[AsmWriter] Ensure getMnemonic doesn't return invalid pointers

[pratlucas]

For instructions that don't map to a mnemonic string, the implementation
of MCInstPrinter::getMnemonic would return an invalid pointer due to the
result of the calculation of the instruction's position in the AsmStrs
table. This patch fixes the issue by ensuring those cases return a
nullptr value instead.

Fixes #74177.

[llvmbot]

@llvm/pr-subscribers-mc

Author: Lucas Duarte Prates (pratlucas)

Changes

For instructions that don't map to a mnemonic string, the implementation
of MCInstPrinter::getMnemonic would return an invalid pointer due to the
result of the calculation of the instruction's position in the AsmStrs
table. This patch fixes the issue by ensuring those cases return a
nullptr value instead.

Fixes #74177.

---

Full diff: https://github.com/llvm/llvm-project/pull/75783.diff

2 Files Affected:

• (modified) llvm/lib/MC/MCAsmStreamer.cpp (+4-1)
• (modified) llvm/utils/TableGen/AsmWriterEmitter.cpp (+4)

diff --git a/llvm/lib/MC/MCAsmStreamer.cpp b/llvm/lib/MC/MCAsmStreamer.cpp
index 9e1d108ac14dc5..532ac89bf9ff76 100644
--- a/llvm/lib/MC/MCAsmStreamer.cpp
+++ b/llvm/lib/MC/MCAsmStreamer.cpp
@@ -154,7 +154,10 @@ class MCAsmStreamer final : public MCStreamer {
   void emitGNUAttribute(unsigned Tag, unsigned Value) override;
 
   StringRef getMnemonic(MCInst &MI) override {
-    return InstPrinter->getMnemonic(&MI).first;
+    std::pair<const char *, uint64_t> M = InstPrinter->getMnemonic(&MI);
+    assert((M.second != 0 || M.first == nullptr) &&
+           "Invalid char pointer for instruction with no mnemonic");
+    return M.first;
   }
 
   void emitLabel(MCSymbol *Symbol, SMLoc Loc = SMLoc()) override;
diff --git a/llvm/utils/TableGen/AsmWriterEmitter.cpp b/llvm/utils/TableGen/AsmWriterEmitter.cpp
index 0220927295cf78..e0cd5fad3254de 100644
--- a/llvm/utils/TableGen/AsmWriterEmitter.cpp
+++ b/llvm/utils/TableGen/AsmWriterEmitter.cpp
@@ -438,6 +438,10 @@ void AsmWriterEmitter::EmitGetMnemonic(
   O << "  // Emit the opcode for the instruction.\n";
   O << BitsString;
 
+  // Make sure we don't return an invalid pointer if bits is 0
+  O << "  if (Bits == 0)\n"
+       "    return {nullptr, Bits};\n";
+
   // Return mnemonic string and bits.
   O << "  return {AsmStrs+(Bits & " << (1 << AsmStrBits) - 1
     << ")-1, Bits};\n\n";

[tmatheson-arm]

LGMT, surprised this didn't blow up earlier.