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digitalmars.D.bugs - [Issue 4046] New: [CTFE] std.intrinsic

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=4046

           Summary: [CTFE] std.intrinsic
           Product: D
           Version: future
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc



import std.intrinsic: bt;
int foo() {
    uint x = uint.max;
    return bt(&x, 5);
}
int _ = foo();
void main() {}


dmd 2.042 gives:

test.d(4): Error: cannot evaluate bt((& x),5u) at compile time
test.d(6): Error: cannot evaluate foo() at compile time
test.d(6): Error: cannot evaluate foo() at compile time


In CTFE the the compiler can replace the intrinsics with little functions with
the same semantics.

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Apr 02 2010
next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=4046




I'm not sure why the btXX functions (bt, btc, btr, etc) exist at all.
Although they are a single instruction, they are MUCH slower than the
equivalent code using shifts or AND/OR/XOR.
For example, on Core i7 (Sandy Bridge), with a memory operand, they take 6
clock cycles!!!! You can execute 24 integer instructions in that time. On AMD
K10, they're even slower. On Pentium 4 they have a latency of EIGHTEEN clock
cycles. They're even slow on VIA processors as well -- they're not good
anywhere.

I think they should be completely removed. There's a case for the intrinsics
mentioned in bug 5703, but I think this should be a WONTFIX. To support them
would just encourage slow, non-portable code.

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Sep 22 2011
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=4046


Dmitry Olshansky <dmitry.olsh gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dmitry.olsh gmail.com



10:42:25 PDT ---


 I'm not sure why the btXX functions (bt, btc, btr, etc) exist at all.
 Although they are a single instruction, they are MUCH slower than the
 equivalent code using shifts or AND/OR/XOR.
 For example, on Core i7 (Sandy Bridge), with a memory operand, they take 6
 clock cycles!!!! You can execute 24 integer instructions in that time. On AMD
 K10, they're even slower. On Pentium 4 they have a latency of EIGHTEEN clock
 cycles. They're even slow on VIA processors as well -- they're not good
 anywhere.


Damn, and I used them at heart of important loops in FReD ....
Thanks, I'm getting rid of them ASAP %)
This should be probably mentioned somewhere, and then there are these
problematic bsr/bsf you mentioned before.


 
 I think they should be completely removed. There's a case for the intrinsics
 mentioned in bug 5703, but I think this should be a WONTFIX. To support them
 would just encourage slow, non-portable code.


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Sep 22 2011
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=4046


bearophile_hugs eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |WONTFIX






 I think they should be completely removed. There's a case for the intrinsics
 mentioned in bug 5703, but I think this should be a WONTFIX. To support them
 would just encourage slow, non-portable code.


OK, then I close this
But I suggest you to open an "enhancement" request that asks to deprecate the
useless/bad intrinsics :-)

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Sep 24 2011