On 25/07/08 01:59PM, Eduard Zingerman wrote: > On Tue, 2025-06-17 at 20:22 -0700, Alexei Starovoitov wrote: > > On Sun, Jun 15, 2025 at 1:55 AM Anton Protopopov > > <a.s.protopopov@xxxxxxxxx> wrote: > > > > > > The final line generates an indirect jump. The > > > format of the indirect jump instruction supported by BPF is > > > > > > BPF_JMP|BPF_X|BPF_JA, SRC=0, DST=Rx, off=0, imm=fd(M) > > > > > [...] > > > Uglier alternatives is to redesign the gotox encoding and > > drop ld_imm64 and *=8 altogether. > > Then gotox jmp_table[R5] will be like jumbo insn that > > does *=8 and load inside and JIT emits all that. > > But it's ugly and likely has other downsides. > > I talked to Alexei and Yonghong off-list, and we seem to be in > agreement that having a single gotox capturing both the map and the > offset looks more elegant. E.g.: > > gotox imm32[dst_reg]; > > Where imm32 is an fd of the map corresponding to the jump table, > and dst-reg is an offset inside the table (it could also be an index). > > So, instead of a current codegen: > > 0000000000000000 <foo>: > ... > 1: w1 = w1 > 2: r1 <<= 0x3 > 3: r2 = 0x0 ll > 0000000000000018: R_BPF_64_64 .BPF.JT.0.0 > 5: r2 += r1 > 6: r1 = *(u64 *)(r2 + 0x0) > 7: gotox r1 > 0000000000000038: R_BPF_64_64 .BPF.JT.0.0 > > LLVM would produce: > > 0000000000000000 <foo>: > ... > 1: w1 = w1 > 2: r1 <<= 0x3 > 3: gotox r1 > 0000000000000038: R_BPF_64_64 .BPF.JT.0.0 > > This sequence leaks a bit less implementation details and avoids a > check for correspondence between load and gotox instructions. > It will require using REG_AX on the jit side. > LLVM side implementation is not hard, as it directly maps to `br_jt` > selection DAG instruction. > > Anton, wdyt? I think that this is exactly what I had proposed originally in [1], so yes, IMO this looks more elegant indeed. (Back then the feedback was that this is too esoteric, and instead the verifier should be taught to eat what LLVM generates (<<3 and load).) The instruction can be extended (SRC and OFF are unused) to support more formats later. > 3: gotox r1 > 0000000000000038: R_BPF_64_64 .BPF.JT.0.0 How hard is to teach the LLVM to generate this? [1] https://lpc.events/event/18/contributions/1941/
