[SOLVED] c to pascal translation (binary operation)

[Phoenix]

hi I would like to translate this piece of code into pascal

Code: C  [Select][+][-]

1. typedef unsigned long long  uint64;
2. #define CACHE_L1_SELECTION_MASK(_bitCount) (~((~(uint64)0) >> (_bitCount)))

but using the following code i get different values ​​and i dont know how to get the correct value (it's probably the binary not)

Code: Pascal  [Select][+][-]

1.  function CACHE_L1_SELECTION_MASK(_bitCount: QWord): NativeInt;
2.  begin
3.   Result:= not ((not QWord(0)) shr _bitCount);
4.  end;

I used lazarus 64bit and Code::Blocks

Code: C  [Select][+][-]

1. printf("CACHE_L1_SELECTION_MASK %d \n", CACHE_L1_SELECTION_MASK(17888));  //-1
2. printf("CACHE_L1_SELECTION_MASK %d \n", CACHE_L1_SELECTION_MASK(255));    //-1
3. printf("CACHE_L1_SELECTION_MASK %d \n", CACHE_L1_SELECTION_MASK(0));      //0
4. printf("CACHE_L1_SELECTION_MASK %d \n", CACHE_L1_SELECTION_MASK(1));      //0
5. printf("CACHE_L1_SELECTION_MASK %d \n", CACHE_L1_SELECTION_MASK(48));     //-65536
6. printf("CACHE_L1_SELECTION_MASK %d \n", CACHE_L1_SELECTION_MASK(128));    //-1
7.  

Code: Pascal  [Select][+][-]

1. ShowMessage(
2.    IntToStr(CACHE_L1_SELECTION_MASK(17888))+LineEnding+  //-4294967296
3.    IntToStr(CACHE_L1_SELECTION_MASK(255))+LineEnding+    //-2
4.    IntToStr(CACHE_L1_SELECTION_MASK(0))+LineEnding+      //0
5.    IntToStr(CACHE_L1_SELECTION_MASK(1))+LineEnding+      //-9223372036...
6.    IntToStr(CACHE_L1_SELECTION_MASK(48))+LineEnding+     //-65536
7.    IntToStr(CACHE_L1_SELECTION_MASK(128))+LineEnding);   //0

[tetrastes]

According to C standard, the behaviour of shift for 17888, 255 and 128 is undefined, and compilers give warnings about that. The behaviour of fpc in these cases is unknown. There was a discussion about that: https://forum.lazarus.freepascal.org/index.php/topic,40340.0.html. In any case you cannot rely on those results.

As for results for 0, 1 and 48, note that in your C code you use %d format, i.e. results are converted to int. If you change your pascal function to return integer longint (edited for accuracy), results for 64-bit program are:

Code: Pascal  [Select][+][-]

1.    IntToStr(CACHE_L1_SELECTION_MASK(17888))+LineEnding+  //0 - do not trust!
2.    IntToStr(CACHE_L1_SELECTION_MASK(255))+LineEnding+    //-2 - do not trust!
3.    IntToStr(CACHE_L1_SELECTION_MASK(0))+LineEnding+      //0
4.    IntToStr(CACHE_L1_SELECTION_MASK(1))+LineEnding+      //0
5.    IntToStr(CACHE_L1_SELECTION_MASK(48))+LineEnding+     //-65536
6.    IntToStr(CACHE_L1_SELECTION_MASK(128))+LineEnding);   //0 - do not trust!

what corresponds to your C results for those arguments, which we can trust.

And if you change format to %lld in C code, you'll get for 64-bit program

Code: C  [Select][+][-]

1. printf("CACHE_L1_SELECTION_MASK %lld \n", CACHE_L1_SELECTION_MASK(17888));  //-1 - undefined!
2. printf("CACHE_L1_SELECTION_MASK %lld \n", CACHE_L1_SELECTION_MASK(255));    //-1 - undefined!
3. printf("CACHE_L1_SELECTION_MASK %lld \n", CACHE_L1_SELECTION_MASK(0));      //0
4. printf("CACHE_L1_SELECTION_MASK %lld \n", CACHE_L1_SELECTION_MASK(1));      //-9223372036854775808
5. printf("CACHE_L1_SELECTION_MASK %lld \n", CACHE_L1_SELECTION_MASK(48));     //-65536
6. printf("CACHE_L1_SELECTION_MASK %lld \n", CACHE_L1_SELECTION_MASK(128));    //-1 - undefined!

what corresponds to your pascal results for those arguments, for which the behaviour is defined.

[jamie]

Try this

Code: Pascal  [Select][+][-]

1. function CACHE_L1_SELECTION_MASK(_bitCount:Integer):Integer;
2.  
3. begin
4.   if _BitCount > 63 Then Exit(-1);  {it may need to be 64?}
5.  Result:= not (UInt64(Not 0) shr _BitCount);
6. end;                                                              
7.  
8.  

There are two things wrong with the original code.
First of., Normal Integers should be assumed here since the C code didn't specify the return type. This corrects a lot.

Also you can't shift more bits than you have so I assume the C compiler is generating a -1 for a default value.

I ran the numbers you gave, and it seems to be ok.

[Phoenix]

Thank you both! 

[tetrastes]

First of., Normal Integers should be assumed here since the C code didn't specify the return type.

No, the type is explicitly defined in macro, otherwise the output of printf for %d and %lld would be identical.

MSC warns about that:

warning C4477: 'printf' : format string '%d' requires an argument of type 'int', but variadic argument 1 has type 'unsigned __int64'
note: consider using '%lld' in the format string

though gcc doesn't.

Also the results of msc in undefined cases are the same as of fpc (not -1, as of gcc).