[lld] [RISCV][LLD] Zcmt RISC-V extension in lld (PR #163142)

[Robin Kastberg via llvm-commits]

RobinKastberg wrote:

I tried zephyr with this..
```
user at b7f43aa2a597:/workdir/zephyr/build$ /workdir/llvm/build/bin/clang --target=riscv32-unknown-elf  -Oz zephyr/CMakeFiles/zephyr_final.dir/misc/empty_file.c.obj zephyr/CMakeFiles/zephyr_final.dir/isr_tables.c.obj -o zephyr/zephyr.elf  zephyr/CMakeFiles/offsets.dir/./arch/riscv/core/offsets/offsets.c.obj  -Wl,-T  zephyr/linker.cmd  -Wl,-Map=/workdir/zephyr/build/zephyr/zephyr_final.map  -Wl,--whole-archive  app/libapp.a  zephyr/libzephyr.a  zephyr/arch/common/libarch__common.a  zephyr/arch/arch/riscv/core/libarch__riscv__core.a  zephyr/lib/libc/picolibc/liblib__libc__picolibc.a  zephyr/lib/libc/common/liblib__libc__common.a  zephyr/drivers/interrupt_controller/libdrivers__interrupt_controller.a  zephyr/drivers/console/libdrivers__console.a  zephyr/drivers/serial/libdrivers__serial.a  zephyr/drivers/timer/libdrivers__timer.a  -Wl,--no-whole-archive  zephyr/kernel/libkernel.a  -L/workdir/zephyr/build/zephyr  -Wl,--print-memory-usage  zephyr/arch/common/libisr_tables.a  --config=/workdir/zephyr/cmake/toolchain/llvm/clang_compiler_rt.cfg  -fuse-ld=lld  -mabi=ilp32  -march=rv32imac_zicsr_zifencei  -mcmodel=medlow  -Wl,--gc-sections -flto -Wl,--build-id=none  -Wl,--relax-gp  -Wl,--sort-section=alignment  -Wl,-u,_OffsetAbsSyms  -Wl,-u,_ConfigAbsSyms  -nostdlib  -static  -Wl,-X  -Wl,-N  -Wl,--orphan-handling=warn  -Wl,--no-pie -L/w
orkdir/llvm/build/lib/clang/22/lib/riscv32-unknown-unknown-elf /workdir/zephyr/build/modules/picolibc/libc.a -lclang_rt.builtins -Wl,--relax-tbljal
ld.lld: warning: <internal>:(.riscv.jvt) is being placed in '.riscv.jvt'
ld.lld: warning: Code size reduction: 0
Memory region         Used Size  Region Size  %age Used
             RAM:       28800 B       256 MB      0.01%
        IDT_LIST:          0 GB         4 KB      0.00%
```

It seems with `--gc-sections` we get a code-size reduction of 0 for some reason.

Without `--gc-sections`
```
ld.lld: warning: zephyr/libzephyr.a(last_section_id.c.obj):(.last_section) is being placed in '.last_section'
ld.lld: warning: <internal>:(.riscv.jvt) is being placed in '.riscv.jvt'
ld.lld: warning: Code size reduction: 44
Memory region         Used Size  Region Size  %age Used
             RAM:       57552 B       256 MB      0.02%
        IDT_LIST:          0 GB         4 KB      0.00%
user at b7f43aa2a597:/workdir/zephyr/build$ python3
Python 3.10.12 (main, Sep 11 2024, 15:47:36) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 44 / 57552
0.0007645259938837921
>>> quit()
```
We get a `44 / 57552 = 0.07%` code-size reduction, which is underwhelming, since removing `--gc-sections` doubled the size of the result :(

https://github.com/llvm/llvm-project/pull/163142