[Bug target/99836] New: aarch64: -fpatchable-function-entry=N[,0] should place .cfi_startproc before NOPs

["i at maskray dot me" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99836

            Bug ID: 99836
           Summary: aarch64: -fpatchable-function-entry=N[,0] should place
                    .cfi_startproc before NOPs
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: i at maskray dot me
  Target Milestone: ---

Extracted from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92424#c8

% echo 'int main() {}' > a.c
% clang --target=aarch64 -fpatchable-function-entry=2
-mbranch-protection=standard -S a.c -o -
...
main:                                   // @main
.Lfunc_begin0:
        .cfi_startproc
// %bb.0:                               // %entry
        hint    #34
.Lpatch0:
        nop
        nop

%
/tmp/glibc-many/install/compilers/aarch64-linux-gnu/bin/aarch64-glibc-linux-gnu-g++
-fpatchable-function-entry=2 -mbranch-protection=standard -S a.c -o -
        .arch armv8-a
        .file   "a.c"
        .text
        .align  2
        .global main
        .type   main, %function
main:
        hint    34 // bti c
        .section        __patchable_function_entries,"aw",@progbits
        .align  3
        .8byte  .LPFE1
        .text
.LPFE1:
        nop
        nop
.LFB0:
        .cfi_startproc


For -fpatchable-function-entry=N[,0], placing .cfi_startproc before NOPs makes
more sense and can make unwinding work in that region.

For N[,M] where M>0, that is a very narrow use case by the Linux kernel. I
prefer not to place .cfi_startproc above the function label.