Re: [PATCH bpf-next v5 00/10] BTF: BPF Type Format

[Yonghong Song <yhs@fb.com>]

On 6/14/18 11:00 AM, Arnaldo Carvalho de Melo wrote:
> Em Thu, Jun 14, 2018 at 02:47:59PM -0300, Arnaldo Carvalho de Melo escreveu:
>> Em Thu, Jun 14, 2018 at 10:21:30AM -0700, Alexei Starovoitov escreveu:
>>> On 6/14/18 10:18 AM, Arnaldo Carvalho de Melo wrote:
>>>> Just out of curiosity, is there any plan to have this as a clang option?
>    
>>> I think
>>> clang ... -mllvm -mattr=dwarfris
>>> should work.
>   
>> The message "(LLVM option parsing)" implies what you suggest, but didn't
>> worked :-\
>   
>>    -mllvm <value>          Additional arguments to forward to LLVM's option processing
>   
>> Almost there tho :-\
> 
> So I thought that this -mattr=dwarfris would be available only after I
> set the target, because I tried 'llc -mattr=help' and dwarfris wasn't
> there:
> 
> [acme@jouet perf]$ llc -mattr=help |& grep dwarf
> [acme@jouet perf]$
> 
> Only after I set the arch it appears:
> 
> [acme@jouet perf]$ llc -march=bpf -mattr=help |& grep dwarf
>    dwarfris - Disable MCAsmInfo DwarfUsesRelocationsAcrossSections.
>    dwarfris - Disable MCAsmInfo DwarfUsesRelocationsAcrossSections.
>    dwarfris - Disable MCAsmInfo DwarfUsesRelocationsAcrossSections.
> [acme@jouet perf]$
> 
> But even after moving the '-mllvm -mattr=dwarfris' to after '-target
> bpf' it still can't grok it :-\
> 
> /usr/local/bin/clang -D__KERNEL__ -D__NR_CPUS__=4 -DLINUX_VERSION_CODE=0x41100 -g -nostdinc -isystem /usr/lib/gcc/x86_64-redhat-linux/7/include -I/home/acme/git/linux/arch/x86/include -I./arch/x86/include/generated  -I/home/acme/git/linux/include -I./include -I/home/acme/git/linux/arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I/home/acme/git/linux/include/uapi -I./include/generated/uapi -include /home/acme/git/linux/include/linux/kconfig.h  -I/home/acme/lib/include/perf/bpf -Wno-unused-value -Wno-pointer-sign -working-directory /lib/modules/4.17.0-rc5/build -c /home/acme/bpf/hello.c -target bpf -mllvm -mattr=dwarfris -O2 -o hello.o
> 
> So onlye with 'clang ... -target bpf -emit-llvm -O2 -o - | llc -march=bpf -mattr=dwarfris ...'
> things work as we expect.

Right. Currently, the only way to use option -mattr=dwarfris is through 
llc. The "clang -mllvm -mattr=dwarfris" won't work since
    -mllvm <value> Additional arguments to forward to LLVM's option 
processing
and -mattr=dwarfris is not in LLVM auto option processing system.
Those options, in llvm source code, typically have a pattern like below:
===
static cl::opt<unsigned> MemCmpEqZeroNumLoadsPerBlock(
     "memcmp-num-loads-per-block", cl::Hidden, cl::init(1),
     cl::desc("The number of loads per basic block for inline expansion of "
              "memcmp that is only being compared against zero."));
===

I really want to get rid of this option as well. To make pahole work
with the default default format, I need to add bpf support to
libdwfl in elfutils repo. I will work on that.



> - Arnaldo
>