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

From: <>
Date: Tue, 14 Sep 2021 08:07:11 UTC
Bugzilla Automation <> has asked freebsd-erlang (Nobody)
<> for maintainer-feedback:
Bug 258494: lang/erlang lang/erlang-runtime21 lang/erlang-runtime23: clang 13
build breaks dtrace if PGO is enabled

--- 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:

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
if utils/gen_git_version amd64-portbld-freebsd14.0/; then
touch beam/erl_bif_info.c; fi
echo " PROFILE"
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 >

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
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 
   -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

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

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
("erts: Implement the BeamAsm JIT"): 

--- a/erts/
+++ b/erts/
@@ -704,6 +719,9 @@ else

+dnl Disable pgo for now

(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.