OpenEuphoria

1. Phix: peek8u deprecated.

• Posted by petelomax Dec 26, 2016
• 1770 views

I have just spent a couple of days chasing down an intermittent bug, only to finally realise that 80-bit floats can store all signed 64-bit integers, but not unsigned.
The fly in the ointment, as I now understand it, is that there is an implicit 1 in 16 and 32 bit floats which is explicit in 80-bit - thanks for that should go to IEEE 754.
The main change is that C_PTR and friends have become signed on 64-bit Phix, mainly to cope with GetStockObject yielding handles such as FFFF_FFFF_B130_104F.
Given that peek8s can cope, I have somewhat reluctantly decided to deprecate peek8u (or at least add dire warnings to the docs).
I would love to be proved wrong on this, but I somehow doubt that will happen.

Pete

3. Re: Phix: peek8u deprecated.

• Posted by ChrisB (moderator) Dec 28, 2016
• 1683 views

This must be the Pokemon Deprecation then.

Chris

5. Re: Phix: peek8u deprecated.

• Posted by ghaberek (admin) Dec 29, 2016
• 1615 views

"PEEK EIGHT YOU" sounds like Pikachu.

-Greg

7. Re: Phix: peek8u deprecated.

• Posted by petelomax Jan 01, 2017
• 1532 views

Oh, ffs - I read something on the internet and it was WRONG!
I knew there was a problem with unsigned 64 bit ints in 80-bit floats, so just accepted his word (/diagram) as the gospel truth.
(Figure 2.3 shows a literal 1, which I thought was odd, but y'know the whole "well, that explains it" thing.. It ain't a fixed 1.)
To my shame, I already knew there was a problem getting them in, what I missed was the (simple) problem in getting them out.

There may be a better way (suggestions welcome) but this will load an unsigned int (already in rax):

    -- to load unsigned, right shift rax by 1, save odd bit in rcx, then *2+[0|1] 
    xor rcx,rcx 
    shr rax,1 
    rcl rcx,1 
    push rax 
    push rcx 
    fild qword[rsp] 
    fild qword[rsp+8] 
    add rsp,16 
    fadd st0,st0        -- *2 
    faddp               -- +0|1 

Extracting an unsigned int (from an 80-bit float already in st0) is not really any better or worse:

    -- if uint>#7FFF... then uint-=#1_0000... 
    push r15            -- #4000_0000_0000_0000 
    fild qword[rsp] 
    fadd st0,st0        -- #8000_0000_0000_0000 
    fld st1 
    fcomp 
    fnstsw ax 
    sahf 
    jb @f 
        fadd st0,st0    -- #1_0000_0000_0000_0000 
        fsub st1,st0 
  @@: 
    fstp st0            -- discard 
    fistp qword[rsp] 
    pop rax 

^{(Obviously, do a mov r15,h4 first when unsure, but that will be unneccessary more often than not.)}
_{[Some further care may also be needed when dealing with already signed values, and it may be easier in hll]}

So that just needs applying in ten dozen or so places throughout the backend, and peek8u can live on

Pete