[ELF] Implement --stats

[davide]

Summary

I used this quite a bit for some internal benchmarking, and seems to be working (at least for the time side). GetMallocUsage() appears to be broken on FreeBSD-11 + tcmalloc(), and I'll investigate on the reason, but I would like to get another pair of eye on this patch in the meanwhile.
I can write an unit test, if needed.

[davide]

D7663 takes care of GetMallocUsage() bits. My previous comment should be read as GetMallocUsage() appears to be broken on FreeBSD-11 + jemalloc" and not as "GetMallocUsage() appears to be broken on FreeBSD-11 + tcmalloc".

[davide]

r229824 is in llvm, so this can be reviewed now.

[ruiu]

LGTM

lib/Driver/GnuLdDriver.cpp

181

total -> Total

"data size" may be confusing -- this can be read as the total number of bytes read from disk, for example. Needs a bit of wordsmithing.

[shankarke]

should this be a general option in LinkingContext ? Why is this specific to the GnuFlavor ? Are you planning to add the stats collection for individual passes that run during Linkstep ?

[davide]

lib/Driver/GnuLdDriver.cpp

181

This is what GNU ld uses. I agree with you it's a poorly chosen sequence of words, so we need to decide if we want to stick with what they currently output or introduce our own notion of statistics. At this point I can also say that we can output something more meaningful/fine-grained.
Considering this isn't a very common option for ld(1) we can probably go for the path of breaking compatibility but I would like to have a more detailed discussion about it before taking any actions.

[ruiu]

LGTM

lib/Driver/GnuLdDriver.cpp

181

Ah, it's okay then. It's better to keep the same wording as the GNU linker if there's no reason to change them. I'm also okay to keep it for the GNU driver only for now as we don't know what command line option should be used to enable this feature for other drivers. (For PE/COFF, it should be enabled when /verbose is given? Perhaps. But I don't know.) It's pretty easy to move this to Driver in future.

include/lld/ReaderWriter/ELFLinkingContext.h

   bool stripSymbols() const { return _stripSymbols; }
   void setStripSymbols(bool strip) { _stripSymbols = strip; }
 
+  /// \brief Collect statistics.
+  bool collectStats() const { return _collectStats; }
+  void setCollectStats(bool s) { _collectStats = s; }
+
   // We can parse several linker scripts via command line whose ASTs are stored
   // in the current linking context via addLinkerScript().
   void addLinkerScript(std::unique_ptr<script::Parser> script) {

   bool _stripSymbols;
   bool _alignSegments;
   bool _nostdlib;
+  bool _collectStats;
   llvm::Optional<uint64_t> _maxPageSize;
 
   OutputMagic _outputMagic;

lib/Driver/GnuLdDriver.cpp

 #include "llvm/Support/Path.h"
 #include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/Signals.h"
+#include "llvm/Support/Timer.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cstring>
 #include <tuple>

     return false;
   if (!options)
     return true;
-  return link(*options, diag);
+  bool linked = link(*options, diag);
+
+  // Handle --stats.
+  if (options->collectStats()) {
+    llvm::TimeRecord t = llvm::TimeRecord::getCurrentTime(true);
+    diag << "total time in link " << t.getProcessTime() << "\n";
+    diag << "data size " << t.getMemUsed() << "\n";
+  }
+  return linked;
 }
 
 static llvm::Optional<llvm::Triple::ArchType>

     ctx->setAllowRemainingUndefines(true);
   }
 
+  // Handle --stats.
+  if (parsedArgs->hasArg(OPT_stats)) {
+    ctx->setCollectStats(true);
+  }
+
   // Figure out if the output type is nmagic/omagic
   if (auto *arg = parsedArgs->getLastArg(
         OPT_nmagic, OPT_omagic, OPT_no_omagic)) {

lib/Driver/GnuLdOptions.td

 def t : Flag<["-"], "t">,
      HelpText<"Print the names of the input files as ld processes them">,
      Group<grp_tracingopts>;
+def stats : Flag<["--"], "stats">,
+     HelpText<"Print time and memory usage stats">, Group<grp_tracingopts>;
 
 //===----------------------------------------------------------------------===//
 /// Extensions

lib/ReaderWriter/ELF/ELFLinkingContext.cpp

       _mergeCommonStrings(false), _useShlibUndefines(true),
       _dynamicLinkerArg(false), _noAllowDynamicLibraries(false),
       _mergeRODataToTextSegment(true), _demangle(true),
-      _stripSymbols(false), _alignSegments(true),
+      _stripSymbols(false), _collectStats(false), _alignSegments(true),
       _outputMagic(OutputMagic::DEFAULT), _initFunction("_init"),
       _finiFunction("_fini"), _sysrootPath("") {}