MC: Skip names of temporary symbols in object streamer

Don't create names for temporary symbols when using an object streamer.
The names never make it to the output anyway.  From the starting point
of r236629, my heap profile says this drops peak memory usage from 1100
MB to 1058 MB for CodeGen of `verify-uselistorder`, a savings of almost
4% on peak memory, and removes `StringMap<bool, BumpPtrAllocator...>`
from the profile entirely.

(I'm looking at `llc` memory usage on `verify-uselistorder.lto.opt.bc`;
see r236629 for details.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236642 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCContext.h

@@ -158,6 +158,7 @@ namespace llvm {
     /// differences between temporary and non-temporary labels (primarily on
     /// Darwin).
     bool AllowTemporaryLabels;
+    bool UseNamesOnTempLabels = true;
 
     /// The Compile Unit ID that we are currently processing.
     unsigned DwarfCompileUnitID;
@@ -224,6 +225,7 @@ namespace llvm {
     const MCObjectFileInfo *getObjectFileInfo() const { return MOFI; }
 
     void setAllowTemporaryLabels(bool Value) { AllowTemporaryLabels = Value; }
+    void setUseNamesOnTempLabels(bool Value) { UseNamesOnTempLabels = Value; }
 
     /// \name Module Lifetime Management
     /// @{

lib/CodeGen/LLVMTargetMachine.cpp

@@ -198,6 +198,9 @@ bool LLVMTargetMachine::addPassesToEmitFile(
     if (!MCE || !MAB)
       return true;
 
+    // Don't waste memory on names of temp labels.
+    Context->setUseNamesOnTempLabels(false);
+
     Triple T(getTargetTriple());
     AsmStreamer.reset(getTarget().createMCObjectStreamer(
         T, *Context, *MAB, Out, MCE, STI, Options.MCOptions.MCRelaxAll,

lib/MC/MCContext.cpp

@@ -150,6 +150,9 @@ MCSymbol *MCContext::CreateSymbol(StringRef Name, bool AlwaysAddSuffix) {
   if (AllowTemporaryLabels)
     IsTemporary = Name.startswith(MAI->getPrivateGlobalPrefix());
 
+  if (IsTemporary && AlwaysAddSuffix && !UseNamesOnTempLabels)
+    return new (*this) MCSymbol("", true);
+
   SmallString<128> NewName = Name;
   bool AddSuffix = AlwaysAddSuffix;
   unsigned &NextUniqueID = NextID[Name];