[NFC][MIR] Fix extra whitespace in MIR printing (#162928)

Fix a whitespace regression in MIR printing that was introduced in
#137361.

The default value for `ListSeparator` is `", "`, so we don't need to
print an additional space in front of tokens for optional symbols and
other things printed after operands.

Note, the modified LIT test will fail at trunk without the fix,
demonstrating that the extra space before `, pre-instr-symbol <mcsymbol
>` on Line 63 exists currently and is fixed with this change.

Author: jurahul

llvm/include/llvm/ADT/StringExtras.h

@@ -529,13 +529,15 @@ inline std::string join_items(Sep Separator, Args &&... Items) {
 class ListSeparator {
   bool First = true;
   StringRef Separator;
+  StringRef Prefix;
 
 public:
-  ListSeparator(StringRef Separator = ", ") : Separator(Separator) {}
+  ListSeparator(StringRef Separator = ", ", StringRef Prefix = "")
+      : Separator(Separator), Prefix(Prefix) {}
   operator StringRef() {
     if (First) {
       First = false;
-      return {};
+      return Prefix;
     }
     return Separator;
   }

llvm/lib/CodeGen/MIRPrinter.cpp

@@ -862,48 +862,46 @@ static void printMI(raw_ostream &OS, MFPrintState &State,
 
   OS << TII->getName(MI.getOpcode());
 
-  LS = ListSeparator();
+  // Print a space after the opcode if any additional tokens are printed.
+  LS = ListSeparator(", ", " ");
 
-  if (I < E) {
-    OS << ' ';
-    for (; I < E; ++I) {
-      OS << LS;
-      printMIOperand(OS, State, MI, I, TRI, TII, ShouldPrintRegisterTies,
-                     PrintedTypes, MRI, /*PrintDef=*/true);
-    }
+  for (; I < E; ++I) {
+    OS << LS;
+    printMIOperand(OS, State, MI, I, TRI, TII, ShouldPrintRegisterTies,
+                   PrintedTypes, MRI, /*PrintDef=*/true);
   }
 
   // Print any optional symbols attached to this instruction as-if they were
   // operands.
   if (MCSymbol *PreInstrSymbol = MI.getPreInstrSymbol()) {
-    OS << LS << " pre-instr-symbol ";
+    OS << LS << "pre-instr-symbol ";
     MachineOperand::printSymbol(OS, *PreInstrSymbol);
   }
   if (MCSymbol *PostInstrSymbol = MI.getPostInstrSymbol()) {
-    OS << LS << " post-instr-symbol ";
+    OS << LS << "post-instr-symbol ";
     MachineOperand::printSymbol(OS, *PostInstrSymbol);
   }
   if (MDNode *HeapAllocMarker = MI.getHeapAllocMarker()) {
-    OS << LS << " heap-alloc-marker ";
+    OS << LS << "heap-alloc-marker ";
     HeapAllocMarker->printAsOperand(OS, State.MST);
   }
   if (MDNode *PCSections = MI.getPCSections()) {
-    OS << LS << " pcsections ";
+    OS << LS << "pcsections ";
     PCSections->printAsOperand(OS, State.MST);
   }
   if (MDNode *MMRA = MI.getMMRAMetadata()) {
-    OS << LS << " mmra ";
+    OS << LS << "mmra ";
     MMRA->printAsOperand(OS, State.MST);
   }
   if (uint32_t CFIType = MI.getCFIType())
-    OS << LS << " cfi-type " << CFIType;
+    OS << LS << "cfi-type " << CFIType;
 
   if (auto Num = MI.peekDebugInstrNum())
-    OS << LS << " debug-instr-number " << Num;
+    OS << LS << "debug-instr-number " << Num;
 
   if (PrintLocations) {
     if (const DebugLoc &DL = MI.getDebugLoc()) {
-      OS << LS << " debug-location ";
+      OS << LS << "debug-location ";
       DL->printAsOperand(OS, State.MST);
     }
   }

llvm/test/CodeGen/MIR/AArch64/return-address-signing.mir

@@ -1,4 +1,4 @@
-# RUN: llc -mtriple=aarch64 -run-pass=prologepilog -run-pass=aarch64-ptrauth -o - %s 2>&1 | FileCheck %s
+# RUN: llc -mtriple=aarch64 -run-pass=prologepilog -run-pass=aarch64-ptrauth -o - %s 2>&1 | FileCheck --strict-whitespace %s
 --- |
   target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
   target triple = "aarch64"

llvm/unittests/ADT/StringExtrasTest.cpp

@@ -290,6 +290,12 @@ TEST(StringExtrasTest, ListSeparator) {
   EXPECT_EQ(S, "");
   S = LS2;
   EXPECT_EQ(S, " ");
+
+  ListSeparator LS3(",", "{");
+  S = LS3;
+  EXPECT_EQ(S, "{");
+  S = LS3;
+  EXPECT_EQ(S, ",");
 }
 
 TEST(StringExtrasTest, toStringAPInt) {