[lld] [llvm] [llvm-objdump][ARM] Find ELF file PLT entries for arm, thumb (PR #130764)

[Peter Smith via llvm-commits]

================
@@ -431,6 +431,112 @@ class ARMMCInstrAnalysis : public MCInstrAnalysis {
   std::optional<uint64_t>
   evaluateMemoryOperandAddress(const MCInst &Inst, const MCSubtargetInfo *STI,
                                uint64_t Addr, uint64_t Size) const override;
+
+  std::vector<std::pair<uint64_t, uint64_t>> findPltEntries(
+      uint64_t PltSectionVA, ArrayRef<uint8_t> PltContents,
+      const Triple &TargetTriple,
+      std::optional<llvm::endianness> InstrEndiannessHint) const override {
+    llvm::endianness DataEndianness =
+        TargetTriple.isLittleEndian() ? endianness::little : endianness::big;
+    llvm::endianness InstrEndianness =
+        InstrEndiannessHint.value_or(DataEndianness);
+
+    std::vector<std::pair<uint64_t, uint64_t>> Result;
+    // Do a lightweight parsing of PLT entries.
+    for (uint64_t Byte = 0, End = PltContents.size(); Byte + 12 < End;
+         Byte += 4) {
+      uint32_t InsnPart1 =
+          support::endian::read16(PltContents.data() + Byte, InstrEndianness);
+
+      // Is Thumb? Check for movw.
----------------
smithp35 wrote:

I think this kind test can be hoisted out of the loop, and cached, although the first entry of the Thumb2 PLT won't be a mov as it will be the PLT header. 

All PLT entries will be Arm state or Thumb2. If you've access to the symbol table, the canonical way of determining whether Arm or Thumb is from the mapping symbol at address 0, $a for Arm or $t for Thumb.

It may even be a bit easier to read if we have something like (pseudo code)
```
test if Arm or Thumb
// Arm is the most common case.
if Arm {
  for(...) {

  }
}
else {
  for (...) {

  }
}
```

https://github.com/llvm/llvm-project/pull/130764