maintainer-feedback requested: [Bug 258494] lang/erlang lang/erlang-runtime21 lang/erlang-runtime23: clang 13 build breaks dtrace if PGO is enabled

Bugzilla Automation <bugzilla@FreeBSD.org> has asked freebsd-erlang (Nobody)
<erlang@FreeBSD.org> for maintainer-feedback:
Bug 258494: lang/erlang lang/erlang-runtime21 lang/erlang-runtime23: clang 13
build breaks dtrace if PGO is enabled
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=258494



--- Description ---
During an exp-run for llvm 13 (see bug 258209), it turned out that lang/erlang
and lang/erlang-runtime2[1-3] fail to build with clang 13:

http://gohan04.nyi.freebsd.org/data/mainamd64PR258209-default/2021-09-05_20h27m
09s/logs/errors/erlang-21.3.8.24_1,4.log
http://gohan04.nyi.freebsd.org/data/mainamd64PR258209-default/2021-09-05_20h27m
09s/logs/errors/erlang-runtime21-21.3.8.24_1.log
http://gohan04.nyi.freebsd.org/data/mainamd64PR258209-default/2021-09-05_20h27m
09s/logs/errors/erlang-runtime23-23.3.4.6.log

What appears to happen is that for these versions of erlang, PGO is enabled,
and it first builds a PGO-enabled beam.smp:

gmake[5]: Entering directory
'/wrkdirs/usr/ports/lang/erlang/work/otp-OTP-21.3.8.24/erts/emulator'
if utils/gen_git_version amd64-portbld-freebsd14.0/gen_git_version.mk; then
touch beam/erl_bif_info.c; fi
echo " PROFILE beam.prof.smp"
 PROFILE beam.prof.smp
rm -f obj/amd64-portbld-freebsd14.0/opt/smp/erl*.profraw
set -e;
LLVM_PROFILE_FILE="obj/amd64-portbld-freebsd14.0/opt/smp/erlc-%m.profraw" \
	  ERL_FLAGS="-emu_type prof +S 1" erlc -W  -DPGO \
  -o obj/amd64-portbld-freebsd14.0/opt/smp test/estone_SUITE.erl >
obj/amd64-portbld-freebsd14.0/opt/smp/PROFILE_LOG

after which it does a test run, and uses llvm-profdata to merge the profiling
data into beam_emu_pu.o:

llvm-profdata merge -output
obj/amd64-portbld-freebsd14.0/opt/smp/default.profdata
obj/amd64-portbld-freebsd14.0/opt/smp/*.profraw
cc -fprofile-instr-use=obj/amd64-portbld-freebsd14.0/opt/smp/default.profdata 
-Werror=undef -Werror=implicit -Werror=return-type   -O3 -fomit-frame-pointer
-pipe  -fno-omit-frame-pointer -DMAP_N
ORESERVE=0 -fstack-protector-strong -fno-strict-aliasing 
-I/wrkdirs/usr/ports/lang/erlang/work/otp-OTP-21.3.8.24/erts/amd64-portbld-free
bsd14.0
   -DHAVE_CONFIG_H -Wall -Wstrict-prototypes -Wmis
sing-prototypes -Wdeclaration-after-statement -DUSE_THREADS -D_THREAD_SAFE
-D_REENTRANT -DPOSIX_THREADS   -Iamd64-portbld-freebsd14.0/opt/smp -Ibeam
-Isys/unix -Isys/common -Iamd64-portbld-freebs
d14.0 -Ipcre -Ihipe -I../include -I../include/amd64-portbld-freebsd14.0
-I../include/internal -I../include/internal/amd64-portbld-freebsd14.0 -c
beam/beam_emu.c -o obj/amd64-portbld-freebsd14.0/o
pt/smp/beam_emu_pu.o

Later, it runs dtrace over all the collected objects, and this dies:

dtrace -G -C -Ibeam \
  -s beam/erlang_dtrace.d \
  -o obj/amd64-portbld-freebsd14.0/opt/smp/erlang_pu_dtrace.o
  ... long list of objects ...
dtrace: failed to link script beam/erlang_dtrace.d: an error was encountered
while processing obj/amd64-portbld-freebsd14.0/opt/smp/beam_emu_pu.o
gmake[5]: *** [amd64-portbld-freebsd14.0/Makefile:1005:
obj/amd64-portbld-freebsd14.0/opt/smp/erlang_pu_dtrace.o] Error 1
gmake[5]: Leaving directory
'/wrkdirs/usr/ports/lang/erlang/work/otp-OTP-21.3.8.24/erts/emulator'

Something in beam_emu_pu.o (emitted by clang or llvm 13) is tripping up dtrace,
but I have very little knowledge about dtrace so I need help here. :)

Now some other erlang runtimes such as lang/erlang-runtime24 *do* build
successfully with clang 13, but this is only because upstream disabled the PGO
feature, as a side effect of https://github.com/erlang/otp/commit/b165524c732
("erts: Implement the BeamAsm JIT"): 

--- a/erts/configure.in
+++ b/erts/configure.in
...
@@ -704,6 +719,9 @@ else
   fi
 fi

+dnl Disable pgo for now
+USE_PGO=false
+
 AC_SUBST(USE_PGO)
 AC_SUBST(PROFILE_COMPILER)

(This took a *lot* of time to bisect due to erlang's afwul non-linear
history... :)

So, I'm unsure why upstream disabled this "for now", as it has been disabled
for more than a year. We could work around the other erlang failures by also
disabling PGO there, or with help from someone knowledgeable about dtrace, try
to get to the bottom of why dtrace dies on clang 13 produced PGO object files.