On 25/07/07 12:07PM, 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 Awesome! I will try to use it tomorrow. > 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. > $ cat jump-table-test.c > struct simple_ctx { int x; }; > > int bar(int v); > > int foo(struct simple_ctx *ctx) > { > int ret_user; > > switch (ctx->x) { > case 0: > ret_user = 2; > break; > case 11: > ret_user = 3; > break; > case 27: > ret_user = 4; > break; > case 31: > ret_user = 5; > break; > default: > ret_user = 19; > break; > } > > switch (bar(ret_user)) { > case 1: > ret_user = 5; > break; > case 12: > ret_user = 7; > break; > case 27: > ret_user = 23; > break; > case 32: > ret_user = 37; > break; > case 44: > ret_user = 77; > break; > default: > ret_user = 11; > break; > } > > return ret_user; > } > > $ 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 > [ 0] NULL 0000000000000000 000000 000000 00 0 0 0 > [ 1] .strtab STRTAB 0000000000000000 000a31 00006b 00 0 0 1 > [ 2] .text PROGBITS 0000000000000000 000040 0001e0 00 AX 0 0 8 > [ 3] .rel.text REL 0000000000000000 000540 000030 10 I 7 2 8 > [ 4] .jumptables PROGBITS 0000000000000000 000220 000260 00 0 0 1 > [ 5] .rel.jumptables REL 0000000000000000 000570 0004c0 10 I 7 4 8 > [ 6] .llvm_addrsig LLVM_ADDRSIG 0000000000000000 000a30 000001 00 E 7 0 1 > [ 7] .symtab SYMTAB 0000000000000000 000480 0000c0 18 1 6 8 > Key to Flags: > W (write), A (alloc), X (execute), M (merge), S (strings), I (info), > L (link order), O (extra OS processing required), G (group), T (TLS), > C (compressed), x (unknown), o (OS specific), E (exclude), > R (retain), p (processor specific) > > Symbol table '.symtab' contains 8 entries: > Num: Value Size Type Bind Vis Ndx Name > 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND > 1: 0000000000000000 0 FILE LOCAL DEFAULT ABS jump-table-test.c > 2: 0000000000000000 0 SECTION LOCAL DEFAULT 2 .text > 3: 0000000000000000 256 NOTYPE LOCAL DEFAULT 4 .BPF.JT.0.0 > 4: 0000000000000100 352 NOTYPE LOCAL DEFAULT 4 .BPF.JT.0.1 > 5: 0000000000000000 0 SECTION LOCAL DEFAULT 4 .jumptables > 6: 0000000000000000 480 FUNC GLOBAL DEFAULT 2 foo > 7: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND bar > > $ 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>: > 0: *(u64 *)(r10 - 0x8) = r1 > 1: r1 = *(u64 *)(r10 - 0x8) > 2: w1 = *(u32 *)(r1 + 0x0) > 3: *(u64 *)(r10 - 0x18) = r1 > 4: if w1 > 0x1f goto +0x13 <foo+0xc0> > 5: r2 = *(u64 *)(r10 - 0x18) > 6: r2 <<= 0x3 > 7: r1 = 0x0 ll > 0000000000000038: R_BPF_64_64 .jumptables > 9: r1 += r2 > 10: r1 = *(u64 *)(r1 + 0x0) > 11: gotox r1 > 12: w1 = 0x2 > 13: *(u32 *)(r10 - 0xc) = w1 > 14: goto +0xc <foo+0xd8> > 15: w1 = 0x3 > 16: *(u32 *)(r10 - 0xc) = w1 > 17: goto +0x9 <foo+0xd8> > 18: w1 = 0x4 > 19: *(u32 *)(r10 - 0xc) = w1 > 20: goto +0x6 <foo+0xd8> > 21: w1 = 0x5 > 22: *(u32 *)(r10 - 0xc) = w1 > 23: goto +0x3 <foo+0xd8> > 24: w1 = 0x13 > 25: *(u32 *)(r10 - 0xc) = w1 > 26: goto +0x0 <foo+0xd8> > 27: w1 = *(u32 *)(r10 - 0xc) > 28: call -0x1 > 00000000000000e0: R_BPF_64_32 bar > 29: w0 += -0x1 > 30: w1 = w0 > 31: *(u64 *)(r10 - 0x20) = r1 > 32: if w0 > 0x2b goto +0x16 <foo+0x1b8> > 33: r2 = *(u64 *)(r10 - 0x20) > 34: r2 <<= 0x3 > 35: r1 = 0x100 ll > 0000000000000118: R_BPF_64_64 .jumptables > 37: r1 += r2 > 38: r1 = *(u64 *)(r1 + 0x0) > 39: gotox r1 > 40: w1 = 0x5 > 41: *(u32 *)(r10 - 0xc) = w1 > 42: goto +0xf <foo+0x1d0> > 43: w1 = 0x7 > 44: *(u32 *)(r10 - 0xc) = w1 > 45: goto +0xc <foo+0x1d0> > 46: w1 = 0x17 > 47: *(u32 *)(r10 - 0xc) = w1 > 48: goto +0x9 <foo+0x1d0> > 49: w1 = 0x25 > 50: *(u32 *)(r10 - 0xc) = w1 > 51: goto +0x6 <foo+0x1d0> > 52: w1 = 0x4d > 53: *(u32 *)(r10 - 0xc) = w1 > 54: goto +0x3 <foo+0x1d0> > 55: w1 = 0xb > 56: *(u32 *)(r10 - 0xc) = w1 > 57: goto +0x0 <foo+0x1d0> > 58: w0 = *(u32 *)(r10 - 0xc) > 59: exit >
