Re: [fpc-pascal] Conversion from C to Pascal - Left bit shift

[Jean SUZINEAU via fpc-pascal]

I made a few tests on Ubuntu 64 bits (arch x86_64) with variations on a 
small test program:

var
   E2: Byte= 3;
   E1: LongWord= 1;
   E: QWord;
begin
 E:= (1000*E1) shl E2;
 writeln( 'E2', E2);
 writeln( 'E1', E1);
 writeln( 'E', E);
end.

In the assembly window, shl is computed on 64 bits %rax, I get :

project1.lpr:132 E:= (1000*E1) shl E2;
004011B2 8b05d8f50c00 mov 0xcf5d8(%rip),%eax    
# 0x4d0790 

004011B8 4869c0e803   imul $0x3e8,%rax,%rax
004011BF 0fb60dbaf50c00   movzbl 
0xcf5ba(%rip),%ecx    # 0x4d0780 

004011C6 48d3e0   shl    %cl,%rax
004011C9 48890580b51000   mov %rax,0x10b580(%rip)    
# 0x50c750 


Changing the formula to E:= E1 shl E2, shl computed on 32 bits %edx, (I 
don't understand the "and    %edx,%edx", may be just to clear the carry ?)

I get :

project1.lpr:132 E:= E1 shl E2;
004011B2 0fb605c7f50c00   movzbl 
0xcf5c7(%rip),%eax    # 0x4d0780 
004011B9 8b15d1f50c00 mov 0xcf5d1(%rip),%edx    
# 0x4d0790 

004011BF 89c1 mov    %eax,%ecx
004011C1 d3e2 shl    %cl,%edx
004011C3 21d2 and    %edx,%edx
004011C5 48891584b51000   mov %rdx,0x10b584(%rip)    
# 0x50c750 


Changing  E1 to QWord ( E1: QWord= 1; ), shl is computed on 64 bits 
%rax, I get :


project1.lpr:132 E:= E1 shl E2;
004011B2 0fb605c7f50c00   movzbl 
0xcf5c7(%rip),%eax    # 0x4d0780 
004011B9 488b15d0f50c00   mov 0xcf5d0(%rip),%rdx    
# 0x4d0790 

004011C0 4889c1   mov    %rax,%rcx
004011C3 48d3e2   shl    %cl,%rdx
004011C6 48891583b51000   mov %rdx,0x10b583(%rip)    
# 0x50c750 



___
fpc-pascal maillist  -  fpc-pascal@lists.freepascal.org
https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal

Re: [fpc-pascal] Hello, new Pascal programmer. had a question!

[Peter via fpc-pascal]

Sorry, my bad.

I hadn't got Thunderbird configured right. All mail from this list just showed 
the list address in the 'From' field.
Now fixed it.

 Edit > Preferences > Advanced >
"Show only display name for people in my address book"
which was checked.


Cheers,
Pete
___
fpc-pascal maillist  -  fpc-pascal@lists.freepascal.org
https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal

Re: [fpc-pascal] Conversion from C to Pascal - Left bit shift

[LacaK via fpc-pascal]

Can we say that in Pascal the result of:
    E1 shl E2
is of same type as E1 ?
(so if E1 is LongWord then result is LongWord also?)

What if there is an expression on left side:
    (E1*x) shl E2
Will E1*x promote to 64 bits (on 64 bit target)?


See documentation on automatic type conversion (the remarks section 
below table 3.3): 
https://www.freepascal.org/docs-html/ref/refsu4.html#x26-26004r3 

While this doesn't explicitly mention shift behaviour, it implies that 
E1 and (E1*x) will be promoted to native sized integer if smaller.  For 
the first example, if E1 is a longword on a 32 bit machine, the result 
should also be a longword.




But look at Delphi documentation: 
https://docwiki.embarcadero.com/RADStudio/Sydney/en/Expressions_(Delphi)#Logical_.28Bitwise.29_Operators


"The operations x shl y and x shr y shift the value of x to the left or 
right by y bits, which (if x is an unsigned integer) is equivalent to 
multiplying or dividing x by 2^y; the result is of the same type as x"


My understanding is, that in case:
  E1 shl E2 result is of E1 type (so if E1 is LongWord on 64 bit 
platform result is still LongWord?)


Case
  (E1*x) shl E2 is subject to native integer promotion I guess?

L.
___
fpc-pascal maillist  -  fpc-pascal@lists.freepascal.org
https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal

Re: [fpc-pascal] Conversion from C to Pascal - Left bit shift

[Christo Crause via fpc-pascal]

On Fri, Sep 3, 2021 at 8:02 AM LacaK via fpc-pascal <
fpc-pascal@lists.freepascal.org> wrote:

> Can we say that in Pascal the result of:
>E1 shl E2
> is of same type as E1 ?
> (so if E1 is LongWord then result is LongWord also?)
>
> What if there is an expression on left side:
>(E1*x) shl E2
> Will E1*x promote to 64 bits (on 64 bit target)?
>

See documentation on automatic type conversion (the remarks section below
table 3.3): https://www.freepascal.org/docs-html/ref/refsu4.html#x26-26004r3
While this doesn't explicitly mention shift behaviour, it implies that E1
and (E1*x) will be promoted to native sized integer if smaller.  For the
first example, if E1 is a longword on a 32 bit machine, the result should
also be a longword.

Personal note:  I find the naming of ordinal types confusing.  Is shortint
smaller than smallint, or smallint shorter than shortint? In FPC a word is
2 bytes in size, however on the hardware side a word refers to "the natural
unit of data used by a particular processor design" (
https://en.wikipedia.org/wiki/Word_(computer_architecture)).  I prefer to
use the type aliases with explicit mention of size, such as int16, uint8
etc. I assume the flip side of this is when trying to write portable code
that uses the most efficient size available on a target.
___
fpc-pascal maillist  -  fpc-pascal@lists.freepascal.org
https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal

Re: [fpc-pascal] Conversion from C to Pascal - Left bit shift

[LacaK via fpc-pascal]

Can we say that in Pascal the result of:
  E1 shl E2
is of same type as E1 ?
(so if E1 is LongWord then result is LongWord also?)

What if there is an expression on left side:
  (E1*x) shl E2
Will E1*x promote to 64 bits (on 64 bit target)?

-Laco.


Hello *,

I have code in C like this:
  E1 << E2
If E1 is of unsigned type then "The value of E1 << E2 is E1 
left-shifted E2 bit positions; vacated bits are zero-filled. If E1 has 
an unsigned type, the value of the result is E1 × 2^E2, reduced modulo 
one more than the maximum value representable in the result type."


I understand this as result of such operation does not grow over 32 
bits, right?


In Pascal I have:
  E1 shl E2
where E1 is of LongWord type.

On 64-bit target this result of shl can overcome 32 bits (iow operates 
on 32 bits)?
(does also bit shift operations works on 64 bits or this only affect 
arithmetic operations like multiply?)


Assembler suggests that shl works on 32 bits: shl    %cl,%r11d

-Laco.



___
fpc-pascal maillist  -  fpc-pascal@lists.freepascal.org
https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal