert (a.length == a3.length);
 [SNIP]

This is just a fact of life regarding how floating point types work. Here is a well documented explanation. It pertains to C++, but applies. http://www.parashift.com/c++-faq/floating-point-arith2.html As a rule of thumb, NEVER use opEqual with floating point types aniways. You need to use some sort of comparison with leway for error, such as std.math.approxEqual.
Sep 30 2012
prev sibling next sibling parent "so" <so so.so> writes:
On Sunday, 30 September 2012 at 17:07:19 UTC, monarch_dodra wrote:
 On Sunday, 30 September 2012 at 01:29:24 UTC, Ivan Agafonov 
 wrote:
 // Tell me about this sutation, may be it is a bug?

 [SNIP]
 	// all of this fails!!!
 	assert (a.length == a.length); // This is really shocking
 	assert (a.length == a3.length);
 [SNIP]

This is just a fact of life regarding how floating point types work. Here is a well documented explanation. It pertains to C++, but applies. http://www.parashift.com/c++-faq/floating-point-arith2.html As a rule of thumb, NEVER use opEqual with floating point types aniways. You need to use some sort of comparison with leway for error, such as std.math.approxEqual.

Floating point types are trouble enough without these optimization failures. There are many unsolved problems, things like approxEqual are far from answering them. Whatever the justifications they come up with, "a.len == a.len" failure is IMO unacceptable, an opEqual like this must not fail. A suggestion: do what i do and have this in your config files. alias real evil;
Sep 30 2012
prev sibling next sibling parent "monarch_dodra" <monarchdodra gmail.com> writes:
On Sunday, 30 September 2012 at 18:31:17 UTC, so wrote:
 On Sunday, 30 September 2012 at 17:07:19 UTC, monarch_dodra 
 wrote:
 On Sunday, 30 September 2012 at 01:29:24 UTC, Ivan Agafonov 
 wrote:
 // Tell me about this sutation, may be it is a bug?

 [SNIP]
 	// all of this fails!!!
 	assert (a.length == a.length); // This is really shocking
 	assert (a.length == a3.length);
 [SNIP]

This is just a fact of life regarding how floating point types work. Here is a well documented explanation. It pertains to C++, but applies. http://www.parashift.com/c++-faq/floating-point-arith2.html As a rule of thumb, NEVER use opEqual with floating point types aniways. You need to use some sort of comparison with leway for error, such as std.math.approxEqual.

Floating point types are trouble enough without these optimization failures. There are many unsolved problems, things like approxEqual are far from answering them. Whatever the justifications they come up with, "a.len == a.len" failure is IMO unacceptable, an opEqual like this must not fail. A suggestion: do what i do and have this in your config files. alias real evil;

I don't really agree with that. floating point operations are just inexact, regardless of optimizations. That's how they work, period. Either you can work with inexact results, and you use them, or you can't, and don't. Banks don't use floating point types for exactly this reason. You have to know what you are getting into before you begin. The real troubles really only start when you start using floating point type, but you expect exact results.
Sep 30 2012
prev sibling next sibling parent Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
On 9/30/12, Tommi <tommitissari hotmail.com> wrote:
 On Sunday, 30 September 2012 at 01:48:04 UTC, Andrej Mitrovic
 wrote:
 Dissasembly:
 __Dmain:; Function begin, communal
         enter   12, 0                                   ; 0000
 _ C8, 000C, 00
         call    _D4test8getFloatFNdZf                   ; 0004
 _ E8,
 00000000(rel)
 ...

Can I tell DMD to produce the assembly, or what did you do to get that?

I use objconv (can run on win32 only methinks) on an .obj file on win32 via a batch file: objconv -fnasm %~nx1 %~n1_disasm.asm && %~n1_disasm.asm http://www.agner.org/optimize/objconv.zip
Sep 30 2012
prev sibling next sibling parent "jerro" <a a.com> writes:
 I don't really agree with that. floating point operations are 
 just inexact, regardless of optimizations. That's how they 
 work, period.

It is true that they are inexact, but inexact and non-deterministic are not the same thing. Floating point operations are deterministic. Doing the same operation twice on the same machine with the same rounding mode and parameters will give you the same result. This should also be true when you do those operations using D, and using == on the two results should return true.
Sep 30 2012
prev sibling next sibling parent "monarch_dodra" <monarchdodra gmail.com> writes:
On Sunday, 30 September 2012 at 20:47:41 UTC, jerro wrote:
 I don't really agree with that. floating point operations are 
 just inexact, regardless of optimizations. That's how they 
 work, period.

