Removing undefined behavior of bitshifts

[Paul D. Anderson]

Timon Gehr Wrote:

> Timon Gehr wrote:
> > On 07/06/2011 00:20, Timon Gehr wrote:
> > <snip>
> >> I'd much prefer the behavior to be defined as 1<<x; being equivalent to
> >> 1<<(0x1f&x); (That's what D effectively does during runtime. It is also what
> >> the machine code supports, at least in x87).
> >
> > Defining the behaviour to match that of one brand of processor would be
> arbitrary and
> > confusing.
> 
> Well, not too much. It is the easiest behavior to implement in hardware.
> I thought that it is the most common behavior on different brands of processors,
> but I might be mistaken.
> Is there any processor that badly needs D support that handles it differently?
> 
> > Why not define it just to shift by the requested number of bits?
> Because it would turn code that is currently
> 
> z = x << y;
> 
> to
> 
> z = y < (8*sizeof(x)) ? x << y : 0;
> 
> On many platforms. Given that you seldom want to shift by a custom amount, and
> that it could eliminate some bugs, it might be a reasonable trade-off.
> 
> 
> >
> > Any extra processor instructions to make it behave correctly for cases where
> this number
> >  >= 32 would be the part of the backend code generation.  And if the right
> operand is a
> > compile-time constant (as it probably is usually), these extra instructions can be
> > eliminated or at least optimised to the particular value.
> >
> >> Are there any practical downsides to making the behavior defined? (Except that
> >> the CTFE Code would have to be fixed). I think Java does it too.
> >
> > Apparently Java shifts are modulo the number of bits in the type of the left
> operand.  Or
> > something like that.  You'd think it was an oversight in the original
> implementation that
> > was kept for bug compatibility, but you could well ask how they dealt with
> finding the
> > behaviour to be machine dependent (contrary to the whole philosophy of Java).
> >
> > Stewart.
> 
> I don't even care so much about what the result is, but I feel that saying "the
> program is in error"/"the behavior is undefined", when actually you'd just get
> back some number is not optimal. (it allows the compiler to do anything if that
> case occurs)
> I would prefer to make the behavior at least implementation-defined (just a formal
> change on the D website) or even defined with some runtime-overhead.
> 
> 
> Timon

FWIW, the General Decimal Arithmetic specification (which I'm getting close to implementing in D: it's always just one week away!) specifies that a shift or rotate by more than the current precision is disallowed (returns a NaN).

I suppose the clarity on this point is possible because the context, including the precision, is part of the specification, and the means to indicate failure is also defined.

Unfortunately no such specification exists for integers (at least none that doesn't start religious wars).

The clarity is offset, however, by  the requirement to shift by decimal digits, not bits. Just a heads up that this won't be in the first release.

Paul