On Fri, Jan 5, 2018 at 7:01 AM, Charlemagne Lasse <charlemagnela...@gmail.com> wrote: > Hi, > > just tested the clang eBPF backend. Very cool thing :) > > But I've noticed that the codegen creates really odd things which I > cannot explain from my understanding of the ISA. My test is attached > and was compiled via: > > clang -O2 -target bpf -x c -c ebpf_opimizer_problems.o -o > ebpf_opimizer_problems.o > llvm-objdump -S ebpf_opimizer_problems.o > > First thing is the odd way how 8 bit loads to an uint8_t are handled > (see bug1_sec): > > 0: bf 16 00 00 00 00 00 00 r6 = r1 > 1: 30 00 00 00 00 00 00 00 r0 = *(u8 *)skb[0] > 2: 57 00 00 00 ff ff 00 00 r0 &= 65535 > 3: 95 00 00 00 00 00 00 00 exit > > I have no idea why the second instruction (line "2:") would be > required when there was just a load of 8 bit to the 64 bit register. > This isn't generated when it is stored in an uint32_t or uint64_t > variable (but also when storing it in an uint16_t). This can be seen > in ok1_sec, ok1_too_a_sec and ok1_too_b_sec. > > The other problem is the chaining of tests which results in an extra > store for the temporary output register (r7, see bug2_sec): > > 0: bf 16 00 00 00 00 00 00 r6 = r1 > 1: b7 07 00 00 00 00 00 00 r7 = 0 > 2: 30 00 00 00 00 00 00 00 r0 = *(u8 *)skb[0] > 3: 55 00 04 00 01 00 00 00 if r0 != 1 goto 4 > 4: 30 00 00 00 01 00 00 00 r0 = *(u8 *)skb[1] > 5: b7 07 00 00 15 00 00 00 r7 = 21 > 6: 15 00 01 00 01 00 00 00 if r0 == 1 goto 1 > 7: b7 07 00 00 00 00 00 00 r7 = 0 > > LBB4_3: > 8: bf 70 00 00 00 00 00 00 r0 = r7 > 9: 95 00 00 00 00 00 00 00 exit > > For unknown reasons, the line "6:" was changed from an JNE to an JEQ. > This makes it necessary to introduce link "5:" and line "7:". It would > have been better (in my eyes) to use keep it an JNE, move line "5:" > after the comparison and drop line "7:". This would look like this > (removed the bytecode to make it easier for me): > > 0: r6 = r1 > 1: r7 = 0 > 2: r0 = *(u8 *)skb[0] > 3: if r0 != 1 goto 3 > 4: r0 = *(u8 *)skb[1] > 5: if r0 != 1 goto 1 > 6: r7 = 21 > LBB4_3: > 7: r0 = r7 > 8: exit > > Or a different version (not so easily readable because it is > asymmetric) would have been: > > 0: bf 16 00 00 00 00 00 00 r6 = r1 > 1: 30 00 00 00 00 00 00 00 r0 = *(u8 *)skb[0] > 2: 55 00 03 00 01 00 00 00 if r0 != 1 goto 3 > 3: b7 07 00 00 15 00 00 00 r7 = 21 > 4: 30 00 00 00 01 00 00 00 r0 = *(u8 *)skb[1] > 5: 15 00 01 00 01 00 00 00 if r0 == 1 goto 1 > > LBB4_2: > 6: b7 07 00 00 00 00 00 00 r7 = 0 > > LBB4_3: > 7: bf 70 00 00 00 00 00 00 r0 = r7 > 8: 95 00 00 00 00 00 00 00 exit > > > The used version (Debian buster amd64) is: > > clang version 5.0.1-2 (tags/RELEASE_501/final) > Target: x86_64-pc-linux-gnu > Thread model: posix > InstalledDir: /usr/bin
thank you for reporting these issues. It does look odd. Not sure whether it's related to any recent changes. Needs more investigation. _______________________________________________ cfe-users mailing list cfe-users@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
