[Bug lto/83967] LTO removes C functions declared as weak in assembler(depending on files order in linking)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83967

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW
                 CC|                            |hjl.tools at gmail dot com

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Rafał Mszal from comment #4)
> Created attachment 43210 [details]
> Basic example of main funtion with LTO failure.
> 
> This is simple example of observed LTO issue. RTC1_IRQHandler is removed
> despite it is used to change volatile variable. RTC1_IRQHandler is defined
> weak in the assembler startup file.

I don't see it being defined in the startup file.  But I take it should be
similar
to

    .weak   NMI_Handler
    .type   NMI_Handler, %function
NMI_Handler:
    b       .
    .size   NMI_Handler, . - NMI_Handler

and the only use is in the startup file as well:

    .section .isr_vector
    .align 2
    .globl __isr_vector
__isr_vector:
    .long   __StackTop                  /* Top of Stack */
    .long   Reset_Handler
    .long   NMI_Handler

and you'd define it in a C file as non-weak (what it does doesn't matter)

void RTC1_IRQHandler(void)
{
}

So a testcase is:

t1.c:
-----

int i;
void Handler()
{
  i = 1;
}
void *Dispatch;
int main(){ ((void (*)(void))Dispatch)(); return i;}

t2.s:
-----

        .file   "t2.s"
        .text
        .weak Handler
        .type   Handler, @function
Handler:
.LFB0:
        .cfi_startproc
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        popq    %rbp
        .cfi_def_cfa 7, 8
        ret
        .cfi_endproc
.LFE0:
        .size   Handler, .-Handler
        .globl  Dispatch
        .data
        .align 8
        .type   Dispatch, @object
        .size   Dispatch, 8
Dispatch:
        .quad   Handler
        .ident  "GCC: (SUSE Linux) 7.2.1 20171020 [gcc-7-branch revision
253932]"
        .section        .note.GNU-stack,"",@progbits

and then I get

> gcc-7 t2.s t1.c -flto
/tmp/ccGhH7Cp.o:(.data+0x0): undefined reference to `Handler'
collect2: error: ld returned 1 exit status
> gcc-7 t1.c t2.s -flto

in the working case:

1
t1.o 4
190 16b2980a71930688 PREEMPTED_REG Handler
198 16b2980a71930688 PREVAILING_DEF_IRONLY i
194 16b2980a71930688 PREVAILING_DEF main
202 16b2980a71930688 RESOLVED_EXEC Dispatch

in the failing case (GNU ld):

1
t1.o 4
190 801577c96f9ccef3 PREVAILING_DEF_IRONLY Handler
198 801577c96f9ccef3 PREVAILING_DEF_IRONLY i
194 801577c96f9ccef3 PREVAILING_DEF main
202 801577c96f9ccef3 RESOLVED_EXEC Dispatch

which works with gold:

1
t1.o 4
190 680bbd652c7cfe04 PREVAILING_DEF Handler
198 680bbd652c7cfe04 PREVAILING_DEF_IRONLY i
194 680bbd652c7cfe04 PREVAILING_DEF main
202 680bbd652c7cfe04 RESOLVED_EXEC Dispatch

HJ?  This is with binutils 2.29.1, thus confirmed but as a GNU ld issue.