[LVer][profcheck] explicitly set unknown branch weights for the versioned/unversioned selector

[mtrofin]

We don't have sufficient information to know when the versioned (or unversioned) loop variant will be taken, so we mark the branch as having "unknown" probabilities.

Issue #147390

[mtrofin]

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• [LSCFG][profcheck] Add dummy branch weights for the dummy switch to dead exits #164714
• [LIR][profcheck] Reuse the loop's exit condition profile #164523
• [LVer][profcheck] explicitly set unknown branch weights for the versioned/unversioned selector #164507 👈 (View in Graphite)
• [SLU][profcheck] Propagate profile for branches on injected conditions. #164476
• [SLU][profcheck] create likely branch weights for guard->branch #164271
• [NFC][SLU] Update SimpleLoopUnswitch/guards.ll #166285
• main

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[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Mircea Trofin (mtrofin)

Changes

We don't have sufficient information to know when the versioned (or unversioned) loop variant will be taken, so we mark the branch as having "unknown" probabilities.

Issue #147390

---

Full diff: https://github.com/llvm/llvm-project/pull/164507.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Utils/LoopVersioning.cpp (+8-2)
• (modified) llvm/test/Transforms/LoopDistribute/basic-with-memchecks.ll (+3-2)

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"}

[fhahn]

Suggested change

	*BI, *BI->getParent()->getParent(), DEBUG_TYPE);
	*BI, *BI->getFunction(), DEBUG_TYPE);

[fhahn]

Actually, looks like the argument can be removed alltogether #166028

[fhahn]

Or not, looks like InstCombine passes disconnected instructions