>> 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. By the way, for doing things like that (further object transformations by linkers and the like) we will need to have the ELF files annotated with: - The BPF cpu version the object was compiled for: v1, v2, v3, v4, and - Individual flags specifying the BPF cpu capabilities (alu32, bswap, jmp32, etc) required/expected by the code in the object. Note it is interesting to being able to denote both, for flexibility. There are 32 bits available for machine-specific flags in e_flags, which are commonly used for this purpose by other arches. For BPF I would suggest something like: #define EF_BPF_ALU32 0x00000001 #define EF_BPF_JMP32 0x00000002 #define EF_BPF_BSWAP 0x00000004 #define EF_BPF_SDIV 0x00000008 #define EF_BPF_CPUVER 0x00FF0000 >> >>> 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! >>>
