llvm dev - Mar 2008 - [LLVMdev] Wrong calling convention?

When my llvm code calls certain function, either it receives a wrong
result or the application ends on what it seems a corrupt stack.

The function is a C++ static method that returns a class with just a
`double' data member:

class Foo {
  double data;
};

My compiler abstracts this as a [8 x i8] array. This is the llvm code:

define internal i1 @Addr_045442A0() {
	alloca [8 x i8], align 4		; <[8 x i8]*>:1 [#uses=2]
	alloca i1, align 4		; <i1*>:2 [#uses=2]
	tail call void @F95478DA5_FFI_FN( [8 x i8]* %1 sret  )
	bitcast [8 x i8]* %1 to i8*		; <i8*>:3 [#uses=1]
	tail call void @Addr_004DDA18( i8* inttoptr (i32 14545104 to i8*), i8* %3 )
	store i1 true, i1* %2
	load i1* %2		; <i1>:4 [#uses=1]
	ret i1 %4
}

It is a function that just calls the above mentioned C++ static method,
calls another external function which is known to be "safe" (as it
does
nothing and is called thousands of times everywhere without problems)
and returns bool.

This is the generated machine code, obtained with gdb:

0x0e8d9370:     push   %esi
0x0e8d9371:     sub    $0x18,%esp
0x0e8d9374:     lea    0x10(%esp),%esi
0x0e8d9378:     mov    %esi,(%esp)
0x0e8d937b:     call   0x5053b0
<_ZN3lp04OpF0IN5IncDB9TDateTimeEXadL_ZNS2_3NowEvEEE1wEv>
0x0e8d9380:     sub    $0x4,%esp
0x0e8d9383:     mov    %esi,0x4(%esp)
0x0e8d9387:     movl   $0x8def210,(%esp)
0x0e8d938e:     call   0x4b3ca0
<_ZN3lp015BasicDestructorIN5IncDB9TDateTimeEEEvRNS_8TipoInfoEPv>
0x0e8d9393:     movb   $0x1,0x8(%esp)
0x0e8d9398:     movzbl 0x8(%esp),%eax
0x0e8d939d:     add    $0x18,%esp
0x0e8d93a0:     pop    %esi
0x0e8d93a1:     ret

I'm using setCallingConv(CallingConv::C). After hours and hours of
debugging, I'm lost. Do you see something wrong here?

My setup is llvm 2.1 on Windows XPSP2. Everything is compiled with MinGW
g++.exe (GCC) 4.2.1-dw2 (mingw32-2)

-- 
Oscar

Hi,
> define internal i1 @Addr_045442A0() {
> 	alloca [8 x i8], align 4		; <[8 x i8]*>:1 [#uses=2]
> 	alloca i1, align 4		; <i1*>:2 [#uses=2]
> 	tail call void @F95478DA5_FFI_FN( [8 x i8]* %1 sret  )
this call uses the "struct-return" convention (due to the sret
attribute).
On x86 this means that the caller is responsible for adjusting the stack
pointer after the call for the sret parameter.  If the callee is not following
the sret convention then the stack pointer will be adjusted wrongly and your
program will die horribly.
> 0x0e8d937b:     call   0x5053b0
<_ZN3lp04OpF0IN5IncDB9TDateTimeEXadL_ZNS2_3NowEvEEE1wEv>
> 0x0e8d9380:     sub    $0x4,%esp
Here you see the sret stack adjustment.

Ciao,

Duncan.

Hi Duncan.

Duncan Sands <baldrick at free.fr> writes:
>> define internal i1 @Addr_045442A0() {
>> 	alloca [8 x i8], align 4		; <[8 x i8]*>:1 [#uses=2]
>> 	alloca i1, align 4		; <i1*>:2 [#uses=2]
>> 	tail call void @F95478DA5_FFI_FN( [8 x i8]* %1 sret  )
>
> this call uses the "struct-return" convention (due to the sret
attribute).
> On x86 this means that the caller is responsible for adjusting the stack
> pointer after the call for the sret parameter.  If the callee is not
following
> the sret convention then the stack pointer will be adjusted wrongly and
your
> program will die horribly.
>
>> 0x0e8d937b:     call   0x5053b0
<_ZN3lp04OpF0IN5IncDB9TDateTimeEXadL_ZNS2_3NowEvEEE1wEv>
>> 0x0e8d9380:     sub    $0x4,%esp
>
> Here you see the sret stack adjustment.
This looks like the opposite: it is making room for passing a parameter
to the next call (the stack grows downwards).

But you put me on the right track. The problem is that the class is
returned on the stack.

This is the class:

class Foo {
public:
  Foo() : data(113.5) {}
  static Foo GetFoo() { return Foo(); }
private:
  double data;
};

This is the assembler code for Foo::GetFoo:

Dump of assembler code for function _ZN3Foo6GetFooEv:
0x6e08b5a4 <_ZN3Foo6GetFooEv+0>:        push   %ebp
0x6e08b5a5 <_ZN3Foo6GetFooEv+1>:        mov    %esp,%ebp
0x6e08b5a7 <_ZN3Foo6GetFooEv+3>:        sub    $0x14,%esp
0x6e08b5aa <_ZN3Foo6GetFooEv+6>:        lea    -0x8(%ebp),%eax
0x6e08b5ad <_ZN3Foo6GetFooEv+9>:        mov    %eax,(%esp)
0x6e08b5b0 <_ZN3Foo6GetFooEv+12>:       call   0x6e08b5bc <Foo>
0x6e08b5b5 <_ZN3Foo6GetFooEv+17>:       fldl   -0x8(%ebp)
0x6e08b5b8 <_ZN3Foo6GetFooEv+20>:       leave
0x6e08b5b9 <_ZN3Foo6GetFooEv+21>:       ret
End of assembler dump.

I guess that g++ exploits its knowledge of Foo's internals for deciding
how to return the class. But my compiler knows nothing about Foo, so I
must disable this "feature" of g++. Suggestions welcome.

-- 
Oscar