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Date: Sat, 03 Feb 2007 23:33:00 +1300
From: Danny Smith <dannysmith AT clear DOT net DOT nz>
Subject: dllimport and __asm__("_alias" )
To: Cygwin <cygwin AT cygwin DOT com>, mingw-users AT lists DOT sourceforge DOT net
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Hello, 

I would appreciate help with 

While working on some dllimport problems with aliased symbols, I ran
across this inconsistency with gcc (all versions)

The use of a literal asmspec to rename a static lib symbol
(say "bar"),  

extern void foo (void) __asm__("_bar");
void call_bar(void)
{ foo(); }

does the right thing.  The literal is treated as a verbatim asm name,
so explicitly adding the '_' USER_LABEL_PREFIX to "bar" is necesssry:

_call_bar:
	pushl	%ebp
	movl	%esp, %ebp
	popl	%ebp
	jmp	_bar 


However,if we try to alias a dllimport'd symbol,
like so

extern __declspec(dllimport) void foo (void) __asm__("_bar");
void call_bar(void)
{ foo(); }

The literal is not treated as verbatim, but _imp_ prefix as well as
additional USER_LABEL_PREFIX to bar is generated

_call_bar:
	pushl	%ebp
	movl	__imp___bar, %ecx  <<< 3 underscores betweem imp and bar
	movl	%esp, %ebp
	popl	%ebp
	jmp	*%ecx

The above will lead to link error if the user name for the exported
symbol is "bar".


My feeling is that to avoid confusion, an asmspec'd alias should
really,really always be verbatim, no adding import prefix, stdcall
suffix, nor C++mangling, nothing.  That is the way gcc works on other
targets.
That is, to do the right thing, we would need to:

extern __declspec(dllimport) void foo (void) __asm__("__imp__bar");
void call_bar(void)
{ foo(); }

to get
_call_bar:
	pushl	%ebp
	movl	__imp__bar, %ecx 
	movl	%esp, %ebp
	popl	%ebp
	jmp	*%ecx

Any comments, before I submit the patch to gcc?

Should gcc warn if the asmspec of a dllimport alias lacks the "__imp__".

Danny


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