[PATCH] D87435: [PGO] Skip if an IndirectBrInst critical edge cannot be split

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
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PGOInstrumentation runs `SplitIndirectBrCriticalEdges` but some IndirectBrInst
critical edge cannot be split. `getInstrBB` will crash when calling `SplitCriticalEdge`, e.g.

  int foo(char *p) {
    void *targets[2];
    targets[0] = &&indirect;
    targets[1] = &&end;
  for (;; p++)
    if (*p == 7) {
  indirect:
      goto *targets[p[1]]; // the self loop is critical in -O
    }
  end:
    return 0;
  }

Skip such critical edges to prevent a crash.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D87435

Files:
  llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp
  llvm/test/Transforms/PGOProfile/split-indirectbr-critical-edges.ll


Index: llvm/test/Transforms/PGOProfile/split-indirectbr-critical-edges.ll
===================================================================
--- llvm/test/Transforms/PGOProfile/split-indirectbr-critical-edges.ll
+++ llvm/test/Transforms/PGOProfile/split-indirectbr-critical-edges.ll
@@ -37,3 +37,27 @@
   indirectbr i8* %2, [label %for.cond2, label %if.end]
 ; CHECK: indirectbr i8* %2, [label %for.cond2, label %if.end]
 }
+
+;; If an indirectbr critical edge cannot be split, ignore it.
+;; The edge will not be profiled.
+; CHECK-LABEL: @cannot_split(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    call void @llvm.instrprof.increment
+; CHECK-NOT:     call void @llvm.instrprof.increment
+define i32 @cannot_split(i8* nocapture readonly %p) {
+entry:
+  %targets = alloca <2 x i8*>, align 16
+  store <2 x i8*> <i8* blockaddress(@cannot_split, %indirect), i8* blockaddress(@cannot_split, %end)>, <2 x i8*>* %targets, align 16
+  %arrayidx2 = getelementptr inbounds i8, i8* %p, i64 1
+  %0 = load i8, i8* %arrayidx2
+  %idxprom = sext i8 %0 to i64
+  %arrayidx3 = getelementptr inbounds <2 x i8*>, <2 x i8*>* %targets, i64 0, i64 %idxprom
+  %1 = load i8*, i8** %arrayidx3, align 8
+  br label %indirect
+
+indirect:                                         ; preds = %entry, %indirect
+  indirectbr i8* %1, [label %indirect, label %end]
+
+end:                                              ; preds = %indirect
+  ret i32 0
+}
Index: llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp
===================================================================
--- llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp
+++ llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp
@@ -140,8 +140,8 @@
   if (!isCriticalEdge(TI, SuccNum, Options.MergeIdenticalEdges))
     return nullptr;
 
-  assert(!isa<IndirectBrInst>(TI) &&
-         "Cannot split critical edge from IndirectBrInst");
+  if (isa<IndirectBrInst>(TI))
+    return nullptr;
 
   BasicBlock *TIBB = TI->getParent();
   BasicBlock *DestBB = TI->getSuccessor(SuccNum);


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