[Xtensa] Add LLD linker support (LLVM-143)

[aykevl]

This is probably not complete and there's a good chance there are bugs. But it works for me to link TinyGo programs.

I wanted to try using it for an esp-idf project but wasn't sure how to hook LLVM ld.lld into the esp-idf build system.

TODO:

• Figure out a way to avoid the ugly hack mentioned in the comments.
• Add tests.

Fixes #11

[aykevl]

Updated. I've added a lot more relocations and it is now a lot closer to link the hello_world example. The only issue remaining (I hope!) is a difference in behavior between GCC and Clang: https://godbolt.org/z/1frb731E6 (with -ffreestanding, Clang compiles main with C++ name mangling while GCC doesn't). I will need to investigate this difference at a later time.

[gerekon]

Updated. I've added a lot more relocations and it is now a lot closer to link the hello_world example. The only issue remaining (I hope!) is a difference in behavior between GCC and Clang: https://godbolt.org/z/1frb731E6 (with -ffreestanding, Clang compiles main with C++ name mangling while GCC doesn't). I will need to investigate this difference at a later time.

Yes, I faced the same issue when cmake compiled test program during config phase. It used simple C++ programs compiled with clang++.
So for our RISCV toolchain I had to exclude crt0.o from linking when -ffreestanding is used, because we do not use it for ESP-IDF. For Xtensa toolchain we do not include crt0.o at all

[gerekon]

Just a clarification to my comment above...
The problem was that crt0.o references main while clang++ -ffreestanding produced mangled main name. My comment from ESP clang RISCV toolchain implementation.

    /* Espressif toolcahin uses newlib. crt0.o from it refers to 'main' symbol.
       In 'freestanding' mode 'main' is not marked as special symbol by clang,
       so when compiling C++ program with 'clang++' 'main' gets mmangled
       (if not decalred as 'extern "C"' ) and linker can not resolve it.
       The problem can happen, for example, when cmake checks C++ compiler by buiding simple C++ code,
       unfortunately 'main' function in that code is not decalred as 'extern "C"'. */

[MabezDev]

Hi @aykevl, I recently tried a custom rust build with this LLD enabled. Unfortunately, I ran into some issue trying to link our bare metal Rust Xtensa projects (not esp-idf). The link failed with the following output:

l/target/xtensa-esp32s2-none-elf/release/examples/hello_world-85f73e95417de02e" "--gc-sections" "-O1" "-Tlinkall.x"
  = note: lld: error: /Users/mabez/Development/rust/embedded/hal/esp-hal/target/xtensa-esp32s2-none-elf/release/build/esp32s2-hal-f787ee6a1beba60a/out/memory.x:71: unable to move location counter backward for: .rtc_fast.dummy
          lld: error: section '.rtc_fast.dummy' will not fit in region 'rtc_fast_dram_seg': overflowed by 18446744072636203008 bytes
          lld: error: section '.rtc_fast.data' will not fit in region 'rtc_fast_dram_seg': overflowed by 18446744072636203008 bytes
          lld: error: section '.rtc_fast.data' will not fit in region 'rtc_fast_dram_seg': overflowed by 18446744072636203008 bytes
          lld: error: section '.rtc_fast.bss' will not fit in region 'rtc_fast_dram_seg': overflowed by 18446744072636203008 bytes
          lld: error: section '.rtc_fast.bss' will not fit in region 'rtc_fast_dram_seg': overflowed by 18446744072636203008 bytes
          lld: error: section '.rtc_fast.noinit' will not fit in region 'rtc_fast_dram_seg': overflowed by 18446744072636203008 bytes
          lld: error: section '.rtc_fast.noinit' will not fit in region 'rtc_fast_dram_seg': overflowed by 18446744072636203008 bytes
          lld: error: /Users/mabez/Development/rust/embedded/hal/esp-hal/target/xtensa-esp32s2-none-elf/release/build/xtensa-lx-rt-dfc61fdc190349a8/out/exception.x:46: unable to move location counter backward for: .vectors
          lld: error: section '.vectors' will not fit in region 'vectors_seg': overflowed by 18446744072635669504 bytes
          lld: error: section '.vectors' will not fit in region 'vectors_seg': overflowed by 18446744072635669504 bytes
          lld: error: section '.vectors' will not fit in region 'vectors_seg': overflowed by 18446744072635669522 bytes
          lld: error: section '.vectors' will not fit in region 'vectors_seg': overflowed by 18446744072635669568 bytes
          lld: error: section '.vectors' will not fit in region 'vectors_seg': overflowed by 18446744072635669632 bytes
          lld: error: section '.vectors' will not fit in region 'vectors_seg': overflowed by 18446744072635669632 bytes
          lld: error: section '.vectors' will not fit in region 'vectors_seg': overflowed by 18446744072635669665 bytes
          lld: error: section '.vectors' will not fit in region 'vectors_seg': overflowed by 18446744072635669696 bytes
          lld: error: section '.vectors' will not fit in region 'vectors_seg': overflowed by 18446744072635669729 bytes
          lld: error: section '.vectors' will not fit in region 'vectors_seg': overflowed by 18446744072635669760 bytes
          lld: error: section '.vectors' will not fit in region 'vectors_seg': overflowed by 18446744072635669805 bytes
          lld: error: too many errors emitted, stopping now (use -error-limit=0 to see all errors)
          

error: could not compile `esp32s2-hal` due to previous error

The linker scripts can be found here and here. Any ideas?

[gerekon]

Hi @aykevl
We released LLD Xtensa support https://github.com/espressif/llvm-project/releases/tag/esp-15.0.0-20221201.

Thanks a lot for your help!