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digitalmars.D - asm code and an inout function argument

reply Vladimir A. Reznichenko <kalessil gmail.com> writes:
I have a function:

void test (inout uint a)
{
	asm
	{
		mov a, 0x25;
	}
}

The trouble is that the function's call doesn't change the a variable.
Any ideas?
May 15 2009
next sibling parent reply Robert Fraser <fraserofthenight gmail.com> writes:
Vladimir A. Reznichenko wrote:

 I have a function:
 
 void test (inout uint a)
 {
 	asm
 	{
 		mov a, 0x25;
 	}
 }
 
 The trouble is that the function's call doesn't change the a variable.
 Any ideas?



Inout variables are pointers.
May 15 2009
parent reply "Tim Matthews" <tim.matthews7 gmail.com> writes:
On Fri, 15 May 2009 22:29:07 +1200, Robert Fraser  
<fraserofthenight gmail.com> wrote:


 Vladimir A. Reznichenko wrote:

 I have a function:
  void test (inout uint a)
 {
 	asm
 	{
 		mov a, 0x25;
 	}
 }
  The trouble is that the function's call doesn't change the a variable.
 Any ideas?



 Inout variables are pointers.




Why is it 'inout' and not 'ref' ?
May 15 2009
parent Vladimir A. Reznichenko <kalessil gmail.com> writes:
Tim Matthews Wrote:


 On Fri, 15 May 2009 22:29:07 +1200, Robert Fraser  
 <fraserofthenight gmail.com> wrote:
 

 Vladimir A. Reznichenko wrote:

 I have a function:
  void test (inout uint a)
 {
 	asm
 	{
 		mov a, 0x25;
 	}
 }
  The trouble is that the function's call doesn't change the a variable.
 Any ideas?



 Inout variables are pointers.



 
 Why is it 'inout' and not 'ref' ?



Aren't they the same?
May 15 2009
prev sibling next sibling parent reply "Denis Koroskin" <2korden gmail.com> writes:
On Fri, 15 May 2009 14:24:16 +0400, Vladimir A. Reznichenko
<kalessil gmail.com> wrote:


 I have a function:

 void test (inout uint a)
 {
 	asm
 	{
 		mov a, 0x25;
 	}
 }

 The trouble is that the function's call doesn't change the a variable.
 Any ideas?



I believe your code is incorrect. This is how it should be done:

import std.stdio;

void test (out uint a)
{
    asm
    {
        mov EDX, a;
        mov [EDX], 0x25;
    }
}

void main()
{
    uint a = 0;
    test(a);
    
    writefln("0x%x", a);
}

Perhaps, errors like yours could be flagged at compile time? If so, an
enhancement request would be nice.
May 15 2009
parent reply Vladimir A. Reznichenko <kalessil gmail.com> writes:
Denis Koroskin Wrote:


 On Fri, 15 May 2009 14:24:16 +0400, Vladimir A. Reznichenko
<kalessil gmail.com> wrote:
 

 I have a function:

 void test (inout uint a)
 {
 	asm
 	{
 		mov a, 0x25;
 	}
 }

 The trouble is that the function's call doesn't change the a variable.
 Any ideas?



 I believe your code is incorrect. This is how it should be done:
 
 import std.stdio;
 
 void test (out uint a)
 {
     asm
     {
         mov EDX, a;
         mov [EDX], 0x25;
     }
 }
 
 void main()
 {
     uint a = 0;
     test(a);
     
     writefln("0x%x", a);
 }
 
 Perhaps, errors like yours could be flagged at compile time? If so, an
enhancement request would be nice.




Thank you, Denis.
May 15 2009
parent Vladimir A. Reznichenko <kalessil gmail.com> writes:
Denis Koroskin Wrote:


 On Fri, 15 May 2009 14:41:35 +0400, Vladimir A. Reznichenko
<kalessil gmail.com> wrote:
 

 Denis Koroskin Wrote:


 On Fri, 15 May 2009 14:24:16 +0400, Vladimir A. Reznichenko  
 <kalessil gmail.com> wrote:


 I have a function:

 void test (inout uint a)
 {
 	asm
 	{
 		mov a, 0x25;
 	}
 }

 The trouble is that the function's call doesn't change the a variable.
 Any ideas?



