www.digitalmars.com         C & C++   DMDScript  

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.
Yes the #2809 is present in 2.026. 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