llvm dev - Jun 2009 - [LLVMdev] Structure Alignment

struct xfirstStru
{
    int                var1;
    double            var2;
    int                var3;
    char    *var4;
};
%FIRSTSTRU_PLUSPLUS_TOTVS. = type { i32, double, i32, i8* }

Hi,

I had not applied the layout at run time, only during the emission of the .bc.
Now it's running ok when I allocate the structure point at LLVM side, but,
when I allocate the structure  pointer in the C side, the LLVM function does not
acquire the double value correctly.
Manoel Teixeira
> Message: 1
> Date: Thu,  4 Jun 2009 10:39:24 -0300
> From: " Manoel Teixeira " <manoel at fonetica.com.br>
> Subject: [LLVMdev] Structure Alignment
> To: llvmdev at cs.uiuc.edu
> Message-ID: <20090604133924.4200.qmail at hm885.locaweb.com.br>
> Content-Type: text/plain; charset=ISO-8859-1
> 
> Hi,
> I have a problem using structs, when calling a function written in C from
> a LLVM function or calling a LLVM function from a C one.
> I noticed that the LLVM align double with 4 bytes and our run time routines
> use 8, in MS Windows.
> If a use #pragma pack(4) in my C programs, the routines run fine, but
> I must ensure that all external call should use the #pragma directive.
> 
> If I change the llvm layout f64:64:64, the LLVM calls C functions and
> the struct is OK, in the C side, but the getelemntptr fails in LLVM side.
> Is there on way to ensure that the LLVM reads the correct memory position
> in this case?
> 
> //#pragma pack(4)
> struct xfirstStru
> {
>     int                var1;
>     double            var2;
>     int                var3;
>     char    *var4;
> };
> typedef struct xfirstStru osX ;
> typedef struct xfirstStru* PosX ;
> //#pragma pack()
> //
> extern "C" int testOSX(    PosX osx    )
> {
>     printf("\nChamada C %d %f %d
%s",++osx->var1,++osx->var2,++osx->var3,osx->var4);
>     return 999;
> }
> 
> it was
> ... f64:32:64 ...
> and I changed it to
> ... f64:64:64 ...
> 
> %struct.VAR1. = getelementptr %FIRSTSTRU.* %OUTRASTRU., i32 0, i32 0       
; <i32*> [#uses=2]    %loadX = load i32* %struct.el.VAR1., align 4       
; <i32> [#uses=1]
> here the first element is OK
> ...
> %struct.el.VAR2. = getelementptr %FIRSTSTRU.* %OUTRASTRU., i32 0, i32 1    
; <i32*> [#uses=2]    %loadX = load double* %struct.el.VAR2., align 8     
; <double> [#uses=1]
> here, the second is wrong
> ...
> 
> 
> Thanks,
> Manoel Teixeira
>

Hello,
> I had not applied the layout at run time, only during the emission of the
.bc.
> Now it's running ok when I allocate the structure point at LLVM side,
but, when
> I allocate the structure  pointer in the C side,
> the LLVM function does not acquire the double value correctly.
LLVM has no idea about any weird requirements of struct fields padding /
alignment. You have to insert necessary padding fields by yourself (as
all frontends do).

-- 
With best regards, Anton Korobeynikov.

Faculty of Mathematics & Mechanics, Saint Petersburg State University.