[llvm] [AsmPrint] Correctly factor function entry count when dumping MBB frequencies (PR #67826)

[Aiden Grossman via llvm-commits]

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@@ -1929,18 +1929,35 @@ void AsmPrinter::emitFunctionBody() {
 
   // Output MBB ids, function names, and frequencies if the flag to dump
   // MBB profile information has been set
-  if (MBBProfileDumpFileOutput) {
+  if (MBBProfileDumpFileOutput && !MF->empty()) {
     if (!MF->hasBBLabels())
       MF->getContext().reportError(
           SMLoc(),
           "Unable to find BB labels for MBB profile dump. -mbb-profile-dump "
           "must be called with -basic-block-sections=labels");
     MachineBlockFrequencyInfo &MBFI =
         getAnalysis<LazyMachineBlockFrequencyInfoPass>().getBFI();
+    // The entry count and the entry basic block frequency aren't the same. We
+    // want to capture "absolute" frequencies, i.e. the frequency with which a
+    // MBB is executed when the program is executed - from there, we can derive
+    // Function-relative frequencies (divide by the value for the first MBB),
+    // and we also have the information about frequency with which functions
+    // were called. This helps, for example, in a type of integration tests
+    // where we want to cross-validate the compiler's profile with a real
+    // profile.
+    // Using double precision because uint64 values used to encode mbb
+    // "frequencies" may be quite large.
+    const double EntryCount =
+        static_cast<double>(MF->getFunction().getEntryCount()->getCount());
+    const double EntryFrequency =
+        static_cast<double>(MBFI.getBlockFreq(&*MF->begin()).getFrequency());
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boomanaiden154 wrote:

Looks like I didn't submit my review fast enough.

I'm not sure if it matters in practice, but `getBlockFreqRelativeToEntryBlock` is used in the register allocator for calculating spill weights (and in a couple other places, but mostly things like the ML register allocator stuff, PBQP). I don't think the compile time difference would be noticeable at all or if the increase in precision would end up impacting anything (maybe given how some cold branch weights are set by default), but it's something I think is worth considering.

https://github.com/llvm/llvm-project/pull/67826