 I believe your code is incorrect. This is how it should be done:

 import std.stdio;

 void test (out uint a)
 {
     asm
     {
         mov EDX, a;
         mov [EDX], 0x25;
     }
 }

 void main()
 {
     uint a = 0;
     test(a);

     writefln("0x%x", a);
 }

 Perhaps, errors like yours could be flagged at compile time? If so, an  
 enhancement request would be nice.




 Thank you, Denis.



 You are wellcome.
 
 But I stand corrected - your original code was correct, it just didn't do what
you expected (I replaced inout with pointer for clarity):
 
 void test (uint* a)
 {
     writefln("0x%x", a); // prints 0x12FE88, may differ
     asm {
         mov a, 0x25;
     }
     writefln("0x%x", a); // prints 0x25, i.e. you were modifying 'a', not '*a'
 }
 
 The following would be correct, but it is disallowed and silently ignored:
 void test (uint* a)
 {
     asm {
         mov [a], 0x25; // no warning is issued, works as if there were no
brackets around a, is that correct behavior?
     }
 }



It looks like "inout/ref uint a" is equal to "uint* a" but the situation when
we write D's code "a = 5" means "*a = 5". This is not obvious, at all. So when
I wrote asm code, it wouldn't work.

Interesting implementation of inout arguments )
What's more interesting is that it wasn't reflected in inline asm documentation.
May 15 2009
prev sibling next sibling parent "Denis Koroskin" <2korden gmail.com> writes:
On Fri, 15 May 2009 14:41:35 +0400, Vladimir A. Reznichenko
<kalessil gmail.com> wrote:


 Denis Koroskin Wrote:


 On Fri, 15 May 2009 14:24:16 +0400, Vladimir A. Reznichenko  
 <kalessil gmail.com> wrote:


 I have a function:

 void test (inout uint a)
 {
 	asm
 	{
 		mov a, 0x25;
 	}
 }

 The trouble is that the function's call doesn't change the a variable.
 Any ideas?



 I believe your code is incorrect. This is how it should be done:

 import std.stdio;

 void test (out uint a)
 {
     asm
     {
         mov EDX, a;
         mov [EDX], 0x25;
     }
 }

 void main()
 {
     uint a = 0;
     test(a);

     writefln("0x%x", a);
 }

 Perhaps, errors like yours could be flagged at compile time? If so, an  
 enhancement request would be nice.




 Thank you, Denis.



You are wellcome.

But I stand corrected - your original code was correct, it just didn't do what
you expected (I replaced inout with pointer for clarity):

void test (uint* a)
{
    writefln("0x%x", a); // prints 0x12FE88, may differ
    asm {
        mov a, 0x25;
    }
    writefln("0x%x", a); // prints 0x25, i.e. you were modifying 'a', not '*a'
}

The following would be correct, but it is disallowed and silently ignored:
void test (uint* a)
{
    asm {
        mov [a], 0x25; // no warning is issued, works as if there were no
brackets around a, is that correct behavior?
    }
}
May 15 2009
prev sibling next sibling parent Jesse Phillips <jessekphillips gmail.com> writes:
On Fri, 15 May 2009 07:14:50 -0400, Vladimir A. Reznichenko wrote:


 It looks like "inout/ref uint a" is equal to "uint* a" but the situation
 when we write D's code "a = 5" means "*a = 5". This is not obvious, at
 all. So when I wrote asm code, it wouldn't work.



Isn't that the point of a reference, that you don't have to dereference 
it? In fact I believe "*a = 5" would be an error when using references.
May 15 2009
prev sibling parent "Denis Koroskin" <2korden gmail.com> writes:
On Fri, 15 May 2009 19:11:55 +0400, Jesse Phillips <jessekphillips gmail.com>
wrote:


 On Fri, 15 May 2009 07:14:50 -0400, Vladimir A. Reznichenko wrote:


 It looks like "inout/ref uint a" is equal to "uint* a" but the situation
 when we write D's code "a = 5" means "*a = 5". This is not obvious, at
 all. So when I wrote asm code, it wouldn't work.



 Isn't that the point of a reference, that you don't have to dereference
 it? In fact I believe "*a = 5" would be an error when using references.



Isn't it an error right now?
May 15 2009