[XRay] Fix computation of function size subject to XRay threshold

[rSerge]

Summary

Currently XRay compares its threshold against Function::size() . However, Function::size() returns the number of basic blocks (as I understand, such as cycle bodies, if/else bodies, switch-case bodies, etc.), rather than the number of instructions.

The name of the parameter -fxray-instruction-threshold=N, as well as XRay documentation at http://llvm.org/docs/XRay.html , suggests that instructions should be counted, rather than the number of basic blocks.

I see two options:

1. Count the number of MachineInstr`s in MachineFunction : this gives better estimate for the number of assembly instructions on the target. So a user can check in disassembly that the threshold works more or less correctly.
2. Count the number of Instruction`s in a Function : AFAIK, this gives correct number of IR instructions, which the user can check in IR listing. However, this number may be far (several times for small functions) from the number of assembly instructions finally emitted.

Option 1 is implemented in this patch because I think that having the closer estimate for the number of assembly instructions emitted is more important than to have a clear definition of the metric.

[dberris]

Thanks @rSerge -- just one suggestion for readability.

lib/CodeGen/XRayInstrumentation.cpp

146–148

Could this be cleaner as a range-based for-loop?

int64_t MICount = 0;
for (const auto& MBB : MF)
  MICount += MBB.size();

[rSerge]

lib/CodeGen/XRayInstrumentation.cpp

146–148

Sure, thanks, changing it.

[rSerge]

Reached mainline in https://reviews.llvm.org/rL305072 . (with Dean's comment implemented)

lib/CodeGen/XRayInstrumentation.cpp

bool XRayInstrumentation::runOnMachineFunction(MachineFunction &MF) {
   Attribute Attr = F.getFnAttribute("xray-instruction-threshold");
   unsigned XRayThreshold = 0;
   if (!AlwaysInstrument) {
     if (Attr.hasAttribute(Attribute::None) || !Attr.isStringAttribute())
       return false; // XRay threshold attribute not found.
     if (Attr.getValueAsString().getAsInteger(10, XRayThreshold))
       return false; // Invalid value for threshold.
 
+    // Count the number of MachineInstr`s in MachineFunction
+    int64_t MICount = 0;
+    typedef MachineFunction::const_iterator BasicBlockIterator;
+    for (BasicBlockIterator it = MF.begin(); it != MF.end(); it++)
+      MICount += it->size();
+
     // Check if we have a loop.
     // FIXME: Maybe make this smarter, and see whether the loops are dependent
     // on inputs or side-effects?
     MachineLoopInfo &MLI = getAnalysis<MachineLoopInfo>();
-    if (MLI.empty() && F.size() < XRayThreshold)
+    if (MLI.empty() && MICount < XRayThreshold)
       return false; // Function is too small and has no loops.
   }
 
   // We look for the first non-empty MachineBasicBlock, so that we can insert
   // the function instrumentation in the appropriate place.
   auto MBI = llvm::find_if(
       MF, [&](const MachineBasicBlock &MBB) { return !MBB.empty(); });
   if (MBI == MF.end())