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.
$ 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
