in reply to Re: [Win32, C, and way OT] C floats, doubles, and their equivalence
in thread [Win32, C, and way OT] C floats, doubles, and their equivalence

I think your detailed investigation demonstrates why the code I posted in repsonse to creamygoodness's post behaves the way it does.

After much poking and scratching and re-reading of suggestions that have been kindly and thoughtfully presented here, I've eventually come up with using this approach in the PDL code: 
#include <stdio.h>

#if defined _MSC_VER && _MSC_VER < 1400
#pragma optimize("", off)
#endif 

int main(void) {

#if defined _MSC_VER && _MSC_VER < 1400
   double nv = 2.0 / 3;
   float foo = 2.0 / 3;
   float dummy = (float)nv;

   if(foo == dummy) printf("True ");
   else printf("False ");

#else
   double nv = 2.0 / 3;
   float foo = 2.0 / 3;

   if(foo == (float)nv) printf("True ");
   else printf("False ");

#endif

   return 0;
}

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It seems to be doing the right thing with all of my compilers.
I don't think that C script needs to have the optimization turned off - the creation of the dummy variable is alone sufficient to get the behaviour I'm after. But for some reason, in the PDL code, creation of the dummy variable is *not*, by itself, sufficient - optimization also needs to be disabled. (I turn it off for the setvaltobad functions, then turn it back on again.)

Thanks to *all* who replied.

Cheers,
Rob
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Re^3: [Win32, C, and way OT] C floats, doubles, and their equivalence
by BrowserUk (Patriarch) on Jul 19, 2009 at 07:28 UTC
    But for some reason, in the PDL code, creation of the dummy variable is *not*, by itself, sufficient - optimization also needs to be disabled. (I turn it off for the setvaltobad functions, then turn it back on again.)

    That's why I moved the temp var and comparison into a separate function; it forces the compiler to use the temp value from memory for the comparison: 

    ; 5    :     return s == tmp ? 1 : 0;

    fld    DWORD PTR _s$[ebp]
    fcomp    DWORD PTR _tmp$[ebp]
    fnstsw    ax
    test    ah, 68                    ; 00000044H
    jp    SHORT $L809

    [download]

    The problem with the test script is that with optimisations enabled, the newer compiler is able to reduce the whole script to a simple printf( "False" ); printf( "True" ); return 0; (even with the use of the sub) as everything is known at compile time: 

    ; Listing generated by Microsoft (R) Optimizing Compiler Version 15.00
+.30729.01 

    TITLE    C:\test\float.c
    .686P
    .XMM
    include listing.inc
    .model    flat

INCLUDELIB LIBCMT
INCLUDELIB OLDNAMES

_DATA    SEGMENT
$SG2527    DB    'True ', 00H
    ORG $+2
$SG2529    DB    'False ', 00H
    ORG $+1
$SG2531    DB    'True', 0aH, 00H
    ORG $+2
$SG2533    DB    'False', 0aH, 00H
_DATA    ENDS
PUBLIC    _cmpFsFd
EXTRN    __fltused:DWORD
; Function compile flags: /Ogtpy
; File c:\test\float.c
;    COMDAT _cmpFsFd
_TEXT    SEGMENT
tv135 = 8                        ; size = 4
_s$ = 8                            ; size = 4
_d$ = 12                        ; size = 8
_cmpFsFd PROC                        ; COMDAT

; 4    :     float tmp = (float)d;
; 5    :     return s == tmp ? 1 : 0;

    fld    DWORD PTR _s$[esp-4]
    fld    QWORD PTR _d$[esp-4]
    fstp    DWORD PTR tv135[esp-4]
    fld    DWORD PTR tv135[esp-4]
    fucompp
    fnstsw    ax
    test    ah, 68                    ; 00000044H
    jp    SHORT $LN3@cmpFsFd
    mov    eax, 1

; 6    : }

    ret    0
$LN3@cmpFsFd:

; 4    :     float tmp = (float)d;
; 5    :     return s == tmp ? 1 : 0;

    xor    eax, eax

; 6    : }

    ret    0
_cmpFsFd ENDP
_TEXT    ENDS
PUBLIC    _main
EXTRN    _printf:PROC
; Function compile flags: /Ogtpy
_TEXT    SEGMENT
_main    PROC

; 11   :    double nv = 2.0 / 3;              ##
; 12   :    float foo = 2.0 / 3;              ## All this and ...
; 13   :      
; 14   :    if( foo == nv ) printf("True ");  ##
; 15   :    else printf("False ");

    push    OFFSET $SG2529
    call    _printf

; 16   : 
; 17   :    if( cmpFsFd( foo, nv ) ) printf("True\n");  ## this are op
+timised away!

    push    OFFSET $SG2531
    call    _printf
    add    esp, 8

; 18   :    else printf("False\n");
; 19   : 
; 20   :    return 0;

    xor    eax, eax

; 21   : }

    ret    0
_main    ENDP
_TEXT    ENDS
END

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      That's why I moved the temp var and comparison into a separate function

      Having a separate function is appealing, but not so straightforward to implement. It would be fine if we had an xs file to fiddle with, but the fact that the source file to be amended is a pd file (not an xs file) adds some complexity to the problem. I think it is possible to use the "separate function" approach - though it's actually "separate functions" (plural), as the templating dictates that we'll need separate functions to handle each of the different data types (ie byte, short, long, long long, double - not just float). There's also the issue of the second arg that gets supplied to the "separate function" - it could be a UV or an IV, not necessarily an NV, so we need to accommodate that as well (probably not difficult).

      It's a much simpler solution if one instead just adds a dummy variable and turns off optimization - which seems to work quite well.

      Cheers,
      Rob
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