[llvm-objdump][NFC] Make the PrettyPrinter::printInst() output buffered

Summary:
Every time PrettyPrinter::printInst is called, stdout is flushed and it makes llvm-objdump slow. This patches adds a string
buffer to prevent stdout from being flushed.

Benchmark results (./llvm-objdump-master: without this patch,  ./bin/llvm-objcopy: with this patch):

  $ hyperfine --warmup 10 './llvm-objdump-master -d ./bin/llvm-objcopy' './bin/llvm-objdump -d ./bin/llvm-objcopy'
  Benchmark #1: ./llvm-objdump-master -d ./bin/llvm-objcopy
    Time (mean ± σ):      2.230 s ±  0.050 s    [User: 1.533 s, System: 0.682 s]
    Range (min … max):    2.115 s …  2.278 s    10 runs

  Benchmark #2: ./bin/llvm-objdump -d ./bin/llvm-objcopy
    Time (mean ± σ):     386.4 ms ±  13.0 ms    [User: 376.6 ms, System: 6.1 ms]
    Range (min … max):   366.1 ms … 407.0 ms    10 runs

  Summary
    './bin/llvm-objdump -d ./bin/llvm-objcopy' ran
      5.77 ± 0.23 times faster than './llvm-objdump-master -d ./bin/llvm-objcopy'

Reviewers: alexshap, Bigcheese, jhenderson, rupprecht, grimar, MaskRay

Reviewed By: jhenderson, MaskRay

Subscribers: dexonsmith, jhenderson, javed.absar, kristof.beyls, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64969

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366984 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-objdump/llvm-objdump.cpp

@@ -668,24 +668,20 @@ class PrettyPrinter {
     if (SP && (PrintSource || PrintLines))
       SP->printSourceLine(OS, Address);
 
-    {
-      formatted_raw_ostream FOS(OS);
-      if (!NoLeadingAddr)
-        FOS << format("%8" PRIx64 ":", Address.Address);
-      if (!NoShowRawInsn) {
-        FOS << ' ';
-        dumpBytes(Bytes, FOS);
-      }
-      FOS.flush();
-      // The output of printInst starts with a tab. Print some spaces so that
-      // the tab has 1 column and advances to the target tab stop.
-      unsigned TabStop = NoShowRawInsn ? 16 : 40;
-      unsigned Column = FOS.getColumn();
-      FOS.indent(Column < TabStop - 1 ? TabStop - 1 - Column : 7 - Column % 8);
-
-      // The dtor calls flush() to ensure the indent comes before printInst().
+    size_t Start = OS.tell();
+    if (!NoLeadingAddr)
+      OS << format("%8" PRIx64 ":", Address.Address);
+    if (!NoShowRawInsn) {
+      OS << ' ';
+      dumpBytes(Bytes, OS);
     }
 
+    // The output of printInst starts with a tab. Print some spaces so that
+    // the tab has 1 column and advances to the target tab stop.
+    unsigned TabStop = NoShowRawInsn ? 16 : 40;
+    unsigned Column = OS.tell() - Start;
+    OS.indent(Column < TabStop - 1 ? TabStop - 1 - Column : 7 - Column % 8);
+
     if (MI)
       IP.printInst(MI, OS, "", STI);
     else