Add option to skip writing an output file

Summary:
The user may wish to run BOLT for printing statistics only
(i.e. to check that the profile is valid). Add an option to run BOLT
without writing any output file, similar to a dry run. This option
is triggered by supplying -o with "/dev/null".

Reviewed By: maksfb

Differential Revision: D26347237

fbshipit-source-id: 14214eab075

src/RewriteInstance.cpp

@@ -58,6 +58,7 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/DataExtractor.h"
 #include "llvm/Support/Errc.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/TargetRegistry.h"
 #include "llvm/Support/TargetSelect.h"
@@ -836,6 +837,9 @@ void RewriteInstance::run() {
   if (opts::LinuxKernelMode) {
     errs() << "BOLT-WARNING: not writing the output file for Linux Kernel\n";
     return;
+  } else if (opts::OutputFilename == "/dev/null") {
+    outs() << "BOLT-INFO: skipping writing final binary to disk\n";
+    return;
   }
 
   // Rewrite allocatable contents and copy non-allocatable parts with mods.
@@ -2939,9 +2943,10 @@ void RewriteInstance::emitAndLink() {
 
   // This is an object file, which we keep for debugging purposes.
   // Once we decide it's useless, we should create it in memory.
+  SmallString<128> OutObjectPath;
+  sys::fs::getPotentiallyUniqueTempFileName("output", "o", OutObjectPath);
   std::unique_ptr<ToolOutputFile> TempOut =
-    llvm::make_unique<ToolOutputFile>(opts::OutputFilename + ".bolt.o",
-                                      EC, sys::fs::F_None);
+      llvm::make_unique<ToolOutputFile>(OutObjectPath, EC, sys::fs::F_None);
   check_error(EC, "cannot create output object file");
 
   std::unique_ptr<buffer_ostream> BOS =
@@ -3095,8 +3100,12 @@ void RewriteInstance::emitAndLink() {
     CacheMetrics::printAll(BC->getSortedFunctions());
   }
 
-  if (opts::KeepTmp)
+  if (opts::KeepTmp) {
     TempOut->keep();
+    outs() << "BOLT-INFO: intermediary output object file saved for debugging "
+              "purposes: "
+           << OutObjectPath << "\n";
+  }
 }
 
 void RewriteInstance::updateMetadata() {

test/X86/no-output.test

@@ -0,0 +1,29 @@
+# This script checks that BOLT is able to work in dry run mode (no output)
+
+# REQUIRES: system-linux
+
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown \
+# RUN:   %s -o %t.o
+# RUN: link_fdata %s %t.o %t.fdata
+# RUN: strip --strip-unneeded %t.o
+# RUN: %host_cc %t.o -o %t.exe -Wl,-q
+# RUN: llvm-bolt %t.exe -relocs=1 -print-profile-stats -o /dev/null \
+# RUN:   -data %t.fdata | FileCheck %s
+
+  .text
+  .globl  main
+  .type main, %function
+  .p2align  4
+main:
+# FDATA: 0 [unknown] 0 1 main 0 0 510
+  pushq %rbp
+  movq  %rsp, %rbp
+  subq  $0x18, %rsp
+  addq  $0x18, %rsp
+  xorq  %rax, %rax
+  leaveq
+  retq
+.size main, .-main
+
+
+# CHECK: Skipping writing final binary