On 7/7/25 12:07 PM, 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 9: r1 += r2 10: r1 = *(u64 *)(r1 + 0x0) 11: gotox r1 ... 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.
Thanks, Eduard. I actually also explored a little bit and came up with the below patch: https://github.com/yonghong-song/llvm-project/tree/br-jt-v6-seperate-jmptable the top commit is the addition on top of https://github.com/llvm/llvm-project/pull/133856. I tried to leverage existing llvm infrastructure and it will support ELF/XCOFF/COFF and all backends. Anton, besides Eduard's patch, please also take a look at the above patch.