It is true that they are inexact, but inexact and non-deterministic are not the same thing. Floating point operations are deterministic.

Technically (AFAIK), IEEE754 does need require reproducibility, ergo determinism. You can open an ER requesting the ability to specify the FP behavior want (like rounding behavior), the way vstudio does it.
Sep 30 2012
prev sibling next sibling parent "jerro" <a a.com> writes:
 Technically (AFAIK), IEEE754 does need require reproducibility, 
 ergo determinism.

It actually requires more than that: "Algebraic operations covered by IEEE 754, namely + , - , * , / , square root ˆš and Binary <-> Decimal Conversion with rare exceptions, must be Correctly Rounded to the precision of the operation’s destination unless the programmer has specified a rounding other than the default. If it does not Overflow, a correctly rounded operation’s error cannot exceed half the gap between adjacent floating-point numbers astride the operation’s ideal ( unrounded ) result. Half-way cases are rounded to Nearest Even, which means that the neighbor with last digit 0 is chosen." I don't know if implementation conform exactly to IEEE 754, but I doubt there is any commonly used implementation that isn't deterministic.
Sep 30 2012
prev sibling next sibling parent "Ivan Agafonov" <armadil yandex.ru> writes:
On Sunday, 30 September 2012 at 06:20:56 UTC, jerro wrote:
 The second one uses fstp twice, then fld twice. I don't know, 
 maybe
 this could be a bug.

You're right the lack of one fst/fld in the first case is a bug. x87 floating point registers are 80 bit. This: fstp dword [ebp-0CH] Converts the value in ST0 to single precision float and stores it to memory (and pops ST0). When it is later loaded with fld, it is not the same as before storing since some precision is lost (because the D code compares floats and not reals, this is the correct behavior). In the first example, this storing and loading only happens for the first function call. For the second call the value is returned in ST0 and stays in x87 registers until it is compared with fucompp so it is not truncated as the result of the first function call was. That's why the compared values are not equal.

Can you or anyone report this bug? I don't know how to do this.
Sep 30 2012
prev sibling next sibling parent "jerro" <a a.com> writes:
On Monday, 1 October 2012 at 04:10:25 UTC, Ivan Agafonov wrote:
 On Sunday, 30 September 2012 at 06:20:56 UTC, jerro wrote:
 The second one uses fstp twice, then fld twice. I don't know, 
 maybe
 this could be a bug.

You're right the lack of one fst/fld in the first case is a bug. x87 floating point registers are 80 bit. This: fstp dword [ebp-0CH] Converts the value in ST0 to single precision float and stores it to memory (and pops ST0). When it is later loaded with fld, it is not the same as before storing since some precision is lost (because the D code compares floats and not reals, this is the correct behavior). In the first example, this storing and loading only happens for the first function call. For the second call the value is returned in ST0 and stays in x87 registers until it is compared with fucompp so it is not truncated as the result of the first function call was. That's why the compared values are not equal.

Can you or anyone report this bug? I don't know how to do this.

I'm not really sure that this is a bug anymore. Apparently c++ does this like this too (http://www.parashift.com/c++-faq/floating-point-arith2.html). I do think it would be more useful if the result of floating point comparison would be defined in a cases like this, but for floating point operation in general it makes sense to keep intermediate results in registers. So I don't know whether introducing a special case for this is worth it. If you care about this, you should probably make a thread on digitalmars.D, where more people will see it and comment on it.
Sep 30 2012
prev sibling next sibling parent "Maxim Fomin" <maxim maxim-fomin.ru> writes:
On Sunday, 30 September 2012 at 17:07:19 UTC, monarch_dodra wrote:
 As a rule of thumb, NEVER use opEqual with floating point types 
 aniways. You need to use some sort of comparison with leway for 
 error, such as std.math.approxEqual.

