[IR] Add MD_prof to the Keep list of dropUBImplyingAttrsAndMetadata (llvm#154635)

`MD_prof` is safe to keep when e.g. hoisting instructions.

Issue llvm#147390

Author: mtrofin

llvm/lib/IR/Instruction.cpp

@@ -553,16 +553,17 @@ void Instruction::dropUBImplyingAttrsAndUnknownMetadata(
 }
 
 void Instruction::dropUBImplyingAttrsAndMetadata(ArrayRef<unsigned> Keep) {
-  // !annotation metadata does not impact semantics.
+  // !annotation and !prof metadata does not impact semantics.
   // !range, !nonnull and !align produce poison, so they are safe to speculate.
   // !noundef and various AA metadata must be dropped, as it generally produces
   // immediate undefined behavior.
   static const unsigned KnownIDs[] = {
       LLVMContext::MD_annotation, LLVMContext::MD_range,
-      LLVMContext::MD_nonnull, LLVMContext::MD_align};
+      LLVMContext::MD_nonnull, LLVMContext::MD_align, LLVMContext::MD_prof};
   SmallVector<unsigned> KeepIDs;
   KeepIDs.reserve(Keep.size() + std::size(KnownIDs));
-  append_range(KeepIDs, KnownIDs);
+  append_range(KeepIDs, (!ProfcheckDisableMetadataFixes ? KnownIDs
+                                                        : drop_end(KnownIDs)));
   append_range(KeepIDs, Keep);
   dropUBImplyingAttrsAndUnknownMetadata(KeepIDs);
 }

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -1705,10 +1705,7 @@ static void hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
       // time in isGuaranteedToExecute if we don't actually have anything to
       // drop.  It is a compile time optimization, not required for correctness.
       !SafetyInfo->isGuaranteedToExecute(I, DT, CurLoop)) {
-    if (ProfcheckDisableMetadataFixes)
-      I.dropUBImplyingAttrsAndMetadata();
-    else
-      I.dropUBImplyingAttrsAndMetadata({LLVMContext::MD_prof});
+    I.dropUBImplyingAttrsAndMetadata();
   }
 
   if (isa<PHINode>(I))

llvm/test/Transforms/SimplifyCFG/PhiBlockMerge.ll

@@ -1,20 +1,21 @@
-; NOTE: Assertions have been autogenerated by update_test_checks.py
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals all --version 5
 ; Test merging of blocks that only have PHI nodes in them
 ;
 ; RUN: opt < %s -passes=simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S | FileCheck %s
 ;
 
 define i32 @test(i1 %a, i1 %b) {
-; CHECK-LABEL: @test(
-; CHECK:       M:
-; CHECK-NEXT:    [[DOT:%.*]] = select i1 %b, i32 0, i32 1
-; CHECK-NEXT:    [[W:%.*]] = select i1 %a, i32 2, i32 [[DOT]]
+; CHECK-LABEL: define i32 @test(
+; CHECK-SAME: i1 [[A:%.*]], i1 [[B:%.*]]) {
+; CHECK-NEXT:  [[M:.*:]]
+; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[B]], i32 0, i32 1, !prof [[PROF0:![0-9]+]]
+; CHECK-NEXT:    [[W:%.*]] = select i1 [[A]], i32 2, i32 [[SPEC_SELECT]], !prof [[PROF1:![0-9]+]]
 ; CHECK-NEXT:    [[R:%.*]] = add i32 [[W]], 1
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
-  br i1 %a, label %M, label %O
+  br i1 %a, label %M, label %O, !prof !0
 O:              ; preds = %0
-  br i1 %b, label %N, label %Q
+  br i1 %b, label %N, label %Q, !prof !1
 Q:              ; preds = %O
   br label %N
 N:              ; preds = %Q, %O
@@ -27,3 +28,9 @@ M:              ; preds = %N, %0
   ret i32 %R
 }
 
+!0 = !{!"branch_weights", i32 11, i32 7}
+!1 = !{!"branch_weights", i32 3, i32 5}
+;.
+; CHECK: [[PROF0]] = !{!"branch_weights", i32 3, i32 5}
+; CHECK: [[PROF1]] = !{!"branch_weights", i32 11, i32 7}
+;.