Re: GCC and binutils support for BPF V4 instructions

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On 7/28/23 9:41 AM, Jose E. Marchesi wrote:
>> Hello.
>> Just a heads up regarding the new BPF V4 instructions and their
>> support
>> in the GNU Toolchain.
>> V4 sdiv/smod instructions
>>    Binutils has been updated to use the V4 encoding of these
>>    instructions, which used to be part of the xbpf testing dialect used
>>    in GCC.  GCC generates these instructions for signed division when
>>    -mcpu=v4 or higher.
>> V4 sign-extending register move instructions
>> V4 signed load instructions
>> V4 byte swap instructions
>>    Supported in assembler, disassembler and linker.  GCC generates
>> these
>>    instructions when -mcpu=v4 or higher.
>> V4 32-bit unconditional jump instruction
>>    Supported in assembler and disassembler.  GCC doesn't generate
>> that
>>    instruction.
>>    However, the assembler has been expanded in order to perform the
>>    following relaxations when the disp16 field of a jump instruction is
>>    known at assembly time, and is overflown, unless -mno-relax is
>>    specified:
>>      JA disp16  -> JAL disp32
>>      Jxx disp16 -> Jxx +1; JA +1; JAL disp32
>>    Where Jxx is one of the conditional jump instructions such as
>> jeq,
>>    jlt, etc.
>
> Sounds great. The above 'JA/Jxx disp16' transformation matches
> what llvm did as well.

Not by chance ;)

Now what is pending in binutils is to relax these jumps in the linker as
well.  But it is very low priority, compared to get these kernel
selftests building and running.  So it will happen, but probably not
anytime soon.

>
>> So I think we are done with this.  Please let us know if these
>> instructions ever change.
>> Relevant binutils bugzillas (all now resolved as fixed):
>> * Make use of long range calls by relaxation (jal/gotol):
>>    https://sourceware.org/bugzilla/show_bug.cgi?id=30690
>> Relevant GCC bugzillas (all now resolved as fixed):
>> * Make use of signed-load instructions:
>>    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110782
>>    * Make use of signed division/modulus:
>>    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110783
>> * Make use of signed mov instructions:
>>    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110784
>> * Make use of byte swap instructions:
>>    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110786
>> Salud!
>>