It is possible to compare exactly floating point types by binary comparison, if it provides some benefits. import std.stdio; import std.math; property float getFloat() { return sqrt(1.1); } void main() { writeln(getFloat is getFloat); // doesn't fail }
Oct 01 2012
prev sibling next sibling parent "monarch_dodra" <monarchdodra gmail.com> writes:
On Monday, 1 October 2012 at 11:36:43 UTC, Maxim Fomin wrote:
 On Sunday, 30 September 2012 at 17:07:19 UTC, monarch_dodra 
 wrote:
 As a rule of thumb, NEVER use opEqual with floating point 
 types aniways. You need to use some sort of comparison with 
 leway for error, such as std.math.approxEqual.

It is possible to compare exactly floating point types by binary comparison, if it provides some benefits. import std.stdio; import std.math; property float getFloat() { return sqrt(1.1); } void main() { writeln(getFloat is getFloat); // doesn't fail }

I think that what you are comparing here is the functions (the address), and not the results of the call. Try writeln(getFloat() is getFloat()); //*May* fail Also, "is" works like opEqual on built in types, AFAIK, it doesn't use any "binary" magic or anything like that.
Oct 01 2012
prev sibling next sibling parent Maxim Fomin <maxim maxim-fomin.ru> writes:
2012/10/1 monarch_dodra <monarchdodra gmail.com>:
 On Monday, 1 October 2012 at 11:36:43 UTC, Maxim Fomin wrote:
 On Sunday, 30 September 2012 at 17:07:19 UTC, monarch_dodra wrote:
 As a rule of thumb, NEVER use opEqual with floating point types aniways.
 You need to use some sort of comparison with leway for error, such as
 std.math.approxEqual.

It is possible to compare exactly floating point types by binary comparison, if it provides some benefits. import std.stdio; import std.math; property float getFloat() { return sqrt(1.1); } void main() { writeln(getFloat is getFloat); // doesn't fail }

I think that what you are comparing here is the functions (the address), and not the results of the call. Try writeln(getFloat() is getFloat()); //*May* fail

http://dpaste.dzfl.pl/1f94c0b1 It works with -m32 too. _Dmain: 0x0806d0e4 <+0>: push %ebp 0x0806d0e5 <+1>: mov %esp,%ebp 0x0806d0e7 <+3>: sub $0x10,%esp 0x0806d0ea <+6>: push %esi 0x0806d0eb <+7>: push %edi 0x0806d0ec <+8>: call 0x806d0d4 <_D4test8getFloatFNdZf> 0x0806d0f1 <+13>: fstps -0x10(%ebp) 0x0806d0f4 <+16>: lea -0x10(%ebp),%esi 0x0806d0f7 <+19>: call 0x806d0d4 <_D4test8getFloatFNdZf> 0x0806d0fc <+24>: fstps -0xc(%ebp) 0x0806d0ff <+27>: lea -0xc(%ebp),%edi 0x0806d102 <+30>: mov $0x4,%ecx 0x0806d107 <+35>: xor %eax,%eax 0x0806d109 <+37>: repz cmpsb %es:(%edi),%ds:(%esi) 0x0806d10b <+39>: je 0x806d112 <_Dmain+46> 0x0806d10d <+41>: sbb %eax,%eax 0x0806d10f <+43>: sbb $0xffffffff,%eax 0x0806d112 <+46>: neg %eax 0x0806d114 <+48>: sbb %eax,%eax 0x0806d116 <+50>: inc %eax 0x0806d117 <+51>: call 0x806d164 <_D3std5stdio14__T7writelnTbZ7writelnFbZv> 0x0806d11c <+56>: call 0x806d0d4 <_D4test8getFloatFNdZf> 0x0806d121 <+61>: fstps -0x8(%ebp) 0x0806d124 <+64>: lea -0x8(%ebp),%esi 0x0806d127 <+67>: call 0x806d0d4 <_D4test8getFloatFNdZf> 0x0806d12c <+72>: fstps -0x4(%ebp) 0x0806d12f <+75>: lea -0x4(%ebp),%edi 0x0806d132 <+78>: mov $0x4,%ecx 0x0806d137 <+83>: xor %eax,%eax 0x0806d139 <+85>: repz cmpsb %es:(%edi),%ds:(%esi) 0x0806d13b <+87>: je 0x806d142 <_Dmain+94> 0x0806d13d <+89>: sbb %eax,%eax 0x0806d13f <+91>: sbb $0xffffffff,%eax 0x0806d142 <+94>: neg %eax 0x0806d144 <+96>: sbb %eax,%eax 0x0806d146 <+98>: inc %eax 0x0806d147 <+99>: call 0x806d164 <_D3std5stdio14__T7writelnTbZ7writelnFbZv> 0x0806d14c <+104>: call 0x806d0d4 <_D4test8getFloatFNdZf> 0x0806d151 <+109>: sub $0x4,%esp 0x0806d154 <+112>: fstps (%esp) 0x0806d157 <+115>: call 0x806d588 <_D3std5stdio14__T7writelnTfZ7writelnFfZv> 0x0806d15c <+120>: xor %eax,%eax 0x0806d15e <+122>: pop %edi 0x0806d15f <+123>: pop %esi 0x0806d160 <+124>: leave 0x0806d161 <+125>: ret
 Also, "is" works like opEqual on built in types, AFAIK, it doesn't use any
 "binary" magic or anything like that.

