[Bug middle-end/93195] -fpatchable-function-entries : __patchable_function_entries should consider comdat groups

[i at maskray dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93195

--- Comment #1 from Fangrui Song <i at maskray dot me> ---
This is similar to --gc-sections
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93536) but a bit different.

The only reasonable fix I can think of is to place __patchable_function_entries
in the same section group.
The ELF spec says:

> A symbol table entry with STB_LOCAL binding that is defined relative to one of a group's sections, and that is contained in a symbol table section that is not part of the group, must be discarded if the group members are discarded. References to this symbol table entry from outside the group are not allowed.

Both GCC and clang reference a .L local symbol in __patchable_function_entries.
The __patchable_function_entries must be discarded when the associated text
section is discarded.
We don't want __patchable_function_entries.foo __patchable_function_entries.bar
because that can waste lots of bytes in .shstrtab .


clang -fpatchable-function-entry=2 -S a.cc b.cc

# COMDAT and SHF_LINK_ORDER are used at the same time
        .section       
__patchable_function_entries,"awo",@progbits,_Z3barv,unique,0
        .p2align        3
        .quad   .Lfunc_begin0

        .section       
__patchable_function_entries,"aGwo",@progbits,_Z3foov,comdat,_Z3foov,unique,1
        .p2align        3
        .quad   .Lfunc_begin1

Because GNU as and ld don't have the features yet. So when -no-integrated-as is
specified (the output is expected to be consumable by GNU as)

clang -fpatchable-function-entry=2 -no-integrated-as -S a.cc b.cc

## The assembler will combine sections with the same name
## If either .Lfunc_begin0 or .Lfunc_begin1 is discarded, the linker will
report an error.
        .section        __patchable_function_entries,"aw",@progbits
        .p2align        3
        .quad   .Lfunc_begin0

        .section        __patchable_function_entries,"aw",@progbits
        .p2align        3
        .quad   .Lfunc_begin1