[llvm] [LVer][profcheck] explicitly set unknown branch weights for the versioned/unversioned selector (PR #164507)

[Mircea Trofin via llvm-commits]

https://github.com/mtrofin updated https://github.com/llvm/llvm-project/pull/164507

>From af190a9ed588d2e22884618b8d798d26c774b9e6 Mon Sep 17 00:00:00 2001
From: Mircea Trofin <mtrofin at google.com>
Date: Tue, 21 Oct 2025 15:20:14 -0700
Subject: [PATCH] [LVer][profcheck] explicitly set unknown branch weights for
 the versioned/unversioned selector

---
 llvm/lib/Transforms/Utils/LoopVersioning.cpp           | 10 ++++++++--
 .../Transforms/LoopDistribute/basic-with-memchecks.ll  |  5 +++--
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Transforms/Utils/LoopVersioning.cpp b/llvm/lib/Transforms/Utils/LoopVersioning.cpp
index ec2e6c1ab796b..4786819d18fa4 100644
--- a/llvm/lib/Transforms/Utils/LoopVersioning.cpp
+++ b/llvm/lib/Transforms/Utils/LoopVersioning.cpp
@@ -23,6 +23,7 @@
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/MDBuilder.h"
 #include "llvm/IR/PassManager.h"
+#include "llvm/IR/ProfDataUtils.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Cloning.h"
@@ -109,8 +110,13 @@ void LoopVersioning::versionLoop(
   // Insert the conditional branch based on the result of the memchecks.
   Instruction *OrigTerm = RuntimeCheckBB->getTerminator();
   Builder.SetInsertPoint(OrigTerm);
-  Builder.CreateCondBr(RuntimeCheck, NonVersionedLoop->getLoopPreheader(),
-                       VersionedLoop->getLoopPreheader());
+  auto *BI =
+      Builder.CreateCondBr(RuntimeCheck, NonVersionedLoop->getLoopPreheader(),
+                           VersionedLoop->getLoopPreheader());
+  // We don't know what the probability of executing the versioned vs the
+  // unversioned variants is.
+  setExplicitlyUnknownBranchWeightsIfProfiled(
+      *BI, *BI->getParent()->getParent(), DEBUG_TYPE);
   OrigTerm->eraseFromParent();
 
   // The loops merge in the original exit block.  This is now dominated by the
diff --git a/llvm/test/Transforms/LoopDistribute/basic-with-memchecks.ll b/llvm/test/Transforms/LoopDistribute/basic-with-memchecks.ll
index 97ea2c6708dad..2828882afe779 100644
--- a/llvm/test/Transforms/LoopDistribute/basic-with-memchecks.ll
+++ b/llvm/test/Transforms/LoopDistribute/basic-with-memchecks.ll
@@ -28,7 +28,7 @@ target triple = "x86_64-apple-macosx10.10.0"
 @E = common global ptr null, align 8
 
 ; CHECK-LABEL: @f(
-define void @f() {
+define void @f() !prof !{!"function_entry_count", i32 10} {
 entry:
   %a = load ptr, ptr @A, align 8
   %b = load ptr, ptr @B, align 8
@@ -55,7 +55,7 @@ entry:
 ; CHECK:     = icmp
 
 ; CHECK-NOT: = icmp
-; CHECK:     br i1 %conflict.rdx15, label %for.body.ph.lver.orig, label %for.body.ph.ldist1
+; CHECK:     br i1 %conflict.rdx15, label %for.body.ph.lver.orig, label %for.body.ph.ldist1, !prof ![[PROF1:[0-9]]]
 
 ; The non-distributed loop that the memchecks fall back on.
 
@@ -289,3 +289,4 @@ attributes #1 = { nounwind convergent }
 
 !0 = distinct !{!0, !1}
 !1 = !{!"llvm.loop.distribute.enable", i1 true}
+; CHECK: ![[PROF1]] = !{!"unknown", !"loop-versioning"}