https://issues.dlang.org/show_bug.cgi?id=23618

mhh <maxhaton@gmail.com> changed:

           What    |Removed                     |Added
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           Severity|enhancement                 |blocker

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https://issues.dlang.org/show_bug.cgi?id=23618

Iain Buclaw <ibuclaw@gdcproject.org> changed:

           What    |Removed                     |Added
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           Keywords|                            |backend
                 CC|                            |ibuclaw@gdcproject.org

--- Comment #1 from Iain Buclaw <ibuclaw@gdcproject.org> ---
Only affects dmd.

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https://issues.dlang.org/show_bug.cgi?id=23618

Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
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                 CC|                            |bugzilla@digitalmars.com

--- Comment #2 from Walter Bright <bugzilla@digitalmars.com> ---
The issue comes up because (ee >>= 5) is done as a signed right shift, even though ee is an unsigned type.

Step by step:

https://dlang.org/spec/expression.html#assignment_operator_expressions says:

    a op= b
    are semantically equivalent to:
    a = cast(typeof(a))(a op b)

so, (ee >>= 5) is rewritten to:

    ee = cast(ushort)(ee >> b);

https://dlang.org/spec/expression.html#shift_expressions says:

    the operands undergo integral promotions

    ee = cast(ushort)(cast(int)ee >> b)

or:

    ee = cast(ushort)((ee & 0x0000_FFFF) >> b)

then:

    >> is a signed right shift

but the sign bit is 0, so it is, in effect, an unsigned right shift. The compiler generates a signed right shift, though.

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https://issues.dlang.org/show_bug.cgi?id=23618

Salih Dincer <salihdb@hotmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |salihdb@hotmail.com

--- Comment #3 from Salih Dincer <salihdb@hotmail.com> ---
The problem is also seen when the sign extension is not done.

Here is the test code:

struct TestType(T)
{
  import std.traits : Unsigned;
  alias U = Unsigned!T;

  T t = T.min;       // sample: -128 for byte
  U u = U.max/2 + 1; // sample: 128 for ubyte
}

void main()
{
  alias T = long; // int, short, byte

  TestType!T v1, v2;
  enum bits = T.sizeof * 8 - 1;

  v1.t >>= bits;
  assert(v1.t == -1); // okay, because signed type

  v1.u >>= bits;
  assert(v1.u == 1); // okay, because unsigned type

  v2.t >>>= bits;
  assert(v2.t == 1); /* okay, no sign extension
                        but  -1 for byte, short, int */
  v2.u >>>= bits;
  assert(v2.u == 1); // okay, no sign extension
}

SDB@79

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https://issues.dlang.org/show_bug.cgi?id=23618

Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Right Shift equals          |Right Shift equals
                   |expressions on unsigned     |expressions on unsigned
                   |shorts don't behave the     |shorts should be unsigned
                   |same as regular shifts      |right shift

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https://issues.dlang.org/show_bug.cgi?id=23618

Dlang Bot <dlang-bot@dlang.rocks> changed:

           What    |Removed                     |Added
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           Keywords|                            |pull

--- Comment #4 from Dlang Bot <dlang-bot@dlang.rocks> ---
@WalterBright created dlang/dmd pull request #14814 "fix Issue 23618 - Right Shift equals expressions on unsigned shorts s…" fixing this issue:

- fix Issue 23618 - Right Shift equals expressions on unsigned shorts should be unsigned right shift

https://github.com/dlang/dmd/pull/14814

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https://issues.dlang.org/show_bug.cgi?id=23618

Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Hardware|x86_64                      |All
                 OS|Linux                       |All

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https://issues.dlang.org/show_bug.cgi?id=23618

Dlang Bot <dlang-bot@dlang.rocks> changed:

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

--- Comment #5 from Dlang Bot <dlang-bot@dlang.rocks> ---
dlang/dmd pull request #14814 "fix Issue 23618 - Right Shift equals expressions on unsigned shorts s…" was merged into stable:

- 1a7eebc1a52954a56173f098ee5128bfbfe6b746 by Walter Bright:
  fix Issue 23618 - Right Shift equals expressions on unsigned shorts should be
unsigned right shift

https://github.com/dlang/dmd/pull/14814

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https://issues.dlang.org/show_bug.cgi?id=23618

--- Comment #6 from Dlang Bot <dlang-bot@dlang.rocks> ---
dlang/dmd pull request #14815 "Merge stable" was merged into master:

- 579c97ab66ddc2c6a8f3db177937bb4c94353120 by Walter Bright:
  fix Issue 23618 - Right Shift equals expressions on unsigned shorts should be
unsigned right shift (#14814)

https://github.com/dlang/dmd/pull/14815

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