> On 11/30/23 7:13 AM, Jose E. Marchesi wrote: >>> On 11/29/23 2:08 AM, Jose E. Marchesi wrote: >>>>> On 11/28/23 11:23 AM, Jose E. Marchesi wrote: >>>>>> [During LPC 2023 we talked about improving communication between the GCC >>>>>> BPF toolchain port and the kernel side. This is the first periodical >>>>>> report that we plan to publish in the GCC wiki and send to interested >>>>>> parties. Hopefully this will help.] >>>>>> >>>>>> GCC wiki page for the port: https://gcc.gnu.org/wiki/BPFBackEnd >>>>>> IRC channel: #gccbpf at irc.oftc.net. >>>>>> Help on using the port: gcc@xxxxxxxxxxx >>>>>> Patches and/or development discussions: gcc-patches@xxxxxxx >>>>> Thanks a lot for detailed report. Really helpful to nail down >>>>> issues facing one or both compilers. See comments below for >>>>> some mentioned issues. >>>>> >>>>>> Assembler >>>>>> ========= >>>>> [...] >>>>> >>>>>> - In the Pseudo-C syntax register names are not preceded by % characters >>>>>> nor any other prefix. A consequence of that is that in contexts like >>>>>> instruction operands, where both register names and expressions >>>>>> involving symbols are expected, there is no way to disambiguate >>>>>> between them. GAS was allowing symbols like `w3' or `r5' in syntactic >>>>>> contexts where no registers were expected, such as in: >>>>>> >>>>>> r0 = w3 ll ; GAS interpreted w3 as symbol, clang emits error >>>>>> >>>>>> The clang assembler wasn't allowing that. During LPC we agreed that >>>>>> the simplest approach is to not allow any symbol to have the same name >>>>>> than a register, in any context. So we changed GAS so it now doesn't >>>>>> allow to use register names as symbols in any expression, such as: >>>>>> >>>>>> r0 = w3 + 1 ll ; This now fails for both GAS and llvm. >>>>>> r0 = 1 + w3 ll ; NOTE this does not fail with llvm, but it should. >>>>> Could you provide a reproducible case above for llvm? llvm does not >>>>> support syntax like 'r0 = 1 + w3 ll'. For add, it only supports >>>>> 'r1 += r2' or 'r1 += 100' syntax. >>>> It is a 128-bit load with an expression. In compiler explorer, clang: >>>> >>>> int >>>> foo () >>>> { >>>> asm volatile ("r1 = 10 + w3 ll"); >>>> return 0; >>>> } >>>> >>>> I get: >>>> >>>> foo: # @foo >>>> r1 = 10+w3 ll >>>> r0 = 0 >>>> exit >>>> >>>> i.e. `10 + w3' is interpreted as an expression with two operands: the >>>> literal number 10 and a symbol (not a register) `w3'. >>>> >>>> If the expression is `w3+10' instead, your parser recognizes the w3 as a >>>> register name and errors out, as expected. >>>> >>>> I suppose llvm allows to hook on the expression parser to handle >>>> individual operands. That's how we handled this in GAS. >>> Thanks for the code. I can reproduce the result with compiler explorer. >>> The following is the link https://godbolt.org/z/GEGexf1Pj >>> where I added -grecord-gcc-switches to dump compilation flags >>> into .s file. >>> >>> The following is the compiler explorer compilation command line: >>> /opt/compiler-explorer/clang-trunk-20231129/bin/clang-18 -g -o /app/output.s \ >>> -S --target=bpf -fcolor-diagnostics -gen-reproducer=off -O2 \ >>> -g -grecord-command-line /app/example.c >>> >>> I then compile the above C code with >>> clang -g -S --target=bpf -fcolor-diagnostics -gen-reproducer=off -O2 -g -grecord-command-line t.c >>> with identical flags. >>> >>> I tried locally with llvm16/17/18. They all failed compilation since >>> 'r1 = 10+w3 ll' cannot be recognized by the llvm. >>> We will investigate why llvm18 in compiler explorer compiles >>> differently from my local build. >> I updated git llvm master today and I managed to reproduce locally with: >> >> jemarch@termi:~/gnu/src/llvm-project/llvm/build$ clang --version >> clang version 18.0.0 (https://github.com/llvm/llvm-project.git 586986a063ee4b9a7490aac102e103bab121c764) >> Target: unknown >> Thread model: posix >> InstalledDir: /usr/local/bin >> $ cat foo.c >> int >> foo () >> { >> asm volatile ("r1 = 10 + w3 ll"); >> return 0; >> } >> $ clang -target bpf -c foo.c >> $ llvm-objdump -dr foo.o >> >> foo.o: file format elf64-bpf >> >> Disassembly of section .text: >> >> 0000000000000000 <foo>: >> 0: 18 01 00 00 0a 00 00 00 00 00 00 00 00 00 00 00 r1 = 0xa ll >> 0000000000000000: R_BPF_64_64 w3 >> 2: b7 00 00 00 00 00 00 00 r0 = 0x0 >> 3: 95 00 00 00 00 00 00 00 exit > > Could you share the cmake command line options when you build you clang? > My cmake command line looks like > cmake .. -DCMAKE_BUILD_TYPE=Release -G Ninja \ > -DLLVM_ENABLE_PROJECTS="clang;lld;compiler-rt" \ > -DLLVM_TARGETS_TO_BUILD="BPF;X86" \ > -DLLVM_ENABLE_ASSERTIONS=ON \ > -DLLVM_ENABLE_ZLIB=ON \ > -DCMAKE_INSTALL_PREFIX=$PWD/install > > and cannot reproduce the issue. I don't have the original cmake command, I executed it long ago (rebuilding clang/llvm in my laptop takes three days or more so I do it incrementally.) I see this in my CMakeCache.txt: LLVM_ENABLE_PROJECTS:STRING=clang LLVM_TARGETS_TO_BUILD:STRING=BPF LLVM_ENABLE_ASSERTIONS:BOOL=OFF LLVM_ENABLE_ZLIB:STRING=ON CMAKE_INSTALL_PREFIX:PATH=/usr/local
