On Mon, 2025-07-07 at 12:07 -0700, Eduard Zingerman wrote: > On Thu, 2025-07-03 at 11:21 -0700, Eduard Zingerman wrote: > > [...] > > > > > > .jumptables > > > > > <subprog-rel-off-0> > > > > > <subprog-rel-off-1> | <--- jump table #1 symbol: > > > > > <subprog-rel-off-2> | .size = 2 // number of entries in the jump table > > > > > ... .value = 1 // offset within .jumptables > > > > > <subprog-rel-off-N> ^ > > > > > | > > > > > .text | > > > > > ... | > > > > > <insn-N> <------ relocation referencing -' > > > > > ... jump table #1 symbol > > [...] > > I think I got it working in: > https://github.com/eddyz87/llvm-project/tree/separate-jumptables-section > > Changes on top of Yonghong's work. > An example is in the attachment the gist is: > > ------------------------------- > > $ clang --target=bpf -c -o jump-table-test.o jump-table-test.c > There are 8 section headers, starting at offset 0xaa0: > > Section Headers: > [Nr] Name Type Address Off Size ES Flg Lk Inf Al > ... > [ 4] .jumptables PROGBITS 0000000000000000 000220 000260 00 0 0 1 > ... > > Symbol table '.symtab' contains 8 entries: > Num: Value Size Type Bind Vis Ndx Name > ... > 3: 0000000000000000 256 NOTYPE LOCAL DEFAULT 4 .BPF.JT.0.0 > 4: 0000000000000100 352 NOTYPE LOCAL DEFAULT 4 .BPF.JT.0.1 > ... > > $ llvm-objdump --no-show-raw-insn -Sdr jump-table-test.o > jump-table-test.o: file format elf64-bpf > > Disassembly of section .text: > > 0000000000000000 <foo>: > ... > 6: r2 <<= 0x3 > 7: r1 = 0x0 ll > 0000000000000038: R_BPF_64_64 .jumptables I just realized that this relocation references a wrong symbol. Instead of .BPF.JT.0.0 it references jump table itself. Need more time to investigate. > 9: r1 += r2 > 10: r1 = *(u64 *)(r1 + 0x0) > 11: gotox r1 Adding a relocation here requires to bend over backwards a little bit. Need more time to figure this out. > ... > 34: r2 <<= 0x3 > 35: r1 = 0x100 ll > 0000000000000118: R_BPF_64_64 .jumptables > 37: r1 += r2 > 38: r1 = *(u64 *)(r1 + 0x0) > 39: gotox r1 > ... > > ------------------------------- > > The changes only touch BPF backend. Can be simplified a bit if I move > MachineFunction::getJTISymbol to TargetLowering in the shared LLVM > parts.
