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

[Jonas Maebe via fpc-pascal]

On 03/09/2021 08:02, LacaK via fpc-pascal wrote:

>> 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)?

If E1 is 32 bits, the result will be 32 bits. From the compiler source code:

{ calculations for ordinals < 32 bit have to be done in
  32 bit for backwards compatibility. That way 'shl 33' is
  the same as 'shl 1'. It's ugly but compatible with delphi/tp/gcc }

Note that the "shl 33' is the same as 'shl 1'" part of the comment
refers to the specific behaviour of the x86 family of processors. That
is not necessarily the case on other systems (e.g. on 32 bit PowerPC,
the result will be 0 as you would normally expect).

> Can we say that in Pascal the result of:
>   E1 shl E2
> is of same type as E1 ?

No. If E1 is smaller than 32 bits, the result will be promoted to
1) on 32/64 bit platforms: 32 bits
2) on 8/16 bit platforms: the smallest integer type that can represent
both E1 and the native integer type of the platform

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

Expressions are always evaluated independently of how their result is
used. So, yes.


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

[Markus Greim via fpc-pascal]

After 35 years of Pascal experience I would urgently recommend NOT to trust any 
automatic type conversion in the case of shift operators. Alteady Turbo Pascal 
failed here on x386 architectures. Force input and output variables to a 
certain data type before you use the shift operator. 


Just my 5 cents 
Markus Greim

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--- original message ---
On September 3, 2021 at 2:36 PM GMT+2 fpc-pascal@lists.freepascal.org wrote:

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 


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