[llc] Use -filetype=null to disable MIR printing

[foad]

Summary

If you use -stop-after or similar options, llc will normally print MIR.
This patch checks for -filetype=null as a special case to disable MIR
printing. As the comment says, "The Null output is intended for use for
performance analysis ...", and I found this useful for timing a subset
of the passes that llc runs without the significant overhead of printing
MIR just to send it to /dev/null.

[MaskRay]

llvm/lib/CodeGen/LLVMTargetMachine.cpp

203

Just say createPrintMIRPass is redundant for null output?

How much overhead have you measured?

llvm/test/tools/llc/filetype-null-stop-after.ll

2

| count 0

[foad]

llvm/lib/CodeGen/LLVMTargetMachine.cpp

203

How much overhead have you measured?

More than 50% in MIR printing in cases like this:

$ ~/llvm-release/bin/llc -march=aarch64 sqlite3.bc -filetype=null -stop-after=finalize-isel -time-passes > /dev/null
===-------------------------------------------------------------------------===
                      ... Pass execution timing report ...
===-------------------------------------------------------------------------===
  Total Execution Time: 4.7993 seconds (4.8033 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
   2.5278 ( 54.2%)   0.0577 ( 42.8%)   2.5856 ( 53.9%)   2.5887 ( 53.9%)  MIR Printing Pass
   1.3541 ( 29.0%)   0.0530 ( 39.3%)   1.4071 ( 29.3%)   1.4081 ( 29.3%)  AArch64 Instruction Selection
   0.1636 (  3.5%)   0.0024 (  1.8%)   0.1660 (  3.5%)   0.1661 (  3.5%)  CodeGen Prepare
...

[foad]

Address feedback.

[MaskRay]

LGTM.

llvm/lib/CodeGen/LLVMTargetMachine.cpp

bool LLVMTargetMachine::addPassesToEmitFile(
   // Add common CodeGen passes.
   if (!MMIWP)
     MMIWP = new MachineModuleInfoWrapperPass(this);
   TargetPassConfig *PassConfig =
       addPassesToGenerateCode(*this, PM, DisableVerify, *MMIWP);
   if (!PassConfig)
     return true;
 
-  if (!TargetPassConfig::willCompleteCodeGenPipeline())
-    PM.add(createPrintMIRPass(Out));
-  else if (addAsmPrinter(PM, Out, DwoOut, FileType,
-                           MMIWP->getMMI().getContext()))
+  if (TargetPassConfig::willCompleteCodeGenPipeline()) {
+    if (addAsmPrinter(PM, Out, DwoOut, FileType, MMIWP->getMMI().getContext()))
       return true;
+  } else {
+    // MIR printing is redundant with -filetype=null.
+    if (FileType != CGFT_Null)
+      PM.add(createPrintMIRPass(Out));
+  }
 
   PM.add(createFreeMachineFunctionPass());
   return false;
 }
 
 /// addPassesToEmitMC - Add passes to the specified pass manager to get
 /// machine code emitted with the MCJIT. This method returns true if machine
 /// code is not supported. It fills the MCContext Ctx pointer which can be

llvm/test/tools/llc/filetype-null-stop-after.ll

+; -stop-after would normally dump MIR, but with -filetype=null as well check
+; there's no output at all.
+; RUN: llc -filetype=null -stop-after=finalize-isel -o - %s | count 0