[llvm-dev] storage class 0 symbol is generated for a double constant in COFF-x86-64 object file

[Reid Kleckner via llvm-dev]

I can't speak for what past versions of LLVM used to do, but I can see that
today for this IR, we generate an external symbol __real at 40...:

declare dllimport void @my_function(double, double)
define i32 @main() {
  call void @my_function(double 2.000000e+00, double 2.000000e+00)
  ret i32 0
}

$ dumpbin -symbols t.o
...
006 00000000 SECT4  notype       Static       | .rdata
    Section length    8, #relocs    0, #linenums    0, checksum 76DC4190,
selection    2 (pick any)
008 00000000 SECT4  notype       External     | __real at 4000000000000000
...

If you can provide complete IR on which llc produces a 0 storage class,
that would be interesting, but otherwise there's not much we can do.

On Fri, Jun 28, 2019 at 5:59 PM Shiyao Ge via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> I am using the llvm codegen facility (version 7.0.1) to translate LLVM IR
> for different platforms. I have this error particularly in win64 platform.
> In my IR code I have such code snippet:
>
> %50 = call i8* @my_function(i8* %48, double 2.000000e+00, double
> 2.000000e+00)
> ...
> declare dllimport i8* @my_function(i8*, double, double)
>
> By passing it to llc.exe, I find following symbol is declared in object
> file's symbol table.
>
> [10](sec  5)(fl 0x00)(ty   0)(scl   0) (nx 0) 0x00000000
> __real at 4000000000000000
>
> which has storage class 0, and it will lead to an error when I try to link
> it with the rest of objects.
>
> obj1.obj: Unrecognized storage class 0 for .rdata symbol
> `__real at 4000000000000000'.
>
>
> As comparison, I compile an equivalent c file (to my llvm asm file) using
> clang-7.0 and emit the llvm from there. For this particular line, I see no
> difference in terms of syntax, except clang generates bunch of function
> attributes to the function declaration. I pass the clang generated llvm asm
> to llc and find the output object file also has one symbol has the same
> name, but the storage class is set to 2.
>
> [10](sec  5)(fl 0x00)(ty   0)(scl   2) (nx 0) 0x00000000
> __real at 4000000000000000
>
> That will probably resolve the linkage error issue. But I don't understand
> what's the key factor to determine the storage class and what change should
> I put into my generated IR.
>
> Moreover, I looked up the microsoft doc about storage class. The storage
> class 2 is described as "A value that Microsoft tools use for external
> symbols. " It doesn't sound like a symbol represents a floating point
> constant at all.
>
> Due to my limited knowledge of PE COFF and llvm codegen for windows
> platform,  I am totally confused here. Any suggestion and hint are
> desperately welcomed!
> --
> Shiyao Ge (Gracia)
>
>
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