[SOLVED] Integer calc. variation between ASM and C backend

[Oso]

Can anyone help to explain the reason behind this variation between ASM and C please? I'm using PB 6.00, Windows x64. I have been converting some Julian-day-count routines, which were written in C, into PureBasic. When I found that I was unable to obtain the precise result, due to a rounding variation, I decided to try the C backend, without modification to the source. This gave me the precise result I expected from the Julian day conversion formula and I presume the same therefore as the original author's code.

From my perspective, I wouldn't necessarily call it a bug, but it's certainly an inconsistency between the two. Since I'm seeking to obtain the result of the C backend, under ASM, I'd prefer to modify my code so that ASM produces the same result as the C backend — which is 2,477,197. Therefore it might help me first of all, to understand why this is happening. Thanks all.

Code: Select all

EnableExplicit
OpenConsole()

Define JD.d = 2445152.5        ; Julian day count
PrintN("Value = " + JD)

Define a = JD + 32044          ; Returns 2477196 on ASM, 2477197 under C
PrintN("a = " + a)      

Print("Press <ENTER> : ")
Input()

[Little John]

I can't explain why this is happening. However, problems with implicit typecasting have been reported and discussed here repeatedly, see e.g. Different implicit typecasting with different backends.

I strongly recommend not to use implicit typecasting, but to use Int(), IntQ() or Round().

[Oso]

I strongly recommend not to use implicit typecasting, but to use Int(), IntQ() or Round().

Thanks Little John, that answers it for me — now using Round() with #PB_Round_Nearest. Job done

[luis]

The two backends must give the same results, the source code is the same, the language is the same,
If this does not happen it's a bug and the inconsistency should be resolved.

That being said, using PB 6.03 I always get the same result: 2477196

[juergenkulow]

Code: Select all

; cvttsd2si versus fistp
Define a.d, b.q

a = 2451544.5000115740113   ; C                                       ; ASM 401495A162000308B800 2451544.500011574011
b = (a - 2440587.5) * 86400 ; 0 10957.000011574  - 0 946684800.999995 ;     10957.00001157401130 946684800.9999945164
OpenConsole()
Print(Str( b))              ; 00000000386D4380       946684800        ;                          946684801
Input()

; C Backend 6.10 B1 
; 0000000140001067               | F2:0F1005 E1DE0500          | movsd xmm0,qword ptr ds:[14005EF50]                  |
; 000000014000106F               | F2:0F1105 29D90700          | movsd qword ptr ds:[14007E9A0],xmm0                  |
; 0000000140001077               | F2:0F1005 21D90700          | movsd xmm0,qword ptr ds:[14007E9A0]                  |
; 000000014000107F               | F2:0F1015 D1DE0500          | movsd xmm2,qword ptr ds:[14005EF58]                  |
; 0000000140001087               | 66:0F28C8                   | movapd xmm1,xmm0                                     |
; 000000014000108B               | F2:0F5CCA                   | subsd xmm1,xmm2                                      |
; 000000014000108F               | F2:0F1005 C9DE0500          | movsd xmm0,qword ptr ds:[14005EF60]                  |
; 0000000140001097               | F2:0F59C1                   | mulsd xmm0,xmm1                                      |
; 000000014000109B               | F248:0F2CC0                 | cvttsd2si rax,xmm0                                   | rax:00000000386D4380 xmm0:0 946684800.999995
; 00000001400010A0               | 48:8905 01D90700            | mov qword ptr ds:[14007E9A8],rax                     | 000000014007E9A8  80 43 6D 38 00 00 00 00  
;                                                                                                                                          ^

; Asm Backend 6.03 
; 0000000140001054               | DD05 D63F0000               | fld st(0),qword ptr ds:[140005030]                   |
; 000000014000105A               | DD1D 9C440000               | fstp qword ptr ds:[1400054FC],st(0)                  |
; 0000000140001060               | DD05 96440000               | fld st(0),qword ptr ds:[1400054FC]                   |
; 0000000140001066               | DC05 CC3F0000               | fadd st(0),qword ptr ds:[140005038]                  |
; 000000014000106C               | DC0D CE3F0000               | fmul st(0),qword ptr ds:[140005040]                  | st(0):946684800.9999945164
; 0000000140001072               | DF3D 8C440000               | fistp qword ptr ds:[140005504],st(0)                 | 0000000140005504  81 43 6D 38 00 00 00 00
;                                                                                                                                          ^