I don't understand what you are trying to say. Is operator at runtime compares two objects without calling opEquals functions (if applied on user-defined types). For built-in and derived types it is similar to == operator. Although, I am suprised that TDPL and spec doesn't mention it (focused only on CT usage), there is a paragraph (http://ddili.org/ders/d.en/null_is.html) from Turkish D book which clearly shows such usage - so, I think this a valid D feature. Object comparison at low-level (repz cmpsb) means binary comparison.
Oct 01 2012
prev sibling next sibling parent "monarch_dodra" <monarchdodra gmail.com> writes:
On Monday, 1 October 2012 at 13:08:07 UTC, Maxim Fomin wrote:
 2012/10/1 monarch_dodra <monarchdodra gmail.com>:
 On Monday, 1 October 2012 at 11:36:43 UTC, Maxim Fomin wrote:
 On Sunday, 30 September 2012 at 17:07:19 UTC, monarch_dodra 
 wrote:
 As a rule of thumb, NEVER use opEqual with floating point 
 types aniways.
 You need to use some sort of comparison with leway for 
 error, such as
 std.math.approxEqual.

It is possible to compare exactly floating point types by binary comparison, if it provides some benefits. import std.stdio; import std.math; property float getFloat() { return sqrt(1.1); } void main() { writeln(getFloat is getFloat); // doesn't fail }

I think that what you are comparing here is the functions (the address), and not the results of the call. Try writeln(getFloat() is getFloat()); //*May* fail

http://dpaste.dzfl.pl/1f94c0b1 [SNIP]

Hum, yes, I guess I was wrong about the comparison of functions. Sorry!
 Also, "is" works like opEqual on built in types, AFAIK, it 
 doesn't use any
 "binary" magic or anything like that.

I don't understand what you are trying to say. Is operator at runtime compares two objects without calling opEquals functions (if applied on user-defined types). For built-in and derived types it is similar to == operator. Although, I am suprised that TDPL and spec doesn't mention it (focused only on CT usage), there is a paragraph (http://ddili.org/ders/d.en/null_is.html) from Turkish D book which clearly shows such usage - so, I think this a valid D feature. Object comparison at low-level (repz cmpsb) means binary comparison.

What I was saying is that for built in types such a floats, "is" is (should be) no different from "==". But you catch something interesting: the fact that it provides different results is (IMO), a bug. Looking at it, I'd say the bug is probably that "==" is overly sensitive to extended precision. I've filed a BR: http://d.puremagic.com/issues/show_bug.cgi?id=8745 Please feel free to add anything to it. We'll see if Walter will react to it for a more definite answer.
Oct 01 2012
prev sibling parent "Maxim Fomin" <maxim maxim-fomin.ru> writes:
On Monday, 1 October 2012 at 21:23:31 UTC, monarch_dodra wrote:
 What I was saying is that for built in types such a floats, 
 "is" is (should be) no different from "==".

 But you catch something interesting: the fact that it provides 
 different results is (IMO), a bug. Looking at it, I'd say the 
 bug is probably that "==" is overly sensitive to extended 
 precision.

 I've filed a BR:
 http://d.puremagic.com/issues/show_bug.cgi?id=8745

 Please feel free to add anything to it. We'll see if Walter 
 will react to it for a more definite answer.

It looks like dmd uses x87 comparison instructions which are inexact comparing to is. So, similarity of is and == operators on built-in and user-defined types may be subject to float/double/real exception.
Oct 01 2012