[EVM] Add custom implementation of isBlockOnlyReachableByFallthrough.

Default implementation assumes that there should not be non-terminator
instructions between terminator ones. As a result it may mistakenly
conclude that a MBB is fall-through, whereas it's not.

llvm/lib/Target/EVM/EVMAsmPrinter.cpp

@@ -42,6 +42,12 @@ class EVMAsmPrinter : public AsmPrinter {
   void emitInstruction(const MachineInstr *MI) override;
 
   void emitFunctionEntryLabel() override;
+
+  /// Return true if the basic block has exactly one predecessor and the control
+  /// transfer mechanism between the predecessor and this block is a
+  /// fall-through.
+  bool isBlockOnlyReachableByFallthrough(
+      const MachineBasicBlock *MBB) const override;
 };
 } // end of anonymous namespace
 
@@ -70,12 +76,43 @@ void EVMAsmPrinter::emitFunctionEntryLabel() {
 void EVMAsmPrinter::emitInstruction(const MachineInstr *MI) {
   EVMMCInstLower MCInstLowering(OutContext, *this, VRegMapping,
                                 MF->getRegInfo());
-
   MCInst TmpInst;
   MCInstLowering.Lower(MI, TmpInst);
   EmitToStreamer(*OutStreamer, TmpInst);
 }
 
+bool EVMAsmPrinter::isBlockOnlyReachableByFallthrough(
+    const MachineBasicBlock *MBB) const {
+  if (!AsmPrinter::isBlockOnlyReachableByFallthrough(MBB))
+    return false;
+
+  // Check terminators in more details. After the stackification we may have the
+  // following case:
+
+  //   PUSH_LABEL %bb.11
+  //   JUMPI %bb.11, %27:gpr..
+  //   PUSH_LABEL %bb.9
+  //   JUMP %bb.9
+  // bb.11:
+  //
+  // The default implementation checks only the first terminator, JUMP %bb.9
+  // and mistakenly concludes MBB is only reachable by fallthrough.
+  // There should be only one predecessor, otherwise
+  // AsmPrinter::isBlockOnlyReachableByFallthrough() should have returned false.
+  assert(MBB->pred_size() == 1);
+  MachineBasicBlock *Pred = *MBB->pred_begin();
+  MachineBasicBlock::const_reverse_iterator I = Pred->rbegin(),
+                                            E = Pred->rend();
+  while (I != E) {
+    if (I->isUnconditionalBranch() || I->isConditionalBranch())
+      for (const auto &MO : I->explicit_uses())
+        if (MO.isMBB() && MO.getMBB() == MBB)
+          return false;
+    ++I;
+  }
+  return true;
+}
+
 extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeEVMAsmPrinter() {
   const RegisterAsmPrinter<EVMAsmPrinter> X(getTheEVMTarget());
 }

llvm/lib/Target/EVM/MCTargetDesc/EVMAsmBackend.cpp

@@ -138,7 +138,9 @@ bool EVMAsmBackend::evaluateTargetFixup(const MCAssembler &Asm,
   Value = Target.getConstant();
   if (const MCSymbolRefExpr *A = Target.getSymA()) {
     const MCSymbol &Sym = A->getSymbol();
-    assert(Sym.isDefined());
+    if (!Sym.isDefined())
+      report_fatal_error("Undef symbol: " + Sym.getName());
+
     Value += Layout.getSymbolOffset(Sym);
     return true;
   }