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digitalmars.D.learn - Bit operator conversions

reply Kagamin <spam here.lot> writes:
Is it valid for this to compile:
---
ushort a(ushort b) pure nothrow
	{ return b<<10|b; }
---

And for this to not compile:
---
ushort a(ushort b) pure nothrow
	{ return b<<10; }
---
?
Apr 06 2009
next sibling parent reply Sergey Gromov <snake.scaly gmail.com> writes:
Mon, 06 Apr 2009 08:53:41 -0400, Kagamin wrote:


 Is it valid for this to compile:
 ---
 ushort a(ushort b) pure nothrow
 	{ return b<<10|b; }
 ---
 
 And for this to not compile:
 ---
 ushort a(ushort b) pure nothrow
 	{ return b<<10; }
 ---
 ?


They both compile with 2.026.  What's your problem?
Apr 07 2009
parent reply Kagamin <spam here.lot> writes:
Sergey Gromov Wrote:


 Mon, 06 Apr 2009 08:53:41 -0400, Kagamin wrote:
 

 Is it valid for this to compile:
 ---
 ushort a(ushort b) pure nothrow
 	{ return b<<10|b; }
 ---
 
 And for this to not compile:
 ---
 ushort a(ushort b) pure nothrow
 	{ return b<<10; }
 ---
 ?


 
 They both compile with 2.026.  What's your problem?


Could you please check bug http://d.puremagic.com/issues/show_bug.cgi?id=2809
with your dmd 2.026? It can be regression.
Apr 08 2009
parent Sergey Gromov <snake.scaly gmail.com> writes:
Wed, 08 Apr 2009 12:06:48 -0400, Kagamin wrote:


 Sergey Gromov Wrote:
 

 Mon, 06 Apr 2009 08:53:41 -0400, Kagamin wrote:
 

 Is it valid for this to compile:
 ---
 ushort a(ushort b) pure nothrow
 	{ return b<<10|b; }
 ---
 
 And for this to not compile:
 ---
 ushort a(ushort b) pure nothrow
 	{ return b<<10; }
 ---
 ?


 
 They both compile with 2.026.  What's your problem?


 
 Could you please check bug http://d.puremagic.com/issues/show_bug.cgi?id=2809
 with your dmd 2.026? It can be regression.




Actually I didn't realize your example was meant to be compiled with -w
switch.  Without -w it simply compiles, no problem.  With -w, it fails
while I think it shouldn't.
Apr 08 2009
prev sibling parent reply Jarrett Billingsley <jarrett.billingsley gmail.com> writes:
On Mon, Apr 6, 2009 at 8:53 AM, Kagamin <spam here.lot> wrote:

 Is it valid for this to compile:
 ---
 ushort a(ushort b) pure nothrow
 =A0 =A0 =A0 =A0{ return b<<10|b; }
 ---

 And for this to not compile:
 ---
 ushort a(ushort b) pure nothrow
 =A0 =A0 =A0 =A0{ return b<<10; }
 ---
 ?


There was a terribly long conversation about this and other operations here=
:

http://d.puremagic.com/issues/show_bug.cgi?id=3D1977

Basically the idea behind the warning on the left-shift is that you
can't know, at compile-time, whether the shift will overflow the size
of ushort or not.  If you passed in 0xFFFF, for instance, it would
overflow.  So it converts left-shifts to int and complains if you
don't have an explicit cast.

But for many bitwise operators, such as | and &, there is no risk of
an overflow at runtime, so if your function returned "b & 0x3F00", you
wouldn't get such a warning.

That it accepts "b << 10 | b" but rejects "b << 10", however, looks
more like a bug.  It's like the compiler isn't doing enough work to
find out whether the former can overflow or not.
Apr 08 2009
parent Don <nospam nospam.com> writes:
Jarrett Billingsley wrote:

 On Mon, Apr 6, 2009 at 8:53 AM, Kagamin <spam here.lot> wrote:

 Is it valid for this to compile:
 ---
 ushort a(ushort b) pure nothrow
        { return b<<10|b; }
 ---

 And for this to not compile:
 ---
 ushort a(ushort b) pure nothrow
        { return b<<10; }
 ---
 ?


 
 There was a terribly long conversation about this and other operations here:
 
 http://d.puremagic.com/issues/show_bug.cgi?id=1977
 
 Basically the idea behind the warning on the left-shift is that you
 can't know, at compile-time, whether the shift will overflow the size
 of ushort or not.  If you passed in 0xFFFF, for instance, it would
 overflow.  So it converts left-shifts to int and complains if you
 don't have an explicit cast.
 
 But for many bitwise operators, such as | and &, there is no risk of
 an overflow at runtime, so if your function returned "b & 0x3F00", you
 wouldn't get such a warning.
 
 That it accepts "b << 10 | b" but rejects "b << 10", however, looks
 more like a bug.  It's like the compiler isn't doing enough work to
 find out whether the former can overflow or not.


cast.c line 50.

		/* This is really only a semi-kludge fix,
		 * we really should look at the operands of op
		 * and see if they are narrower types.
		 * For example, b=b|b and b=b|7 and s=b+b should be allowed,
		 * but b=b|i should be an error.
		 */

Sure, put it in bugzilla.
Apr 09 2009