Re: Removing undefined behavior of bitshifts
Timon Gehr Wrote: > Timon Gehr wrote: > > On 07/06/2011 00:20, Timon Gehr wrote: > > > >> I'd much prefer the behavior to be defined as 1< >> 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
Re: Removing undefined behavior of bitshifts
Am 09.06.2011 09:08, schrieb Don: s_lange wrote: 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. Sorry, my bad, I should have said that there are only a few RISC-like processors that don't have rotate instructions: MIPS, SPARC... Although you are right that many accumulator CPUs and µC do have rotate instructions: 8051, 6502, Z80 and PIC
Re: Removing undefined behavior of bitshifts
Timon Gehr wrote: > On 07/06/2011 00:20, Timon Gehr wrote: > >> I'd much prefer the behavior to be defined as 1<> 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
Re: Removing undefined behavior of bitshifts
On 07/06/2011 00:20, Timon Gehr wrote: I'd much prefer the behavior to be defined as 1< Defining the behaviour to match that of one brand of processor would be arbitrary and confusing. Why not define it just to shift by the requested number of bits? 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.
Re: Removing undefined behavior of bitshifts
s_lange wrote: Am 07.06.2011 01:20, schrieb Timon Gehr: I'd much prefer the behavior to be defined as 1<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)) 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<= 0x20 ? 0 : 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.
Re: Removing undefined behavior of bitshifts
Am 07.06.2011 01:20, schrieb Timon Gehr: I'd much prefer the behavior to be defined as 1< 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)) 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<= 0x20 ? 0 : 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.
Re: Removing undefined behavior of bitshifts
Vladimir Panteleev wrote:
> On Tue, 07 Jun 2011 02:20:17 +0300, Timon Gehr wrote:
>
>> I'd much prefer the behavior to be defined as 1<> 1<<(0x1f&x); (That's what D effectively does during runtime. It is also
>> what the machine code supports, at least in x87).
>
> Can you think of any cases where this overflow behavior would be expected
> and useful? D can't (cheaply) catch runtime instance of this, but at
> compile-time it should definitely be an error.
>
> --
> Best regards,
> Vladimir
My point is not that this behavior is expected and useful. It is what
effectively
happens. And I agree that it might be a good idea to disallow it during compile
time. But the behavior during runtime should be _defined_! Saying "it's an
error,
but it is not catched" is saying "the behavior is undefined." An optimizing
compiler could use this to ruin the memory safety of SafeD:
int[32] x;
@safe int foo(int shamt){
int a=1<31 or
shamt<0, the compiler can remove the bounds check on the array access. It cannot
screw anything up by doing that. If shamt is not in range, the whole program is
meaningless anyways. The compiler could as well make the program format your
hard
drive whenever an illegal shift amount occurs. ;)
This is a hole in the language specification and needs to be fixed. It is not
about adding useful features that people want to use. It is about removing
undefined behavior while keeping the full performance. (of the shift operation)
BTW: Java handles it the same:
http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.19
Timon
Re: Removing undefined behavior of bitshifts
On Tue, 07 Jun 2011 02:20:17 +0300, Timon Gehr wrote: Currently, the behavior of a shift by more than the size in bytes of the operand is undefined. (Well, it's an 'error', but unchecked.) Also see http://d.puremagic.com/issues/show_bug.cgi?id=4887 -- Best regards, Vladimirmailto:vladi...@thecybershadow.net
Re: Removing undefined behavior of bitshifts
On Tue, 07 Jun 2011 02:20:17 +0300, Timon Gehr wrote: I'd much prefer the behavior to be defined as 1<1<<(0x1f&x); (That's what D effectively does during runtime. It is also what the machine code supports, at least in x87). Can you think of any cases where this overflow behavior would be expected and useful? D can't (cheaply) catch runtime instance of this, but at compile-time it should definitely be an error. -- Best regards, Vladimirmailto:vladi...@thecybershadow.net
Removing undefined behavior of bitshifts
Currently, the behavior of a shift by more than the size in bytes of the operand is undefined. (Well, it's an 'error', but unchecked.) int x=32; x=1<
