[BOLT] Not use hlt as split point when build the CFG

For x86, the halt instruction is defined as a terminator instruction.
When building the CFG, the instruction sequence following the hlt
instruction is treated as an independent MBB. Since there is no
jump information, the predecessor of this MBB cannot be identified,
and it is considered an unreachable MBB that will be removed.

Using this fix, the instruction sequences before and after hlt are
refused to be placed in different blocks.

Author: Jianghibo

bolt/include/bolt/Core/MCPlusBuilder.h

@@ -740,6 +740,10 @@ class MCPlusBuilder {
     return false;
   }
 
+  /// Return true if the hlt instruction under the x86, otherwise, default to
+  /// false.
+  virtual bool isX86HLT(const MCInst &Inst) const { return false; }
+
   /// Return the width, in bytes, of the memory access performed by \p Inst, if
   /// this is a pop instruction. Return zero otherwise.
   virtual int getPopSize(const MCInst &Inst) const {

bolt/lib/Core/MCPlusBuilder.cpp

@@ -132,8 +132,10 @@ bool MCPlusBuilder::equals(const MCSpecifierExpr &A, const MCSpecifierExpr &B,
 }
 
 bool MCPlusBuilder::isTerminator(const MCInst &Inst) const {
-  return Analysis->isTerminator(Inst) ||
-         (opts::TerminalTrap && Info->get(Inst.getOpcode()).isTrap());
+  return (opts::TerminalTrap && Info->get(Inst.getOpcode()).isTrap()) ||
+                 Analysis->isTerminator(Inst)
+             ? !isX86HLT(Inst)
+             : false;
 }
 
 void MCPlusBuilder::setTailCall(MCInst &Inst) const {

bolt/lib/Target/X86/X86MCPlusBuilder.cpp

@@ -223,6 +223,10 @@ class X86MCPlusBuilder : public MCPlusBuilder {
     return Inst.getOpcode() == X86::ENDBR32 || Inst.getOpcode() == X86::ENDBR64;
   }
 
+  bool isX86HLT(const MCInst &Inst) const override {
+    return Inst.getOpcode() == X86::HLT;
+  }
+
   int getPopSize(const MCInst &Inst) const override {
     switch (Inst.getOpcode()) {
     case X86::POP16r:

bolt/test/X86/cfg_build_hlt.s

@@ -0,0 +1,17 @@
+## Check CFG for halt instruction
+
+# RUN: %clang %cflags %s -static -o %t.exe -nostdlib
+# RUN: llvm-bolt %t.exe --print-cfg --print-only=main -o %t 2>&1 | FileCheck %s --check-prefix=CHECK-CFG
+# RUN: llvm-objdump -d %t --print-imm-hex | FileCheck %s --check-prefix=CHECK-BIN
+
+# CHECK-CFG: BB Count    : 1
+# CHECK-BIN: <main>:
+# CHECK-BIN-NEXT: f4                            hlt
+# CHECK-BIN-NEXT: c3                            retq
+
+.global main
+  .type main, %function
+main:
+        hlt
+        retq
+.size main, .-main