• KlausO <oberhofer users.sourceforge.net> Oct 10 2006
• Lionello Lunesu <lio lunesu.remove.com> Oct 10 2006
• Frank Benoit <keinfarbton nospam.xyz> Oct 11 2006
• Lionello Lunesu <lio lunesu.remove.com> Oct 11 2006
• KlausO <oberhofer users.sf.net> Oct 11 2006

KlausO <oberhofer users.sourceforge.net> writes:

Hello D-experts,

I created a little test program which IMHO shows
not the expected behaviour.
Is this a bug or not ?

module testshift;

import std.stdio;

void main()
{
   uint val_a = (1 << 32);

   uint shift = 32;
   uint val_b = (1 << shift);

   writefln("Result: ", val_a, "  ", val_b);
   assert(val_a == val_b);
}

Output is as follows:
Result: 0  1
Error: AssertError Failure testshift(14)

Lionello Lunesu <lio lunesu.remove.com> writes:

KlausO wrote:
 Hello D-experts,
 
 I created a little test program which IMHO shows
 not the expected behaviour.
 Is this a bug or not ?
 
 module testshift;
 
 import std.stdio;
 
 void main()
 {
   uint val_a = (1 << 32);
 
   uint shift = 32;
   uint val_b = (1 << shift);
 
   writefln("Result: ", val_a, "  ", val_b);
   assert(val_a == val_b);
 }
 
 Output is as follows:
 Result: 0  1
 Error: AssertError Failure testshift(14)



The problem here is: what values would you expect?

1<<32 is too big for an uint, so the 0 from the constant folding is 
correct, BUT the "shl" instructions (the one << translates into) only 
looks at the lower 5 bits, which are 0 (32 & 0x1F == 0), so also the 1 
is correct.

I don't think we'd want extra overhead for something like <<, so I 
suppose we let "<<" behave like the instruction "shl". Which means 
there's only one solution: let the compiler complain.

Come to think of it, it should already have complained: 1<<32 does not 
fit into an uint! But DMD gives no error, not even a warning (-w)...

L.

Frank Benoit <keinfarbton nospam.xyz> writes:

http://www.digitalmars.com/d/expression.html#ShiftExpression
It's illegal to shift by more bits than the size of the quantity being
shifted.

That means, 32 is legal for shifting. And therefore it should evaluate
to the correct result.

So I think the example is a bug.

Lionello Lunesu <lio lunesu.remove.com> writes:

Frank Benoit wrote:
 http://www.digitalmars.com/d/expression.html#ShiftExpression
 It's illegal to shift by more bits than the size of the quantity being
 shifted.
 
 That means, 32 is legal for shifting. And therefore it should evaluate
 to the correct result.
 
 So I think the example is a bug.


I think 32 should be illegal also. It would append 32 0-bits, which 
means the result (uint) will always be 0.

L.

KlausO <oberhofer users.sf.net> writes:

Lionello Lunesu wrote:
 
 I think 32 should be illegal also. It would append 32 0-bits, which 
 means the result (uint) will always be 0.
 


Are there practical uses for shifting more than 32 bits ?

Example: Calculating a mask
   uint mask = (1 << nbits) - 1;

Of course, you could express this also as
   uint mask = (0xffffffff >> (32 - nbits));