[AsmPrint] Correctly factor function entry count when dumping MBB frequencies

llvm/include/llvm/CodeGen/MachineBlockFrequencyInfo.h

@@ -65,9 +65,10 @@ class MachineBlockFrequencyInfo : public MachineFunctionPass {
 
   /// Compute the frequency of the block, relative to the entry block.
   /// This API assumes getEntryFreq() is non-zero.
-  float getBlockFreqRelativeToEntryBlock(const MachineBasicBlock *MBB) const {
+  double getBlockFreqRelativeToEntryBlock(const MachineBasicBlock *MBB) const {
     assert(getEntryFreq() != 0 && "getEntryFreq() should not return 0 here!");
-    return getBlockFreq(MBB).getFrequency() * (1.0f / getEntryFreq());
+    return static_cast<double>(getBlockFreq(MBB).getFrequency()) /
+           static_cast<double>(getEntryFreq());
   }
 
   std::optional<uint64_t>

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -1929,18 +1929,32 @@ 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() &&
+      MF->getFunction().getEntryCount()) {
     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).
+    // 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());
     for (const auto &MBB : *MF) {
+      const double MBBRelFreq = MBFI.getBlockFreqRelativeToEntryBlock(&MBB);
+      const double AbsMBBFreq = MBBRelFreq * EntryCount;
       *MBBProfileDumpFileOutput.get()
-          << MF->getName() << "," << MBB.getBBID() << ","
-          << MBFI.getBlockFreq(&MBB).getFrequency() << "\n";
+          << MF->getName() << "," << MBB.getBBID() << "," << AbsMBBFreq << "\n";
     }
   }
 }

llvm/test/CodeGen/Generic/bb-profile-dump.ll

@@ -7,26 +7,48 @@
 
 ; Check that given a simple case, we can return the default MBFI
 
-define i64 @f2(i64 %a, i64 %b) {
+define i64 @f2(i64 %a, i64 %b) !prof !1{
     %sum = add i64 %a, %b
     ret i64 %sum
 }
 
-; CHECK: f2,0,8
+; CHECK: f2,0,1.000000e+03
 
-define i64 @f1() {
+define i64 @f1() !prof !2{
     %sum = call i64 @f2(i64 2, i64 2)
     %isEqual = icmp eq i64 %sum, 4
-    br i1 %isEqual, label %ifEqual, label %ifNotEqual
+    br i1 %isEqual, label %ifEqual, label %ifNotEqual, !prof !3
 ifEqual:
     ret i64 0
 ifNotEqual:
     ret i64 %sum
 }
 
-; CHECK-NEXT: f1,0,16
-; CHECK-NEXT: f1,1,8
-; CHECK-NEXT: f1,2,16
+; CHECK-NEXT: f1,0,1.000000e+01
+; CHECK-NEXT: f1,2,6.000000e+00
+; CHECK-NEXT: f1,1,4.000000e+00
+
+define void @f3(i32 %iter) !prof !4 {
+entry:
+    br label %loop
+loop:
+    %i = phi i32 [0, %entry], [%i_next, %loop]
+    %i_next = add i32 %i, 1
+    %exit_cond = icmp slt i32 %i_next, %iter
+    br i1 %exit_cond, label %loop, label %exit, !prof !5
+exit:
+    ret void
+}
+
+; CHECK-NEXT: f3,0,2.000000e+00
+; CHECK-NEXT: f3,1,2.002000e+03
+; CHECK-NEXT: f3,2,2.000000e+00
+
+!1 = !{!"function_entry_count", i64 1000}
+!2 = !{!"function_entry_count", i64 10}
+!3 = !{!"branch_weights", i32 2, i32 3}
+!4 = !{!"function_entry_count", i64 2}
+!5 = !{!"branch_weights", i32 1000, i32 1}
 
 ; Check that if we pass -mbb-profile-dump but don't set -basic-block-sections,
 ; we get an appropriate error message