Cannot prevent constants placed in sections from being stripped

[cbuttner]

This is an example for Linux ELF:

module test;
import std::io;

extern const String __START_MYSECTION @cname("__start_mysection");
extern const String __STOP_MYSECTION @cname("__stop_mysection");

fn void main() {
  for (String* r = &__START_MYSECTION; r < &__STOP_MYSECTION; r++) {
    io::printfn("Got %s", *r);
  }
  
  // The @nostrip doesn't do anything here
  // Note that if you put these in global scope, they don't get stripped (only for this example though!)
  const String STRING1 @section("mysection") @nostrip = "Hello";
  const String STRING2 @section("mysection") @nostrip = "World";

  // Uncomment this: Section exists, program links fine
  // io::printfn("%s %s", STRING1, STRING2);
}

Run with c3c build --optsize=none && readelf -S build/test | grep mysection

Program linked to executable 'build/test'.
  [29] mysection         PROGBITS         0000000000047a50  00047a50

Run with c3c build --optsize=small

/usr/bin/ld: build/obj/linux-x64/test.o: in function `test.main':
/tmp/test/src/main.c3:8:(.text+0xb): undefined reference to `__start_mysection'
/usr/bin/ld: /tmp/test/src/main.c3:8:(.text+0x12): undefined reference to `__stop_mysection'
/usr/bin/ld: /tmp/test/src/main.c3:(.text+0x2a): undefined reference to `__start_mysection'
/usr/bin/ld: /tmp/test/src/main.c3:8:(.text+0x76): undefined reference to `__stop_mysection'
/usr/bin/ld: build/obj/linux-x64/test.o:(.debug_info+0x38): undefined reference to `__start_mysection'
/usr/bin/ld: build/obj/linux-x64/test.o:(.debug_info+0x99): undefined reference to `__stop_mysection'
collect2: error: ld returned 1 exit status

And readelf -S build/test | grep mysection does not show the section.

I need the constants to always be placed in the section in order to make some string interning optimizations possible. I'd be okay with having to add @nostrip, but maybe @section should already imply this, since adding @section is already quite intentional.

[lerno]

Are they removed on -O0?

[lerno]

(Sorry for the late reply BTW)

[cbuttner]

Are they removed on -O0?

On Linux, no. On macOS, I can't get it to work at all.

If you want to test on macOS, here is the equivalent code:

module test;
import std::io;

extern const String __START_MYSECTION @cname("section$start$__DATA$__mysection");
extern const String __STOP_MYSECTION @cname("section$end$__DATA$__mysection");

fn void main() {
  for (String* r = &__START_MYSECTION; r < &__STOP_MYSECTION; r++) {
    io::printfn("Got %s", *r);
  }
  
  const String STRING1 @section("__DATA,__mysection") @nostrip = "Hello";
  const String STRING2 @section("__DATA,__mysection") @nostrip = "World";

  //io::printfn("%s %s", STRING1, STRING2);
}

Roughly equivalent C code for comparison (using int instead of String)

#include <stdio.h>

// Declare the start and end symbols for the __DATA,__mysection section
extern const int START_MYSECTION __asm("section$start$__DATA$__mysection");
extern const int STOP_MYSECTION  __asm("section$end$__DATA$__mysection");

int main(void) {
    for (const int* r = &START_MYSECTION; r < &STOP_MYSECTION; r++) {
        printf("Got %d\n", *r);
    }

    // Mark variables as used to prevent the linker from stripping the section
    static const int STRING1 __attribute__((used, section("__DATA,__mysection"))) = 1;
    static const int STRING2 __attribute__((used, section("__DATA,__mysection"))) = 2;

    return 0;
}

[lerno]

Note that the trick with start-end does not work on mach-o

[lerno]

This should work properly now. Please test.