Re: GCC and binutils support for BPF V4 instructions

[Yonghong Song <yonghong.song@linux.dev>]

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.

> 
> 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!
>