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

The goal in #66818 was to capture function entry counts, but those are not the same as the frequency of the entry (machine) basic block. This fixes that, and adds explicit profiles to the test.

We also increase the precision of `MachineBlockFrequencyInfo::getBlockFreqRelativeToEntryBlock` to double. Existing code uses it as float so should be unaffected.

Author: mtrofin

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