[Bug target/90826] New: Weak symbol does not work reliably on windows

[yyc1992 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90826

            Bug ID: 90826
           Summary: Weak symbol does not work reliably on windows
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yyc1992 at gmail dot com
  Target Milestone: ---

The following code does not link correctly with all optimization levels on
windows with the mingw-w64-x86_64-g++ compiler.

```
#include <stdint.h>

extern "C" void f() __attribute__((weak));

int main()
{
    return (int)(uintptr_t)f;
}

```

The assembly generated is

```
        .file   "weak.cpp"
        .text
        .def    __main; .scl    2;      .type   32;     .endef
        .section        .text.startup,"x"
        .p2align 4
        .globl  main
        .def    main;   .scl    2;      .type   32;     .endef
        .seh_proc       main
main:
.LFB1:
        subq    $56, %rsp
        .seh_stackalloc 56
        .seh_endprologue
        call    __main
        movq    .refptr.f(%rip), %rax
        movq    %rax, 40(%rsp)
        addq    $56, %rsp
        ret
        .seh_endproc
        .ident  "GCC: (Rev2, Built by MSYS2 project) 9.1.0"
        .section        .rdata$.refptr.f, "dr"
        .globl  .refptr.f
        .linkonce       discard
.refptr.f:
        .quad   f

```

and the error,

```
C:\msys64\tmp\ccQkPfOi.o:weak.cpp:(.rdata$.refptr.f[.refptr.f]+0x0): undefined
reference to `f'
```

This should not happen since `f` is declared weak. (I realized that the symbol
resolution happens at compile time for weak symbol, which is fine for me, but I
just want it to compile...)

Another case where the optimization actually makes this work is,

```
#include <stdio.h>

extern "C" void f() __attribute__((weak));

int main()
{
    printf("%p\n", f);
    return 0;
}
```

With -O0, the assembly generated is

```
        .file   "weak.cpp"
        .text
        .def    __main; .scl    2;      .type   32;     .endef
        .section .rdata,"dr"
.LC0:
        .ascii "%p\12\0"
        .text
        .globl  main
        .def    main;   .scl    2;      .type   32;     .endef
        .seh_proc       main
main:
.LFB28:
        pushq   %rbp
        .seh_pushreg    %rbp
        movq    %rsp, %rbp
        .seh_setframe   %rbp, 0
        subq    $32, %rsp
        .seh_stackalloc 32
        .seh_endprologue
        call    __main
        movq    .refptr.f(%rip), %rdx
        leaq    .LC0(%rip), %rcx
        call    printf
        movl    $0, %eax
        addq    $32, %rsp
        popq    %rbp
        ret
        .seh_endproc
        .ident  "GCC: (Rev2, Built by MSYS2 project) 9.1.0"
        .def    printf; .scl    2;      .type   32;     .endef
        .section        .rdata$.refptr.f, "dr"
        .globl  .refptr.f
        .linkonce       discard
.refptr.f:
        .quad   f
```

with error,

```
C:\msys64\tmp\ccTiwMKh.o:weak.cpp:(.rdata$.refptr.f[.refptr.f]+0x0): undefined
reference to `f'
```

with -O1 or higher, the assembly produced is,
```
        .file   "weak.cpp"
        .text
        .def    __main; .scl    2;      .type   32;     .endef
        .section .rdata,"dr"
.LC0:
        .ascii "%p\12\0"
        .text
        .globl  main
        .def    main;   .scl    2;      .type   32;     .endef
        .seh_proc       main
main:
.LFB30:
        subq    $40, %rsp
        .seh_stackalloc 40
        .seh_endprologue
        call    __main
        leaq    f(%rip), %rdx
        leaq    .LC0(%rip), %rcx
        call    printf
        movl    $0, %eax
        addq    $40, %rsp
        ret
        .seh_endproc
        .weak   f
        .ident  "GCC: (Rev2, Built by MSYS2 project) 9.1.0"
        .def    f;      .scl    2;      .type   32;     .endef
        .def    printf; .scl    2;      .type   32;     .endef
        .section        .rdata$.refptr.f, "dr"
        .globl  .refptr.f
        .linkonce       discard
.refptr.f:
        .quad   f
```


The difference between the two assembly is

```
--- weak1.s     2019-06-10 19:42:27.039467600 -0400
+++ weak0.s     2019-06-10 19:42:23.709467500 -0400
@@ -9,21 +9,24 @@
        .def    main;   .scl    2;      .type   32;     .endef
        .seh_proc       main
 main:
-.LFB30:
-       subq    $40, %rsp
-       .seh_stackalloc 40
+.LFB28:
+       pushq   %rbp
+       .seh_pushreg    %rbp
+       movq    %rsp, %rbp
+       .seh_setframe   %rbp, 0
+       subq    $32, %rsp
+       .seh_stackalloc 32
        .seh_endprologue
        call    __main
-       leaq    f(%rip), %rdx
+       movq    .refptr.f(%rip), %rdx
        leaq    .LC0(%rip), %rcx
        call    printf
        movl    $0, %eax
-       addq    $40, %rsp
+       addq    $32, %rsp
+       popq    %rbp
        ret
        .seh_endproc
-       .weak   f
        .ident  "GCC: (Rev2, Built by MSYS2 project) 9.1.0"
-       .def    f;      .scl    2;      .type   32;     .endef
        .def    printf; .scl    2;      .type   32;     .endef
        .section        .rdata$.refptr.f, "dr"
        .globl  .refptr.f
```

where weak1.s (-) is the working one and weak0.s (+) is the error one.

The missing `.weak f` seems to be the issue. With that added to `weak0.s` or
removed from `weak1.s` the compilation result changes to that of the other
version. So it seems that without optimization on gcc just forgot to put in the
weak attribute into the assembly file??