filterc: Optimize jumps

When one of the branches goes to the next insn, it is not necessary to emit
two eBPF insns. We can either leave out the second jump if the false case
is the next insn, or we can invert the condition and leave out the second
jump if the true case is the next insn.

filterc/bpfc.c

@@ -278,6 +278,36 @@ static int convert_cbpf(struct cbpf_program *cbpf_prog,
 				src_reg = bpf_src == BPF_X ? BPF_REG_X : 0;
 			}
 
+			// Only emit one jump if jump_false is next insn
+			if (fp->jf == 0) {
+				code = BPF_JMP | BPF_OP(fp->code) | bpf_src;
+				target = i + fp->jt + 1;
+				*insn++ = BPF_JMP_INSN(code, dst_reg, src_reg, imm, target);
+				break;
+			}
+
+			// Invert conditons where possible if jump_true is next insn
+			if (fp->jt == 0) {
+				switch (BPF_OP(fp->code)) {
+				case BPF_JEQ:
+					code = BPF_JMP | BPF_JNE | bpf_src;
+					break;
+				case BPF_JGT:
+					code = BPF_JMP | BPF_JLE | bpf_src;
+					break;
+				case BPF_JGE:
+					code = BPF_JMP | BPF_JLT | bpf_src;
+					break;
+				default:
+					goto jmp_rest;
+				}
+
+				target = i + fp->jf + 1;
+				*insn++ = BPF_JMP_INSN(code, dst_reg, src_reg, imm, target);
+				break;
+			}
+
+jmp_rest:
 			/* Other jumps are mapped into two insns: Jxx and JA. */
 			code = BPF_JMP | BPF_OP(fp->code) | bpf_src;
 			target = i + fp->jt + 1;