[X86] Set Int_MemBarrier as a meta-instruction

[skan]

Summary

Compiler only emits a comment for Int_MemBarrier, so it should
be marked as a meta-instruction, which can help improve accuracy
of debug location.

[pengfei]

LGTM.

llvm/lib/Target/X86/X86InstrCompiler.td

 // Memory barriers
 
 let isCodeGenOnly = 1, Defs = [EFLAGS] in
 def OR32mi8Locked  : Ii8<0x83, MRM1m, (outs), (ins i32mem:$dst, i32i8imm:$zero),
                          "or{l}\t{$zero, $dst|$dst, $zero}", []>,
                          Requires<[Not64BitMode]>, OpSize32, LOCK,
                          Sched<[WriteALURMW]>;
 
-let hasSideEffects = 1 in
+let hasSideEffects = 1, isMeta = 1 in
 def Int_MemBarrier : I<0, Pseudo, (outs), (ins),
                      "#MEMBARRIER",
                      [(X86MemBarrier)]>, Sched<[WriteLoad]>;
 
 // RegOpc corresponds to the mr version of the instruction
 // ImmOpc corresponds to the mi version of the instruction
 // ImmOpc8 corresponds to the mi8 version of the instruction
 // ImmMod corresponds to the instruction format of the mi and mi8 versions

llvm/test/DebugInfo/X86/dwarf-mem-barrier.ll

+; Check .loc directive is emitted at correct position
+; RUN: llc -O0 -mtriple x86_64-linux-gnu <%s | FileCheck %s
+
+; CHECK:  #MEMBARRIER
+; CHECK:  #MEMBARRIER
+; CHECK:  movl  $0, -4(%rsp)
+; CHECK:  .loc  1 1 14 prologue_end             # none.c:1:14
+
+define dso_local i32 @main() !dbg !7 {
+entry:
+  fence acquire, !dbg !9
+  fence release, !dbg !9
+  %retval = alloca i32, align 4
+  store i32 0, i32* %retval, align 4
+  ret i32 0, !dbg !9
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang", isOptimized: false, runtimeVersion: 0, emissionKind: DebugDirectivesOnly, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "none.c", directory: "/temp")
+!2 = !{}
+!3 = !{i32 2, !"Tracing", i32 1}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 11.0.0"}
+!7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 1, type: !8, scopeLine: 1, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !2)
+!9 = !DILocation(line: 1, column: 14, scope: !7)

llvm/unittests/MIR/MachineMetadata.cpp

body:             |
   EH_LABEL 0
   GC_LABEL 0
   DBG_VALUE $rax, $noreg, !13, !DIExpression(), debug-location !14
   DBG_LABEL 0
   LIFETIME_START 0
   LIFETIME_END 0
   PSEUDO_PROBE 6699318081062747564, 1, 0, 0
   $xmm0 = ARITH_FENCE $xmm0
+  Int_MemBarrier
 ...
 )MIR";
 
   MachineModuleInfo MMI(TM.get());
   M = parseMIR(*TM, MIRString, "test0", MMI);
   ASSERT_TRUE(M);
 
   auto *MF = MMI.getMachineFunction(*M->getFunction("test0"));