floating-WTF - Compiler-BUG with 64bit

[dennis luehring]

Am 24.01.2012 18:49, schrieb sclytrack:

dmd 2.057 give this result under windows/32bit

calculate1:
1.5
1.5
calculate2:
1.5
1.5
calculate3:
1.5
1.5

so its seems to be an x64 compiler bug

>
>
> Yes, I missed my lessons in clear communication.
>
>
> -------------------------------------------------
> +	The complete source files.
> -------------------------------------------------
>
> import std.stdio;
>
> void calculate1(float a, float b)
> {
> 	float s1 = 1.0f - a;
> 	float s2 = a * b;
> 	writeln(s2);
> 	writeln(a * b);
> }
>
> void calculate2(float a, float b)
> {
> 	float s1 = 1.0 - a;
> 	float s2 = a * b;
> 	writeln(s2);
> 	writeln(a * b);
> }
>
> void calculate3(double a, double b)
> {
> 	double s1 = 1.0 - a;
> 	double s2 = a * b;
> 	writeln(s2);
> 	writeln(a * b);	
> }
>
> int main()
> {	
> 	writeln("calculate1:");
> 	calculate1(0.75f,2.0f);
> 	writeln("calculate2:");
> 	calculate2(0.75f,2.0f);
> 	writeln("calculate3:");
> 	calculate3(0.75f,2.0f);
> 	return 0;
> }
>
> -------------------------------------------------
> +	The complete output
> -------------------------------------------------
>
> calculate1:
> 1.5
> 1.5
> calculate2:
> 0
> 1.5
> calculate3:
> 1.5
> 1.5
>
> -------------------------------------------------
> +	Compiler
> -------------------------------------------------
>
> DMD64 D Compiler v2.057
> Copyright (c) 1999-2011 by Digital Mars written by Walter Bright
> Documentation: http://www.digitalmars.com/d/2.0/index.html
> Usage:
>
> This is from the downloaded zip file.
> I'm using eclipse to compile it. Added the -debug.
>
>
> -------------------------------------------------
> +	Ubuntu 11.10 64 bit
> -------------------------------------------------
>
> uname -r
> 3.0.0-15-generic
>
> -------------------------------------------------
> +	BEHAVIOUR
> -------------------------------------------------
>
> As for the behaviour on 32 bit. I wish
> somebody else would do it. :-)
>
> In the previous message here below is the assembly
> output of calculate2. Pay close attention to the
> cvtss2sd, the cvtsd2ss and the mulss.
>
> 	float s1 = 1.0 - a;
> 	float s2 = a * b;
>
> It converts the float a to double precision.
> then performs the s1 calculation.
>
> Then does the s2 calculation with the high
> precision a and the normal float b.
>
> Also it performs the "cvtsd2ss %xmm2,%xmm2"
> for no reason at all. As it is no longer used
> in the rest the code.
>
> -------------------------------------------------
> +	objdump -S test1.o
> -------------------------------------------------
>
> 0000000000000000<_D4main10calculate2FffZv>:
>      0:	55                   	push   %rbp
>      1:	48 8b ec             	mov    %rsp,%rbp
>      4:	48 83 ec 20          	sub    $0x20,%rsp
>      8:	f3 0f 11 45 f0       	movss  %xmm0,-0x10(%rbp)
>      d:	f3 0f 11 4d f8       	movss  %xmm1,-0x8(%rbp)
>     12:	48 b8 00 00 00 00 00 	movabs $0x3ff0000000000000,%rax
>     19:	00 f0 3f
>     1c:	48 89 45 e0          	mov    %rax,-0x20(%rbp)
>     20:	f2 0f 10 55 e0       	movsd  -0x20(%rbp),%xmm2
>     25:	f3 0f 5a c9          	cvtss2sd %xmm1,%xmm1
>     29:	f2 0f 5c d1          	subsd  %xmm1,%xmm2
>     2d:	f2 0f 5a d2          	cvtsd2ss %xmm2,%xmm2
>     31:	f3 0f 59 c8          	mulss  %xmm0,%xmm1
>     35:	f3 0f 11 4d e8       	movss  %xmm1,-0x18(%rbp)
>     3a:	f3 0f 10 45 e8       	movss  -0x18(%rbp),%xmm0
>     3f:	e8 00 00 00 00       	callq  44<_D4main10calculate2FffZv+0x44>
>     44:	f3 0f 10 45 f8       	movss  -0x8(%rbp),%xmm0
>     49:	f3 0f 10 4d f0       	movss  -0x10(%rbp),%xmm1
>     4e:	f3 0f 59 c1          	mulss  %xmm1,%xmm0
>     52:	e8 00 00 00 00       	callq  57<_D4main10calculate2FffZv+0x57>
>     57:	c9                   	leaveq
>     58:	c3                   	retq
>     59:	90                   	nop
>     5a:	90                   	nop
>     5b:	90                   	nop
>
>
> I'm going to eat now.
>
>
>
> On 01/24/2012 05:01 PM, dennis luehring wrote:
>>  can you give us a complete out-of-the-box compileable example (with
>>  imports,main,etc.) with both examples the working/and nonworking
>>
>>  does it work correct with dmd(32bit)?
>>
>>  is this "calculate2" needed to reproduce the behavior?
>>
>>  and what compiler did you test: dmd64, (dmd32?), ...?
>>
>>  Am 24.01.2012 16:20, schrieb sclytrack:
>>>
>>>
>>>  void calculate2(float a, float b)
>>>  {
>>>  float s1 = 1.0 - a;
>>>  float s2 = a * b;
>>>  writeln(s2);
>>>  writeln(a * b);
>>>  }
>>>
>>>  ----------------
>>>  Output:
>>>  t*b= 0
>>>  t*b= 1.5
>>>  ----------------
>>>
>>>
>>>  assume CS:.text._D4main10calculate2FffZv
>>>  _D4main10calculate2FffZv:
>>>  push RBP
>>>  mov RBP,RSP
>>>  sub RSP,020h
>>>  movss -010h[RBP],XMM0
>>>  movss -8[RBP],XMM1
>>>  mov RAX,03FF0000000000000h
>>>  mov -020h[RBP],RAX
>>>  movsd XMM2,-020h[RBP]
>>>  cvtss2sd XMM1,XMM1
>>>  subsd XMM2,XMM1
>>>  ------conversion but xmm2 is no longer used
>>>  cvtsd2ss XMM2,XMM2
>>>  ------precision error multiplication
>>>  mulss XMM1,XMM0
>>>  movss -018h[RBP],XMM1
>>>  movss XMM0,-018h[RBP]
>>>  call _D3std5stdio14__T7writelnTfZ7writelnFfZv at PC32
>>>  movss XMM0,-8[RBP]
>>>  movss XMM1,-010h[RBP]
>>>  mulss XMM0,XMM1
>>>  call _D3std5stdio14__T7writelnTfZ7writelnFfZv at PC32
>>>  leave
>>>  ret
>>>  nop
>>>  nop
>>>  nop
>>>
>>>  ------------
>>>  DMD64 D Compiler v2.057
>>>  Ubuntu 11.10
>>
>