-Wl,-gc-sections linker switch is never included

[mgrojo]

Checklist

• I've searched for similar issues and found none.
• I've included the output of alr version.
• I've included complete steps to reproduce my issue.

Describe the bug
Alire includes these compiler switches in all its build profiles:

           ,"-ffunction-sections" -- Separate ELF section for each function
           ,"-fdata-sections" -- Separate ELF section for each variable

But it doesn't include -gc-sections to the linker in any of the profiles. Can that be considered a bug, or is there a reason for that? As far as I know, these options only make sense when the linker performs garbage collection. The GCC documentation seems to suggest that you should use these options with care, and the GNAT documentation only mentions the compiler switches in combination to the linker switches.

To Reproduce
Steps to reproduce the behavior:

1. alr init gc_sections
2. alr build -- -v
3. The output does contain -ffunction-sections -fdata-sections in the compilation command, but not -Wl,-gc-sections in the linker command:

$ alr build -- -v
Warning: Ignoring XDG_SESSION_TYPE=wayland on Gnome. Use QT_QPA_PLATFORM=wayland to run on Wayland anyway.
ⓘ Building gc_sections=0.1.0-dev/gc_sections.gpr...
object directory "/home/mgr/tmp/gc_sections/obj/development" created for project Gc_Sections
exec directory "/home/mgr/tmp/gc_sections/bin" created for project Gc_Sections
Changing to object directory of "Gc_Sections": "/home/mgr/tmp/gc_sections/obj/development/"
/home/mgr/.local/share/alire/toolchains/gnat_native_14.2.1_06bb3def/bin/gcc -c -x ada -gnatA -Og -ffunction-sections -fdata-sections -g -gnatwa -gnatw.X -gnatVa -gnaty3 -gnatya -gnatyA -gnatyB -gnatyb -gnatyc -gnaty-d -gnatye -gnatyf -gnatyh -gnatyi -gnatyI -gnatyk -gnatyl -gnatym -gnatyn -gnatyO -gnatyp -gnatyr -gnatyS -gnatyt -gnatyu -gnatyx -gnatW8 -gnatec=/tmp/GNAT-TEMP-000003.TMP -gnatem=/tmp/GNAT-TEMP-000004.TMP /home/mgr/tmp/gc_sections/src/gc_sections.adb
/home/mgr/.local/share/alire/toolchains/gprbuild_22.0.1_24dfc1b5/libexec/gprbuild/gprbind gc_sections.bexch
/home/mgr/.local/share/alire/toolchains/gnat_native_14.2.1_06bb3def/bin/gnatbind -o b__gc_sections.adb /home/mgr/tmp/gc_sections/obj/development/gc_sections.ali -Es -x -F=/tmp/GNAT-TEMP-000005.TMP -O=/tmp/GNAT-TEMP-000007.TMP
/home/mgr/.local/share/alire/toolchains/gnat_native_14.2.1_06bb3def/bin/gcc -c -x ada -gnatA -c -gnatA -gnatWb -gnatiw -gnatws -Og -ffunction-sections -fdata-sections -g -mtune=generic -march=x86-64 b__gc_sections.adb -o b__gc_sections.o
gcc gc_sections.o b__gc_sections.o -L/home/mgr/tmp/gc_sections/obj/development/ -L/home/mgr/tmp/gc_sections/obj/development/ -L/home/mgr/.local/share/alire/toolchains/gnat_native_14.2.1_06bb3def/lib/gcc/x86_64-pc-linux-gnu/14.2.0/adalib/ -static-libgcc /home/mgr/.local/share/alire/toolchains/gnat_native_14.2.1_06bb3def/lib/gcc/x86_64-pc-linux-gnu/14.2.0/adalib/libgnat.a -ldl -Wl,-rpath-link,/home/mgr/.local/share/alire/toolchains/gnat_native_14.2.1_06bb3def/lib/gcc/x86_64-pc-linux-gnu/14.2.0//adalib -Wl,-z,origin,-rpath,$ORIGIN/..//obj/development:$ORIGIN/../../..//.local/share/alire/toolchains/gnat_native_14.2.1_06bb3def/lib/gcc/x86_64-pc-linux-gnu/14.2.0/adalib -o /home/mgr/tmp/gc_sections/bin//gc_sections
✓ Build finished successfully in 0.57 seconds.

