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digitalmars.D.bugs - [Issue 5728] New: "rol" in core.bitop

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=5728

           Summary: "rol" in core.bitop
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc


--- Comment #0 from bearophile_hugs eml.cc 2011-03-10 14:10:33 PST ---
I'd like the "rol" X86 instrinsic added to core.bitop module.

--------------------

Rationale:

I'd like the intrinsic because rotating bits in a 32 bit number is a common
enough operation (there was recently a thread in D.learn of a person in need
for the fastest rotation of the bytes of a uint), and in C-derived languages
there isn't a bitwise operator for it, while most CPUs have four or more
instructions to rotate a register.

In C-like languages the problem is sometimes solved making the compiler a bit
smarter, but this little test shows DMD is not able to optimize two shifts plus
or into a single rot instruction:

dmd -O -release -inline test.d


int main(string[] args) {
    uint x = args.length;
    return (x << 24) | (x >> 8);
}


__Dmain comdat
    mov EAX,4[ESP]
    mov ECX,4[ESP]
    shl EAX,018h
    shr ECX,8
    or  EAX,ECX
    ret

--------------------

Using inline asm is an option:

int main(string[] args) {
    uint x = args.length;
    asm {
        rol x, 8;
    }
    return x;
}

__Dmain    comdat
    push    EBP
    mov    EBP,ESP
    push    EAX
    mov    EAX,8[EBP]
    mov    -4[EBP],EAX
    rol    -4[EBP],8
    mov    EAX,-4[EBP]
    mov    ESP,EBP
    pop    EBP
    ret

-----------------

But in some cases the inline gives problems, so I think an inlined intrinsic is
more handy, safer, shorter, simpler to use:

union Four {
    uint u;
    ubyte[4] a;
}
void main() {
    Four f;
    asm {
        rol f.u, 8;
    }
}


test.d(8): bad type/size of operands 'f.u'

-----------------

Note: an alternative to the intrinsic is to introduce in D/DMD a standard
syntax for inlined asm expressions using __asm(), as present in LDC, but they
are quite less easy to use than a rot() function-like intrinsic (but they are
much more useful than a single intrinsic):

http://www.dsource.org/projects/ldc/wiki/InlineAsmExpressions

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--- Comment #1 from bearophile_hugs eml.cc 2011-07-28 05:35:18 PDT ---
Now DMD too is able to figure by itself when a rol is needed, but I think I'd
like the instrinsic still:

https://github.com/D-Programming-Language/dmd/commit/32ea0206dead162f564689339b6c43212042643c

https://github.com/D-Programming-Language/dmd/commit/25df0466ab3ed9d36c354da7576e39f0c64663a5

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Adam D. Ruppe <destructionator gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |destructionator gmail.com


--- Comment #2 from Adam D. Ruppe <destructionator gmail.com> 2011-08-03
19:18:02 PDT ---
I just want to say that dmd commit is fantastic. We got a little embarrassed by
that in a discussion a couple weeks ago!

Re bearophiles request: it probably doesn't need an intrinsic since the
optimzation should let a plain library function do it just as well.

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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |bugzilla digitalmars.com
         Resolution|                            |FIXED


--- Comment #3 from Walter Bright <bugzilla digitalmars.com> 2011-08-03
22:45:51 PDT ---
Adam's right.

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--- Comment #4 from bearophile_hugs eml.cc 2011-08-07 11:23:10 PDT ---
As reference, DMD 2.055head compiles this D2 function:


T rot(T)(T x, int shift) {
    return (x >> shift) | (x << (T.sizeof * 8 - shift));
}
void main() {
  int a = 0b_1111_1111;
  int b = 0b_0000_0010;
  rot(a, b);
}


To (-O -release):

_D4test10__T3rotTiZ3rotFiiZi    comdat
    push    EAX
    mov EAX,8[ESP]
    mov ECX,[ESP]
    sar EAX,CL
    mov ECX,020h
    mov EDX,8[ESP]
    sub ECX,[ESP]
    shl EDX,CL
    or  EAX,EDX
    pop ECX
    ret 4

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--- Comment #5 from Walter Bright <bugzilla digitalmars.com> 2011-08-09
17:34:51 PDT ---
(In reply to comment #4)

 As reference, DMD 2.055head compiles this D2 function:
 T rot(T)(T x, int shift) {
     return (x >> shift) | (x << (T.sizeof * 8 - shift));
 }


The mistake is this is not a correct rotate function. (x >> shift) is a signed
right shift. Rewrite as (x >>> shift).

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--- Comment #6 from bearophile_hugs eml.cc 2011-08-09 18:08:29 PDT ---
(In reply to comment #5)


 The mistake is this is not a correct rotate function. (x >> shift) is a signed
 right shift. Rewrite as (x >>> shift).


Thank you, I'm wrong all the time. Now it shows the ror:


_D4test10__T3rotTiZ3rotFNaNbxixiZi
    push EAX
    mov  EAX,8[ESP]
    mov  ECX,[ESP]
    ror  EAX,CL
    pop  ECX
    ret  4

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