[licm] clone metadata when hoisting conditional branch

Author: mtrofin

llvm/lib/Analysis/BranchProbabilityInfo.cpp

@@ -59,6 +59,14 @@ static cl::opt<std::string> PrintBranchProbFuncName(
     cl::desc("The option to specify the name of the function "
              "whose branch probability info is printed."));
 
+// FIXME: Flag used for an ablation performance test, Issue #147390. Placing it
+// here because referencing Analysis should be feasible from anywhere. Will be
+// removed after the ablation test.
+cl::opt<bool> ProfcheckDisableMetadataFixes(
+    "profcheck-disable-metadata-fixes", cl::Hidden, cl::init(false),
+    cl::desc(
+        "Disable metadata propagation fixes discovered through Issue #147390"));
+
 INITIALIZE_PASS_BEGIN(BranchProbabilityInfoWrapperPass, "branch-prob",
                       "Branch Probability Analysis", false, true)
 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -218,6 +218,8 @@ using PointersAndHasReadsOutsideSet =
 static SmallVector<PointersAndHasReadsOutsideSet, 0>
 collectPromotionCandidates(MemorySSA *MSSA, AliasAnalysis *AA, Loop *L);
 
+extern cl::opt<bool> ProfcheckDisableMetadataFixes;
+
 namespace {
 struct LoopInvariantCodeMotion {
   bool runOnLoop(Loop *L, AAResults *AA, LoopInfo *LI, DominatorTree *DT,
@@ -857,9 +859,18 @@ class ControlFlowHoister {
     }
 
     // Now finally clone BI.
-    ReplaceInstWithInst(
-        HoistTarget->getTerminator(),
-        BranchInst::Create(HoistTrueDest, HoistFalseDest, BI->getCondition()));
+    auto *NewBI =
+        BranchInst::Create(HoistTrueDest, HoistFalseDest, BI->getCondition(),
+                           HoistTarget->getTerminator()->getIterator());
+    HoistTarget->getTerminator()->eraseFromParent();
+    // Copy all the metadata. In particular:
+    // - debug info (critical to Sample-based profiling) should be the same
+    // as the original branch, not that of HoistTarget->getTerminator(),
+    // which is what ReplaceInstWithInst would use.
+    // - md_prof should also come from the original branch - since the
+    // condition was hoisted, the branch probabilities shouldn't change.
+    if (!ProfcheckDisableMetadataFixes)
+      NewBI->copyMetadata(*BI);
     ++NumClonedBranches;
 
     assert(CurLoop->getLoopPreheader() &&

llvm/test/Transforms/LICM/hoist-phi-metadata.ll

@@ -0,0 +1,62 @@
+; Test that hoisting conditional branches copies the debug and profiling info
+; metadata from the branch being hoisted.
+; RUN: opt -S -passes=licm -licm-control-flow-hoisting=1 %s -o - | FileCheck %s
+
+; CHECK-LABEL: @triangle_phi
+define void @triangle_phi(i32 %x, ptr %p) {
+; CHECK-LABEL: entry:
+; CHECK: %cmp1 = icmp sgt i32 %x, 0
+; CHECK: br i1 %cmp1, label %[[IF_LICM:.*]], label %[[THEN_LICM:.*]], !dbg [[ORIG_DBG:![0-9]+]], !prof [[ORIG_PROF:![0-9]+]]
+entry:
+  br label %loop, !dbg !5
+
+; CHECK: [[IF_LICM]]:
+; CHECK: %add = add i32 %x, 1
+; CHECK: br label %[[THEN_LICM]]
+
+; CHECK: [[THEN_LICM]]:
+; CHECK: phi i32 [ %add, %[[IF_LICM]] ], [ %x, %entry ]
+; CHECK: store i32 %phi, ptr %p
+; CHECK: %cmp2 = icmp ne i32 %phi, 0
+; CHECK: br label %loop
+
+; CHECK-LABEL: loop:
+loop:
+  %cmp1 = icmp sgt i32 %x, 0
+; CHECK: br i1 %cmp1, label %if, label %then, !dbg [[ORIG_DBG]], !prof [[ORIG_PROF]]
+  br i1 %cmp1, label %if, label %then, !dbg !6, !prof !8
+if:
+  %add = add i32 %x, 1
+  br label %then
+
+; CHECK-LABEL: then:
+then:
+  %phi = phi i32 [ %add, %if ], [ %x, %loop ]
+  store i32 %phi, ptr %p
+  %cmp2 = icmp ne i32 %phi, 0
+; CHECK-NEXT: br i1 %cmp2, label %loop, label %end, !dbg [[OTHER_DBG:![0-9]+]], !prof [[OTHER_PROF:![0-9]+]]
+  br i1 %cmp2, label %loop, label %end, !dbg !7, !prof !9
+
+; CHECK-LABEL: end:
+end:
+  ret void
+}
+
+; CHECK-DAG: [[ORIG_DBG]] = !DILocation(line: 2, column: 22
+; CHECK-DAG: [[ORIG_PROF]] = !{!"branch_weights", i32 5, i32 7}
+; CHECK-DAG: [[OTHER_DBG]] = !DILocation(line: 3, column: 22
+; CHECK-DAG: [[OTHER_PROF]] = !{!"branch_weights", i32 13, i32 11}
+
+
+!llvm.module.flags = !{!2, !3}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1)
+!1 = !DIFile(filename: "t", directory: "/")
+!2 = !{i32 7, !"Dwarf Version", i32 5}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = distinct !DISubprogram(name: "triangle_phi", linkageName: "triangle_phi", scope: !1, file: !1, line: 1, unit: !0)
+!5 = !DILocation(line: 1, column: 22, scope: !4)
+!6 = !DILocation(line: 2, column: 22, scope: !4)
+!7 = !DILocation(line: 3, column: 22, scope: !4)
+!8 = !{!"branch_weights", i32 5, i32 7}
+!9 = !{!"branch_weights", i32 13, i32 11}