Expected Behavior
-Wl,--gc-sections is added to the default generated gpr file for binary crates, so that the other switches make something useful. Maybe could be done only in the release profile, but then, why to include the compilation switches in the other profiles?

alr version

Warning: Ignoring XDG_SESSION_TYPE=wayland on Gnome. Use QT_QPA_PLATFORM=wayland to run on Wayland anyway.
APPLICATION
alr version:               2.0.1
libalire version:          2.0.1
compilation date:          2024-03-21 11:05:29
compiled with version:     10.5.0

CONFIGURATION
settings folder:           /home/mgr/.config/alire
cache folder:              /home/mgr/.local/share/alire
vault folder:              /home/mgr/.local/share/alire/releases
build folder:              /home/mgr/.local/share/alire/builds
temp folder:               /run/user/1000
force flag:                FALSE
non-interactive flag:      FALSE
community index branch:    stable-1.3.0
compatible index versions: ^1.1 & <=1.3.0
indexes folder:            /home/mgr/.config/alire/indexes
indexes metadata:          OK
index #1:                  (community) git+https://github.com/alire-project/alire-index#stable-1.3.0
toolchain folder:          /home/mgr/.local/share/alire/toolchains
toolchain assistant:       disabled
tool #1 gnat:              gnat_native=14.2.1
tool #2 gprbuild:          gprbuild=22.0.1
system package manager:    /usr/bin/apt
distro detection disabled: FALSE

WORKSPACE
root status:               VALID
root release:              gc_sections=0.1.0-dev
root load error:           none
root folder:               /home/mgr/tmp/gc_sections
current folder:            /home/mgr/tmp/gc_sections

SYSTEM
distribution:              UBUNTU
host-arch:                 X86_64
os:                        LINUX
target:                    NATIVE
toolchain:                 SYSTEM
word-size:                 BITS_64

[mosteo]

@Fabien-Chouteau do you have input on this? I haven't used these switches deliberately in recent memory.

[simonjwright]

Is there a use case for these switches except on resource-constrained embedded systems?

I write as a Mac user, where the GNU ld option -gc-sections isn’t supported:

$ gnatmake hello.adb -largs -Wl,-gc-sections
gcc -c hello.adb
gnatbind -x hello.ali
gnatlink hello.ali -Wl,-gc-sections
ld: unknown options: -gc-sections 
collect2: error: ld returned 1 exit status

so for me -ffunction-sections & -fdata-sections are useless.

(On the other hand, -dead_strip reduces hello from 422200 to 146392)

[Fabien-Chouteau]

But it doesn't include -gc-sections to the linker in any of the profiles. Can that be considered a bug, or is there a reason for that?

The list of switches generated by Alire are compiler switches not linker switches. -gc-sections is a linker switch so it's up to you if you want to use it or not.

We could change the alr init --bin generated project to include this switch by default...

[JeremyGrosser]

-gc-sections will also remove debug information, so I don't think it should be set by default. Maybe we can enable it only if Build_Profile = "release"?

[simonjwright]

@JeremyGrosser : my embedded projects use -gc-sections (some of them wouldn’t fit otherwise) and can be debugged without issue. At least, I’ve never had a problem related to this.
@Fabien-Chouteau : please don’t do this! unless you can arrange for it not to apply for macOS builds.

[mgrojo]

@JeremyGrosser : my embedded projects use -gc-sections (some of them wouldn’t fit otherwise) and can be debugged without issue. At least, I’ve never had a problem related to this.

Same experience about debugging. But this seems to depend on debug format (from GCC documentation):

On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats.

@Fabien-Chouteau : please don’t do this! unless you can arrange for it not to apply for macOS builds.

Something like this could be generated by default:

   package Linker is

      case Your_Crate_Config.Alire_Host_OS is
      when "macos" =>
         Linker_Options := ();
      when others =>
         Linker_Options := ("-Wl,-gc-sections");
      end case;

      for Default_Switches ("Ada") use Linker_Options;

   end Linker;

It could also take Your_Crate_Config.Build_Profile into account.

[Fabien-Chouteau]

-gc-sections will also remove debug information, so I don't think it should be set by default. Maybe we can enable it only if Build_Profile = "release"?

@JeremyGrosser It's probably something else because, like @simonjwright, I never had an issue with debugging and these options. Also, the documentation clearly says it's for "eliminating the unused code and data": https://gcc.gnu.org/onlinedocs/gnat_ugn/Compilation-options.html

@mgrojo Indeed, we could do something like this. Although I am not a huge fan of adding more complexity to the non-generate project files, another option would be to have Alire generate Linker switches in addition to compiler switches.

[simonjwright]

My opinion: none of these options should be used in Alire-generated project files. There could be a case for including them in embedded RTS’s runtime.xmls; ISTR suggesting this a while back, the idea wasn’t accepted.