llvm dev - Feb 2006 - [LLVMdev] Using llvm-gcc with a simple program and the '-c' option

Hello,

When I compile a "hello.c" program with a printf in "main"
and use
llvm-gcc with a "-c" option:

    llvm-gcc -c t1.c -o t1.bc

and then try to compile t1.bc to native using llc & gcc I get a call to 
"__main" which is undefined.

If I don't use the "-c" option:

    llvm-gcc t1.c -o t1

I don't get a reference to "__main" and I can compile to native
without
problem. Also, I created some simple modules without a "main" and
using
the "-c" option and that works as expected. What am I doing wrong?

Thanks,

Wink Saville

The -c option tells llvm-gcc to build a bytecode file without linking 
in the LLVM runtime library.  This is similar to the -c option for 
regular gcc, which you use to build multiple separate .o files that 
you're going to link into a single executable.  If you want to build 
from a single source file, it's easiest just to compile without the -c 
option.  If you're building from multiple source files, you can either 
give them all to llvm-gcc at the same time with no -c option

	llvm-gcc foo.c bar.c -o foobar

or compile separately with -c and then link them together

	llvm-gcc -c foo.c -o foo.bc
	llvm-gcc -c bar.c -o bar.bc
	llvm-gcc foo.bc bar.bc -o foobar

You can also use llvm-ld for the last step.

Rob

On Sunday, February 26, 2006, at 10:02 PM, Wink Saville wrote:
> Hello,
>
> When I compile a "hello.c" program with a printf in
"main" and use
> llvm-gcc with a "-c" option:
>
>    llvm-gcc -c t1.c -o t1.bc
>
> and then try to compile t1.bc to native using llc & gcc I get a call 
> to "__main" which is undefined.
>
> If I don't use the "-c" option:
>
>    llvm-gcc t1.c -o t1
>
> I don't get a reference to "__main" and I can compile to
native
> without problem. Also, I created some simple modules without a
"main"
> and using the "-c" option and that works as expected. What am I
doing
> wrong?
>
> Thanks,
>
> Wink Saville
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
Robert L. Bocchino Jr.
1950 South Orchard St., Apt. A
Urbana, IL  61801
(217) 979-1053

Robert,

Thanks for the info, you've confirmed what I was trying to do, but when 
I compile:

-----------------------
#include <stdio.h>

int main(int argc, char *argv[])
{
    printf("yo\n");
    return 0;
}
-----------------------

without "-c" (llvm-gcc t1.c -o t1) the dissassembled bytecode does not
call __main:

-----------------------
; ModuleID = '<stdin>'
target endian = little
target pointersize = 32
target triple = "i686-pc-linux-gnu"
deplibs = [ "c", "crtend" ]
    %struct..TorRec = type { int, void ()* }
    %struct.TorRec = type { int, void ()* }
%.str_1 = internal constant [4 x sbyte] c"yo\0A\00"        ; <[4 x 
sbyte]*> [#uses=1]

implementation   ; Functions:

declare int %printf(sbyte*, ...)

int %main(int %argc, sbyte** %argv) {
entry:
    %tmp.0 = tail call int (sbyte*, ...)* %printf( sbyte* getelementptr 
([4 x sbyte]* %.str_1, int 0, int 0) )        ; <int> [#uses=0]
    ret int 0
}
-----------------------

But if I use the "-c" option (llvm-gcc -c t1.c -o t1.bc) the bytecode
is:

-----------------------
; ModuleID = '<stdin>'
target endian = little
target pointersize = 32
target triple = "i686-pc-linux-gnu"
deplibs = [ "c", "crtend" ]
%.str_1 = internal constant [4 x sbyte] c"yo\0A\00"        ; <[4 x 
sbyte]*> [#uses=1]

implementation   ; Functions:

declare int %printf(sbyte*, ...)

int %main(int %argc, sbyte** %argv) {
entry:
    tail call void %__main( )
    %tmp.0 = tail call int (sbyte*, ...)* %printf( sbyte* getelementptr 
([4 x sbyte]* %.str_1, int 0, int 0) )        ; <int> [#uses=0]
    ret int 0
}

declare void %__main()
--------------------------

Note the call to %__main.

Wink