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

[Mircea Trofin via llvm-commits]

https://github.com/mtrofin created https://github.com/llvm/llvm-project/pull/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.

>From 4c11591a42d2132589b886ade415bc1ed2dc542b Mon Sep 17 00:00:00 2001
From: Mircea Trofin <mtrofin at google.com>
Date: Fri, 29 Sep 2023 09:06:12 -0700
Subject: [PATCH] [AsmPrint] Correctly factor function entry count when dumping
 MBB frequencies

The goal in PR #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.
---
 llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp   | 23 +++++++++++--
 llvm/test/CodeGen/Generic/bb-profile-dump.ll | 36 ++++++++++++++++----
 2 files changed, 49 insertions(+), 10 deletions(-)

diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index 0c4ea1b3d9f04d9..a54a5ab75797b4c 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -1929,7 +1929,7 @@ 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(),
@@ -1937,10 +1937,27 @@ void AsmPrinter::emitFunctionBody() {
           "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());
+    const double FreqAdjustmentFactor = EntryCount / EntryFrequency;
     for (const auto &MBB : *MF) {
+      const double MBBFreq =
+          static_cast<double>(MBFI.getBlockFreq(&MBB).getFrequency());
+      const double AbsMBBFreq = MBBFreq * FreqAdjustmentFactor;
       *MBBProfileDumpFileOutput.get()
-          << MF->getName() << "," << MBB.getBBID() << ","
-          << MBFI.getBlockFreq(&MBB).getFrequency() << "\n";
+          << MF->getName() << "," << MBB.getBBID() << "," << AbsMBBFreq << "\n";
     }
   }
 }
diff --git a/llvm/test/CodeGen/Generic/bb-profile-dump.ll b/llvm/test/CodeGen/Generic/bb-profile-dump.ll
index 934c281219d4ae7..7391a6ee6f9128d 100644
--- a/llvm/test/CodeGen/Generic/bb-profile-dump.ll
+++ b/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