[llvm] [SCEVExpander] Mark selects with unknown profile. (PR #224362)

[Florian Hahn via llvm-commits]

https://github.com/fhahn updated https://github.com/llvm/llvm-project/pull/224362

>From d509075ab03cb64281dc6b13b41be9b388620fcb Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Sun, 13 Sep 2026 20:10:28 +0100
Subject: [PATCH] [SCEVExpander] Mark selects created during expansion with
 unknown profile.

SCEVExpander creates selects when expanding min/max expressions for
pointers and predicates. In neither case there is any information on how
likely either arm of the select is. Annotate with unknown profile.

This is part of addressing the remaining profcheck failures in
LoopVectorize: https://github.com/llvm/llvm-project/issues/161235.
---
 .../Transforms/Utils/ScalarEvolutionExpander.cpp    | 10 +++++++---
 .../LoopVectorize/scev-check-unknown-prof.ll        | 13 +++++++------
 2 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index f01a674609325..6c64cfbec044b 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -24,6 +24,7 @@
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/PatternMatch.h"
+#include "llvm/IR/ProfDataUtils.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Utils/Local.h"
@@ -1580,7 +1581,8 @@ Value *SCEVExpander::expandMinMaxExpr(SCEVUseT<const SCEVNAryExpr *> S,
     else {
       Value *ICmp =
           Builder.CreateICmp(MinMaxIntrinsic::getPredicate(IntrinID), LHS, RHS);
-      Sel = Builder.CreateSelect(ICmp, LHS, RHS, Name);
+      Sel = Builder.CreateSelectWithUnknownProfile(ICmp, LHS, RHS,
+                                                   "scev-expander", Name);
     }
     LHS = Sel;
   }
@@ -2340,7 +2342,8 @@ Value *SCEVExpander::generateOverflowCheck(const SCEVAddRecExpr *AR,
   Builder.SetInsertPoint(Loc);
   // Compute |Step|
   Value *StepCompare = Builder.CreateICmp(ICmpInst::ICMP_SLT, StepValue, Zero);
-  Value *AbsStep = Builder.CreateSelect(StepCompare, NegStepValue, StepValue);
+  Value *AbsStep = Builder.CreateSelectWithUnknownProfile(
+      StepCompare, NegStepValue, StepValue, "scev-expander");
 
