Removing undefined behavior of bitshifts

[Don]

s_lange wrote:
> Am 07.06.2011 01:20, schrieb Timon Gehr:
>> 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).
> Well, you probably mean what x86 processors do.
> 
> The behaviour of x86 processors in this regard is (at leas) well 
> defined, but not uniform. It all depends whether you use general-purpose 
> integer instructions or some 64, 128, or 256-bit SIMD instructions (MMX, 
> SSE, AVX).
> 
> Int denotes 32-bit integers, operations on differently sized integer are 
> analoguous.
> 
> For general-purpose integer instructions, the following identities hold 
> true:
> (l<<x == l<<(x & 0x1F)) and (l>>x == l>>(x & 0x1F)) for any (signed | 
> unsigned) int l and x , regardless whether arithmetical or logical right 
> shifts are used.
> 
> However, for SIMD integer instructions, things are a little different 
> and the following identities hold true:
> (l<<x == (unsigned int)x >= 0x20 ? 0 : l<<x) for any (signed | unsigned) 
> int l and x ,
> (l>>x == (unsigned int)x >= 0x20 ? 0 : l>>x) for any (signed | unsigned) 
> int x and any unsigned int l (using logical right shifts)
> (l>>x == (unsigned int)x >= 0x20 ? -1 : l>>x) for any (signed | 
> unsigned) int x and any signed int l (using arithmetic right shifts)
> 
> As of yet, there are only general-pupose integer rotate instructions on 
> x86 processors, and very few other CPUs and µCs actually implement 
> rotate instructions.

Really? Itanium, PowerPC, ARM, 6502, Z80, PIC all have rotate 
instructions. I've never used a processor that didn't.