Add few clang compiler alias for BPF target

[dkm]

Add 3 latest clang version aliases to BPF target.

refs #4408

Signed-off-by: Marc Poulhiès dkm@kataplop.net

[borkmann]

Thanks so much! Not blocking for the PR of course, but could you also add clang trunk as well? That would be very useful too.

[dkm]

@borkmann we usually use binutils for objdump and demangler (c++filt). Is it a valid choice also for bpf?

[borkmann]

@borkmann we usually use binutils for objdump and demangler (c++filt). Is it a valid choice also for bpf?

For BPF, is it possible to use in all cases LLVM pendants?

cat foo.c 
int square(int num)
{
	return num * num;
}
clang -target bpf -c foo.c -o foo.o

Will give:

llvm-objdump --disassemble-symbols=square foo.o 

foo.o:	file format elf64-bpf

Disassembly of section .text:

0000000000000000 <square>:
       0:	63 1a fc ff 00 00 00 00	*(u32 *)(r10 - 4) = r1
       1:	61 a0 fc ff 00 00 00 00	r0 = *(u32 *)(r10 - 4)
       2:	2f 00 00 00 00 00 00 00	r0 *= r0
       3:	95 00 00 00 00 00 00 00	exit

That's the more commonly used disasm output in BPF community.

I know that the BPF gcc trunk uses their own asm syntax at the moment.

Each could use their own by default, but if it is possible, it would actually be really nice for users to pick a choice (LLVM BPF asm syntax vs gcc BPF asm syntax) when they compare gcc vs clang outputs so results are easier to read for each party and compare.

[dkm]

Ok thanks, I'll see what I can do. I don't think I've seen any usage of llvm-objdump yet, so not sure our post-processing will work, or even if we know how to call it.

edit: so we have some llvm-objdump around for mos and hip/rocm, so should be ok. User can't switch it dynamically, so I'll have clang compilers use llvm, and gcc use binutils.

[partouf]

Ok thanks, I'll see what I can do. I don't think I've seen any usage of llvm-objdump yet, so not sure our post-processing will work, or even if we know how to call it.

you'll need to also set objdumperType=llvm

[RubenRBS]

+1 on this

[dkm]

Thank you all for reviewing/advising!

[dkm]

Ok, so this is not working :(
The reason being that our "compile to binary" is really "link to binary" (another reason for me undusting #3232 ), and we use gcc as the linker drivers.

clang version 16.0.0 (https://github.com/llvm/llvm-project.git 8304f6de94872cdcaba63039a61b8860fe367620)
Target: bpf
Thread model: posix
InstalledDir: /opt/compiler-explorer/clang-trunk/bin
 "/opt/compiler-explorer/clang-trunk-20230107/bin/clang-16" -cc1 -triple bpf -emit-obj -mrelax-all -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name example.cpp -mrelocation-model static -mframe-pointer=all -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -mllvm -treat-scalable-fixed-error-as-warning -debug-info-kind=constructor -dwarf-version=5 -debugger-tuning=gdb -v -fcoverage-compilation-dir=/app -resource-dir /opt/compiler-explorer/clang-trunk-20230107/lib/clang/16 -fdeprecated-macro -fdebug-compilation-dir=/app -ferror-limit 19 -fgnuc-version=4.2.1 -fcxx-exceptions -fexceptions -fcolor-diagnostics -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o /tmp/example-87b770.o -x c++ <source>
clang -cc1 version 16.0.0 based upon LLVM 16.0.0git default target x86_64-unknown-linux-gnu
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /opt/compiler-explorer/clang-trunk-20230107/lib/clang/16/include
 /usr/include
End of search list.
 "/usr/bin/gcc" -L. -o /app/output.s /tmp/example-87b770.o -Wl,-rpath,.
/usr/bin/ld: /tmp/example-87b770.o: Relocations in generic ELF (EM: 247)
/usr/bin/ld: /tmp/example-87b770.o: Relocations in generic ELF (EM: 247)
/usr/bin/ld: /tmp/example-87b770.o: Relocations in generic ELF (EM: 247)
/usr/bin/ld: /tmp/example-87b770.o: Relocations in generic ELF (EM: 247)
/usr/bin/ld: /tmp/example-87b770.o: Relocations in generic ELF (EM: 247)
/usr/bin/ld: /tmp/example-87b770.o: Relocations in generic ELF (EM: 247)
/usr/bin/ld: /tmp/example-87b770.o: error adding symbols: file in wrong format
collect2: error: ld returned 1 exit status
clang-16: error: linker (via gcc) command failed with exit code 1 (use -v to see invocation)
Compiler returned: 1

Possible solutions:

• use bpf-gcc for linking. I still feel this is not what we want, but that's the easy fix
• really only compile (no link) to binary (see PR above)
• disable binary altogether.
  I'll try number 1 until 2 is implemented :)