On Wed, Dec 9, 2020 at 5:56 AM 'Fangrui Song' via Clang Built Linux
[off-list ref] wrote:

On 2020-12-08, 'Sami Tolvanen' via Clang Built Linux wrote:

quoted

On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann [off-list ref] wrote:

quoted

So far I have
not managed to get a working build out of it, the main problem so far being
that it is really slow to build because the link stage only uses one CPU.
These are the other issues I've seen so far:

ld.lld ThinLTO uses the number of (physical cores enabled by affinity) by default.

Ah, I see.  Do you know if it's also possible to do something like
-flto=jobserver
to integrate better with the kernel build system?

I tend to run multiple builds under a top-level makefile with 'make
-j30' in order
to use 30 of the 32 threads and leave the scheduling to jobserver instead of
the kernel. If the linker itself is multithreaded but the jobserver
thinks it is a
single thread, could end up with 30 concurrent linkers each trying to use
16 cores.

quoted

quoted

- CONFIG_CPU_BIG_ENDIAN doesn't seem to work with lld, and LTO
  doesn't work with ld.bfd.
  I've added a CPU_LITTLE_ENDIAN dependency to
  ARCH_SUPPORTS_LTO_CLANG{,THIN}

Ah, good point. I'll fix this in v9.

Full/Thin LTO should work with GNU ld and gold with LLVMgold.so built from
llvm-project (https://llvm.org/docs/GoldPlugin.html ). You'll need to make sure
that LLVMgold.so is newer than clang. (Newer clang may introduce bitcode
attributes which are unrecognizable by older LLVMgold.so/ld.lld)

The current patch series requires LLD:

config HAS_LTO_CLANG
       def_bool y
       depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD

Is this something we should change then, or try to keep it simple with the
current approach, leaving LTO disabled for big-endian builds and hosts without
a working lld?

       Arnd

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