[BPF] Emit fatal error if out of range for FK_PCRel_2 branch target

[yonghong-song]

Summary

Currently for the branch insn like

"if $dst "#OpcodeStr#" $imm goto $BrDst"

The $BrDst range needs to be in the range of [INT16_MIN, INT16_MAX].

When running bpf selftest with latest llvm, I found
pyperf600.o generated insn with range outside
of [INT16_MIN, INT16_MAX], which caused verifier failure.
See below insn #12.

0000000000000000 <on_event>:
; { 
       0:       7b 1a 00 ff 00 00 00 00 *(u64 *)(r10 - 256) = r1
;       uint64_t pid_tgid = bpf_get_current_pid_tgid();
       1:       85 00 00 00 0e 00 00 00 call 14
       2:       bf 06 00 00 00 00 00 00 r6 = r0
;       pid_t pid = (pid_t)(pid_tgid >> 32);
       3:       bf 61 00 00 00 00 00 00 r1 = r6
       4:       77 01 00 00 20 00 00 00 r1 >>= 32
       5:       63 1a fc ff 00 00 00 00 *(u32 *)(r10 - 4) = r1
       6:       bf a2 00 00 00 00 00 00 r2 = r10 
       7:       07 02 00 00 fc ff ff ff r2 += -4
;       PidData* pidData = bpf_map_lookup_elem(&pidmap, &pid);
       8:       18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
      10:       85 00 00 00 01 00 00 00 call 1
      11:       bf 08 00 00 00 00 00 00 r8 = r0
;       if (!pidData)
      12:       15 08 15 e8 00 00 00 00 if r8 == 0 goto -6123 <LBB0_27588+0xffffffffffdae100>
      13:       b4 01 00 00 00 00 00 00 w1 = 0

We may need to add new insn to extend the range of $BrDst.
This patch added fatal error if out of range so compiler can warn
the otherwise incorrect code generation.

[ast]

assert is a heavy hammer.
can we exit with report_fatal_error instead?

[yonghong-song]

In D123877#3459461, @ast wrote:

assert is a heavy hammer.
can we exit with report_fatal_error instead?

Okay. Let me implement with a report_fatal_error.

[yonghong-song]

• use report_fatal_error instead of assert.

[ast]

Thanks!

llvm/lib/Target/BPF/MCTargetDesc/BPFAsmBackend.cpp

if (Endian == support::little) {
       Data[Fixup.getOffset() + 1] = 0x10;
       support::endian::write32le(&Data[Fixup.getOffset() + 4], Value);
     } else {
       Data[Fixup.getOffset() + 1] = 0x1;
       support::endian::write32be(&Data[Fixup.getOffset() + 4], Value);
     }
   } else {
     assert(Fixup.getKind() == FK_PCRel_2);
+    assert(((int64_t)Value - 8) <= INT16_MAX * 8 &&
+           ((int64_t)Value - 8) >= INT16_MIN * 8);
+
     Value = (uint16_t)((Value - 8) / 8);
     support::endian::write<uint16_t>(&Data[Fixup.getOffset() + 2], Value,
                                      Endian);
   }
 }
 
 std::unique_ptr<MCObjectTargetWriter>
 BPFAsmBackend::createObjectTargetWriter() const {