[Bug libc/29749] New: clarify usage of .cfi_label in riscv start.S

[Bug libc/29749] New: clarify usage of .cfi_label in riscv start.S

[alex.fan.q at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29749

            Bug ID: 29749
           Summary: clarify usage of .cfi_label in riscv start.S
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: alex.fan.q at gmail dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

riscv's start.S includes a `.cfi_label dummy` directive in [1]. The comment
above it says 
> use a dummy .cfi_label to force starting the FDE.`
but as I understand it, `ENTRY (ENTRY_POINT)` macro includes a
`.cfi_startproc`, which should already do the same thing as comment suggests.

This directive was introduced in
https://sourceware.org/bugzilla/show_bug.cgi?id=23125 , but the original patch
in the first comment didn't include cfi_label and works as reported.

Zig compilation gets stuck on this directive as clang doesn't support it [2].

Can I get some help to clarify this directive and whether it is okay to remove
it?

[1]
https://github.com/bminor/glibc/blob/8c77e26ba8b360c851b2b9485bb4431aacc51ad1/sysdeps/riscv/start.S#L48
[2] https://github.com/ziglang/zig/issues/3340

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[Bug libc/29749] clarify usage of .cfi_label in riscv start.S

[alex.fan.q at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29749

--- Comment #1 from Alex Fan <alex.fan.q at gmail dot com> ---
btw, this issue originally opened at
https://github.com/riscv-collab/riscv-gnu-toolchain/issues/1149 and I was
suggested to report it here.

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[Bug libc/29749] clarify usage of .cfi_label in riscv start.S

[alex.fan.q at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29749

Alex Fan <alex.fan.q at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |schwab@linux-m68k.org

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[Bug libc/29749] clarify usage of .cfi_label in riscv start.S

[alex.fan.q at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29749

Alex Fan <alex.fan.q at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |joseph at codesourcery dot com

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[Bug libc/29749] clarify usage of .cfi_label in riscv start.S

[schwab@linux-m68k.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=29749

--- Comment #2 from Andreas Schwab <schwab@linux-m68k.org> ---
.cfi_startproc does *not* start the FDE.  Without .cfi_label, the
DW_CFA_undefined directive is part of the CIE.

$ readelf -wf csu/start.o
Contents of the .eh_frame section:


00000000 0000000000000014 00000000 CIE
  Version:               3
  Augmentation:          "zR"
  Code alignment factor: 1
  Data alignment factor: -4
  Return address column: 1
  Augmentation data:     1b
  DW_CFA_def_cfa_register: r2 (sp)
  DW_CFA_undefined: r1 (ra)
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop

00000018 0000000000000014 0000001c FDE cie=00000000
pc=0000000000000002..000000000000002c
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop

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[Bug libc/29749] clarify usage of .cfi_label in riscv start.S

[alex.fan.q at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29749

--- Comment #3 from Alex Fan <alex.fan.q at gmail dot com> ---
(In reply to Andreas Schwab from comment #2)
> .cfi_startproc does *not* start the FDE.  Without .cfi_label, the
> DW_CFA_undefined directive is part of the CIE.

I find llvm assembler puts it into fde without needing .cfi_label. Is
cfi_startproc meant to mark start of a function frame and fde is per function?

rv64d-dev0 /tmp # llvm-mc --filetype=obj -o=test.o test.s 
rv64d-dev0 /tmp # objdump --dwarf test.o

test.o:     file format elf64-littleriscv

Contents of the .eh_frame section:


00000000 0000000000000010 00000000 CIE
  Version:               1
  Augmentation:          "zR"
  Code alignment factor: 1
  Data alignment factor: -8
  Return address column: 1
  Augmentation data:     1b
  DW_CFA_def_cfa: r2 (sp) ofs 0

00000014 0000000000000010 00000018 FDE cie=00000000
pc=000000000000001c..0000000000000020
  DW_CFA_undefined: r1 (ra)
  DW_CFA_nop

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