[Bug ld/26766] Support standalone PGO build

[mliska at suse dot cz]

https://sourceware.org/bugzilla/show_bug.cgi?id=26766

--- Comment #10 from Martin Liska <mliska at suse dot cz> ---
(In reply to H.J. Lu from comment #9)
> I updated users/hjl/pgo-lto branch.  Please give it a try.

Looks nice to me!

Few comments:
- using make -j16 leads to invocation of many ltrans units:

$ ps ax | grep lto1 | grep ltrans | wc -l
182

16057 pts/4    R+     0:00 /usr/lib64/gcc/x86_64-suse-linux/10/lto1 -quiet
-dumpdir ./ -dumpbase size.ltrans20 -mtune=generic -march=x86-64 -auxbase-strip
/tmp/size.LZeDmU.ltrans20.ltrans.o -g -g -O2 -O2 -Wextra -Wshadow
-Wstack-usage=262144 -fno-openmp -fno-openacc -fno-pie -fprofile-generate
-fltrans /tmp/size.LZeDmU.ltrans20.o -o /tmp/cctrJGxZ.s
16059 pts/4    R+     0:00 /usr/lib64/gcc/x86_64-suse-linux/10/lto1 -quiet
-dumpdir ./ -dumpbase strings.ltrans1 -mtune=generic -march=x86-64
-auxbase-strip /tmp/strings.hZ0ByX.ltrans1.ltrans.o -g -g -O2 -O2 -Wextra
-Wshadow -Wstack-usage=262144 -fno-openmp -fno-openacc -fno-pie
-fprofile-generate -fltrans /tmp/strings.hZ0ByX.ltrans1.o -o /tmp/ccRyZY47.s
16061 pts/4    R+     0:00 /usr/lib64/gcc/x86_64-suse-linux/10/lto1 -quiet
-dumpdir ./ -dumpbase size.ltrans15 -mtune=generic -march=x86-64 -auxbase-strip
/tmp/size.LZeDmU.ltrans15.ltrans.o -g -g -O2 -O2 -Wextra -Wshadow
-Wstack-usage=262144 -fno-openmp -fno-openacc -fno-pie -fprofile-generate
-fltrans /tmp/size.LZeDmU.ltrans15.o -o /tmp/cctzjdSi.s
16063 pts/4    R+     0:00 /usr/lib64/gcc/x86_64-suse-linux/10/lto1 -quiet
-dumpdir ./ -dumpbase strings.ltrans6 -mtune=generic -march=x86-64
-auxbase-strip /tmp/strings.hZ0ByX.ltrans6.ltrans.o -g -g -O2 -O2 -Wextra
-Wshadow -Wstack-usage=262144 -fno-openmp -fno-openacc -fno-pie
-fprofile-generate -fltrans /tmp/strings.hZ0ByX.ltrans6.o -o /tmp/ccbMqdFo.s

It seems -flto=jobserver is not working as it should be.

- about the --enable-pgo-bootstrap=lto, I would recommend adding an orthogonal
option: --enable-lto-build where it will append -flto=jobserver and will be
independent to --enable-pgo-bootstrap as I asked in the original comment.

Apart from that, it works nice!

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