• pmoore <pmoore_member pathlink.com> May 20 2006
• Thomas Kuehne <thomas-dloop kuehne.cn> May 21 2006
• Thomas Kuehne <thomas-dloop kuehne.cn> May 21 2006
• pmoore <pmoore_member pathlink.com> May 22 2006

pmoore <pmoore_member pathlink.com> writes:

Hi,

It seems that within inline assembly I can read a class variable but I can't set
it. Is this a bug or is there a good reason for this?

uint myvar;

void main()
{
}

class AClass
{
uint myvar2;

this()
{
asm
{
mov EAX,myvar;
mov myvar,EAX;  // this is fine

mov EAX,myvar2; // no problem reading
mov myvar2,EAX;	// doesn't compile - bad type/size of operands 'mov'
}
}
}

Thomas Kuehne <thomas-dloop kuehne.cn> writes:

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Hash: SHA1

pmoore schrieb am 2006-05-20:
 Hi,

 It seems that within inline assembly I can read a class variable but I can't
set
 it. Is this a bug or is there a good reason for this?

 uint myvar;

 void main()
 {
 }

 class AClass
 {
 uint myvar2;

 this()
 {
 asm
 {
 mov EAX,myvar;
 mov myvar,EAX;  // this is fine

 mov EAX,myvar2; // no problem reading
 mov myvar2,EAX;	// doesn't compile - bad type/size of operands 'mov'
 }
 }
 }


mov EAX,myvar2; seems to move the offset of myvar2 - not the value:

# import std.stdio;
# 
# class C{
#    int i;
# 
#    void foo(){
#       int x;
#       
#       asm{
#          mov EAX, i;
#          mov x, EAX;
#       }
# 
#       writefln("%s at 0x%X -> 0x%X, %s", i, &i, x, x);
#       writefln("offset of i: %s", i.offsetof);
#    }
# }
# 
# int main(){
#    C c = new C();
#    c.foo();
# 
#    return 0;
# }

What you are looking for is:
#       void* p = this;
# 
#       asm{
#          mov EBX, p;
#          mov EAX, [EBX+myvar2];
#          // something productive here
#          mov [EBX+myvar2], EBX;
#       }

Thomas


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Thomas Kuehne <thomas-dloop kuehne.cn> writes:

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Thomas Kuehne schrieb am 2006-05-21:
 pmoore schrieb am 2006-05-20:
 Hi,

 It seems that within inline assembly I can read a class variable but I can't
set
 it. Is this a bug or is there a good reason for this?




 What you are looking for is:
 #       void* p = this;
 # 
 #       asm{
 #          mov EBX, p;
 #          mov EAX, [EBX+myvar2];
 #          // something productive here
 #          mov [EBX+myvar2], EBX;
 #       }


The last line should have been:

mov [EBX+myvar2], EAX;

Thomas


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pmoore <pmoore_member pathlink.com> writes:

Thomas,

You are right. Now that I think about it again it would be impossible for the
compiler to know at runtime what the address of a class local variable would be
so it makes sense to be an offset.

Thanks for taking the time to point this out.


In article <1qv6k3-m9f.ln1 birke.kuehne.cn>, Thomas Kuehne says...
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Thomas Kuehne schrieb am 2006-05-21:
 pmoore schrieb am 2006-05-20:
 Hi,

 It seems that within inline assembly I can read a class variable but I can't
set
 it. Is this a bug or is there a good reason for this?




 What you are looking for is:
 #       void* p = this;
 # 
 #       asm{
 #          mov EBX, p;
 #          mov EAX, [EBX+myvar2];
 #          // something productive here
 #          mov [EBX+myvar2], EBX;
 #       }


The last line should have been:

mov [EBX+myvar2], EAX;

Thomas


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