https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111735
Bug ID: 111735 Summary: incorrect BTF representation of forward-declared enums Product: gcc Version: 13.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: debug Assignee: unassigned at gcc dot gnu.org Reporter: gprocida at google dot com Target Milestone: --- The BTF_FWD_KIND node can only distinguish between struct and union types. (Non-standard) forward-declared enum types need to be represented in some different fashion. I queried this on LKML in July 2020 and was told that then current way to represent such types is as BTF_KIND_ENUM with vlen = 0. This is in fact what both Clang and pahole -J do. I think Clang produces slightly better output than pahole (leaving size as 0). test input: $ cat /tmp/xx.c enum Foo; enum Foo* foo; compilation command: $ gcc -gbtf -c /tmp/xx.c -o /tmp/xx.o unexpected output: $ bpftool btf dump file /tmp/xx.o format raw [1] FWD 'Foo' fwd_kind=struct [2] PTR '(anon)' type_id=1 [3] VAR 'foo' type_id=2, linkage=global [4] DATASEC '.bss' size=0 vlen=1 type_id=3 offset=0 size=8 (VAR 'foo') possible expected output (size=0): $ clang-15 -target bpf -g -c /tmp/xx.c -o /tmp/xx.o $ pahole -J /tmp/xx.o $ bpftool btf dump file /tmp/xx.o format raw | grep Foo [2] ENUM 'Foo' encoding=UNSIGNED size=0 vlen=0 other possible expected output (size=4): $ gcc -g -c /tmp/xx.c -o /tmp/xx.o $ pahole -J /tmp/xx.o $ bpftool btf dump file /tmp/xx.o format raw | grep Foo [1] ENUM 'Foo' encoding=UNSIGNED size=4 vlen=0 GCC version: Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/13/lto-wrapper OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa OFFLOAD_TARGET_DEFAULT=1 Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Debian 13.2.0-4' --with-bugurl=file:///usr/share/doc/gcc-13/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-13 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-13-oyarai/gcc-13-13.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-13-oyarai/gcc-13-13.2.0/debian/tmp-gcn/usr --enable-offload-defaulted --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=28 Thread model: posix Supported LTO compression algorithms: zlib zstd gcc version 13.2.0 (Debian 13.2.0-4)