   // Compute |Step| * Backedge
   // Compute:
@@ -2399,7 +2402,8 @@ Value *SCEVExpander::generateOverflowCheck(const SCEVAddRecExpr *AR,
           Signed ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT, Sub, StartValue);
     if (NeedPosCheck && NeedNegCheck) {
       // Select the answer based on the sign of Step.
-      EndCheck = Builder.CreateSelect(StepCompare, EndCompareGT, EndCompareLT);
+      EndCheck = Builder.CreateSelectWithUnknownProfile(
+          StepCompare, EndCompareGT, EndCompareLT, "scev-expander");
     }
     return Builder.CreateOr(EndCheck, OfMul);
   };
diff --git a/llvm/test/Transforms/LoopVectorize/scev-check-unknown-prof.ll b/llvm/test/Transforms/LoopVectorize/scev-check-unknown-prof.ll
index cb145d88c8b05..f2d2f13a39a6b 100644
--- a/llvm/test/Transforms/LoopVectorize/scev-check-unknown-prof.ll
+++ b/llvm/test/Transforms/LoopVectorize/scev-check-unknown-prof.ll
@@ -11,8 +11,8 @@ define void @wrap_check(i32 %n, i32 %step) !prof !0 {
 ; CHECK:  [[LOOP_PREHEADER]]:
 ; CHECK:    br i1 [[MIN_ITERS_CHECK:%.*]], label %[[SCALAR_PH:.*]], label %[[VECTOR_SCEVCHECK:.*]]
 ; CHECK:  [[VECTOR_SCEVCHECK]]:
-; CHECK:    [[TMP4:%.*]] = select i1 [[TMP3:%.*]], i8 [[TMP2:%.*]], i8 [[TMP1:%.*]]
-; CHECK:    [[TMP9:%.*]] = select i1 [[TMP3]], i1 [[TMP8:%.*]], i1 [[TMP7:%.*]]
+; CHECK:    [[TMP4:%.*]] = select i1 [[TMP3:%.*]], i8 [[TMP2:%.*]], i8 [[TMP1:%.*]], !prof [[PROF1:![0-9]+]]
+; CHECK:    [[TMP9:%.*]] = select i1 [[TMP3]], i1 [[TMP8:%.*]], i1 [[TMP7:%.*]], !prof [[PROF1]]
 ; CHECK:    br i1 [[TMP16:%.*]], label %[[SCALAR_PH]], label %[[VECTOR_PH:.*]]
 ; CHECK:  [[VECTOR_PH]]:
 ; CHECK:  [[VECTOR_BODY:.*]]:
@@ -57,12 +57,12 @@ define void @runtime_step_memcheck(ptr %in, ptr %out, i64 %n, i64 %step) !prof !
 ; CHECK:  [[LOOP_PREHEADER]]:
 ; CHECK:    br i1 [[MIN_ITERS_CHECK:%.*]], label %[[SCALAR_PH:.*]], label %[[VECTOR_SCEVCHECK:.*]]
 ; CHECK:  [[VECTOR_SCEVCHECK]]:
-; CHECK:    [[TMP4:%.*]] = select i1 [[TMP3:%.*]], i32 [[TMP2:%.*]], i32 [[TMP1:%.*]]
-; CHECK:    [[TMP9:%.*]] = select i1 [[TMP3]], i1 [[TMP8:%.*]], i1 [[TMP7:%.*]]
+; CHECK:    [[TMP4:%.*]] = select i1 [[TMP3:%.*]], i32 [[TMP2:%.*]], i32 [[TMP1:%.*]], !prof [[PROF1]]
+; CHECK:    [[TMP9:%.*]] = select i1 [[TMP3]], i1 [[TMP8:%.*]], i1 [[TMP7:%.*]], !prof [[PROF1]]
 ; CHECK:    br i1 [[TMP16:%.*]], label %[[SCALAR_PH]], label %[[VECTOR_MEMCHECK:.*]]
 ; CHECK:  [[VECTOR_MEMCHECK]]:
-; CHECK:    [[UMIN:%.*]] = select i1 [[TMP23:%.*]], ptr [[IN]], ptr [[SCEVGEP1:%.*]]
-; CHECK:    [[UMAX:%.*]] = select i1 [[TMP24:%.*]], ptr [[IN]], ptr [[SCEVGEP1]]
+; CHECK:    [[UMIN:%.*]] = select i1 [[TMP23:%.*]], ptr [[IN]], ptr [[SCEVGEP1:%.*]], !prof [[PROF1]]
+; CHECK:    [[UMAX:%.*]] = select i1 [[TMP24:%.*]], ptr [[IN]], ptr [[SCEVGEP1]], !prof [[PROF1]]
 ; CHECK:    br i1 [[FOUND_CONFLICT:%.*]], label %[[SCALAR_PH]], label %[[VECTOR_PH:.*]]
 ; CHECK:  [[VECTOR_PH]]:
 ; CHECK:  [[VECTOR_BODY:.*]]:
@@ -149,6 +149,7 @@ exit:
 !0 = !{!"function_entry_count", i64 1000}
 ;.
 ; CHECK: [[PROF0]] = !{!"function_entry_count", i64 1000}
+; CHECK: [[PROF1]] = !{!"unknown", !"scev-expander"}
 ; CHECK: [[LOOP1]] = distinct !{[[LOOP1]], [[META2:![0-9]+]], [[META3:![0-9]+]]}
 ; CHECK: [[META2]] = !{!"llvm.loop.isvectorized", i32 1}
 ; CHECK: [[META3]] = !{!"llvm.loop.unroll.runtime.disable"}