On Fri, Jan 31, 2020 at 9:16 AM John Fastabend <john.fastabend@xxxxxxxxx> wrote:
>
> Also don't mind to build pseudo instruction here for signed extension
> but its not clear to me why we are getting different instruction
> selections? Its not clear to me why sext is being chosen in your case?
Sign extension has to be there if jmp64 is used.
So the difference is due to -mcpu=v2 vs -mcpu=v3
v2 does alu32, but not jmp32
v3 does both.
By default selftests are using -mcpu=probe which
detects v2/v3 depending on running kernel.
llc -mattr=dwarfris -march=bpf -mcpu=v3 -mattr=+alu32
; usize = bpf_get_stack(ctx, raw_data, max_len, BPF_F_USER_STACK);
48: bf 61 00 00 00 00 00 00 r1 = r6
49: bf 72 00 00 00 00 00 00 r2 = r7
50: b4 03 00 00 20 03 00 00 w3 = 800
51: b7 04 00 00 00 01 00 00 r4 = 256
52: 85 00 00 00 43 00 00 00 call 67
53: bc 08 00 00 00 00 00 00 w8 = w0
; if (usize < 0)
54: c6 08 16 00 00 00 00 00 if w8 s< 0 goto +22 <LBB0_6>
; ksize = bpf_get_stack(ctx, raw_data + usize, max_len - usize, 0);
55: 1c 89 00 00 00 00 00 00 w9 -= w8
56: bc 81 00 00 00 00 00 00 w1 = w8
57: 67 01 00 00 20 00 00 00 r1 <<= 32
58: 77 01 00 00 20 00 00 00 r1 >>= 32
59: bf 72 00 00 00 00 00 00 r2 = r7
60: 0f 12 00 00 00 00 00 00 r2 += r1
61: bf 61 00 00 00 00 00 00 r1 = r6
62: bc 93 00 00 00 00 00 00 w3 = w9
63: b7 04 00 00 00 00 00 00 r4 = 0
64: 85 00 00 00 43 00 00 00 call 67
; if (ksize < 0)
65: c6 00 0b 00 00 00 00 00 if w0 s< 0 goto +11 <LBB0_6>
llc -mattr=dwarfris -march=bpf -mcpu=v2 -mattr=+alu32
; usize = bpf_get_stack(ctx, raw_data, max_len, BPF_F_USER_STACK);
48: bf 61 00 00 00 00 00 00 r1 = r6
49: bf 72 00 00 00 00 00 00 r2 = r7
50: b4 03 00 00 20 03 00 00 w3 = 800
51: b7 04 00 00 00 01 00 00 r4 = 256
52: 85 00 00 00 43 00 00 00 call 67
53: bc 08 00 00 00 00 00 00 w8 = w0
; if (usize < 0)
54: bc 81 00 00 00 00 00 00 w1 = w8
55: 67 01 00 00 20 00 00 00 r1 <<= 32
56: c7 01 00 00 20 00 00 00 r1 s>>= 32
57: c5 01 19 00 00 00 00 00 if r1 s< 0 goto +25 <LBB0_6>
; ksize = bpf_get_stack(ctx, raw_data + usize, max_len - usize, 0);
58: 1c 89 00 00 00 00 00 00 w9 -= w8
59: bc 81 00 00 00 00 00 00 w1 = w8
60: 67 01 00 00 20 00 00 00 r1 <<= 32
61: 77 01 00 00 20 00 00 00 r1 >>= 32
62: bf 72 00 00 00 00 00 00 r2 = r7
63: 0f 12 00 00 00 00 00 00 r2 += r1
64: bf 61 00 00 00 00 00 00 r1 = r6
65: bc 93 00 00 00 00 00 00 w3 = w9
66: b7 04 00 00 00 00 00 00 r4 = 0
67: 85 00 00 00 43 00 00 00 call 67
; if (ksize < 0)
68: bc 01 00 00 00 00 00 00 w1 = w0
69: 67 01 00 00 20 00 00 00 r1 <<= 32
70: c7 01 00 00 20 00 00 00 r1 s>>= 32
71: c5 01 0b 00 00 00 00 00 if r1 s< 0 goto +11 <LBB0_6>
zext is there both cases and it will be optimized with your llvm patch.
So please send it. Don't delay :)
