Fix LTO warning by enabling parallel compilation

[eleanorLYJ]

When using make, the following warning occurs:
lto-wrapper: warning: using serial compilation of 2 LTRANS jobs.

To resolve this and enable parallel compilation, -flto=auto is now used.

According to the GCC documentation:
Use -flto=auto to use GNU make’s job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system.

[visitorckw]

Nit: Missing a period in the commit message. Otherwise, LGTM. Thanks!

[ChinYikMing]

The warning remains when using older version of gcc. @eleanorLYJ Can you check? (maybe provide some fallback mechanism for older gcc)

Note: I tried gcc-12 or above and the warning are gone.

Compiler:

gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Build log of make check:

  CC	build/map.o
  CC	build/utils.o
  CC	build/decode.o
  CC	build/io.o
  CC	build/syscall.o
  CC	build/emulate.o
  CC	build/riscv.o
  CC	build/elf.o
  CC	build/cache.o
  CC	build/mpool.o
  CC	build/main.o
  LD	build/rv32emu
lto-wrapper: warning: using serial compilation of 2 LTRANS jobs
[OK]
Checking SHA-1 of prebuilt binaries ... [OK]
Running hello.elf ... [OK]
Running puzzle ... [OK]
Running fcalc ... [OK]
Running pi ... [OK]

[jserv]

Always rebase the latest master branch when you submit pull requests.

[visitorckw]

The warning remains when using older version of gcc. @eleanorLYJ Can you check? (maybe provide some fallback mechanism for older gcc)

Note: I tried gcc-12 or above and the warning are gone.

Hmm, that's strange.
Is there a difference in the functionality of -flto=auto between gcc v11.4 and now?
Or does GCC v11.4 not even support the auto parameter?

[ChinYikMing]

The warning remains when using older version of gcc. @eleanorLYJ Can you check? (maybe provide some fallback mechanism for older gcc)
Note: I tried gcc-12 or above and the warning are gone.

Hmm, that's strange. Is there a difference in the functionality of -flto=auto between gcc v11.4 and now? Or does GCC v11.4 not even support the auto parameter?

Found this at stackoverflow. The warning was introduced from v11.4 . Thus, I believe it might be specific to version 11.4 or later but below version 12.

Actually, flto=auto is supported in v11.4, and that's where the odd behavior originates.

I think we can mention this in the commit message and move on.

[ChinYikMing]

Don't use markdown syntax in the body of git commit messages since not every parser recognizes the format (I meant the gcc docs link here). Instead, always use plaintext. Thanks!

[jserv]

Don't use markdown syntax in the body of git commit messages since not every parser recognizes the format (I meant the gcc docs link here). Instead, always use plaintext. Thanks!

Change the following:

According to the GCC documentation:
Use -flto=auto to use GNU make’s job server, if available, or otherwise
fall back to autodetection of the number of CPU threads present in your
system.

to

According to GCC optimization options [1], ...
[1] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

[ChinYikMing]

LGTM.

[jserv]

Thank @eleanorLYJ for contributing!