BreakCriticalEdges for callbr indirect dests

Summary:
llvm::SplitEdge was failing an assertion that the BasicBlock only had
one successor (for BasicBlocks terminated by CallBrInst, we typically
have multiple successors).  It was surprising that the earlier call to
SplitCriticalEdge did not handle the critical edge (there was an early
return).  Removing that triggered another assertion relating to creating
a BlockAddress for a BasicBlock that did not (yet) have a parent, which
is a simple order of operations issue in llvm::SplitCriticalEdge (a
freshly constructed BasicBlock must be inserted into a Function's basic
block list to have a parent).

Thanks to @nathanchance for the report.
Fixes: https://github.com/ClangBuiltLinux/linux/issues/1018

Reviewers: craig.topper, jyknight, void, fhahn, efriedma

Reviewed By: efriedma

Subscribers: eli.friedman, rnk, efriedma, fhahn, hiraditya, llvm-commits, nathanchance, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81607

llvm/lib/Transforms/Utils/BreakCriticalEdges.cpp

@@ -150,10 +150,6 @@ llvm::SplitCriticalEdge(Instruction *TI, unsigned SuccNum,
   // it in this generic function.
   if (DestBB->isEHPad()) return nullptr;
 
-  // Don't split the non-fallthrough edge from a callbr.
-  if (isa<CallBrInst>(TI) && SuccNum > 0)
-    return nullptr;
-
   if (Options.IgnoreUnreachableDests &&
       isa<UnreachableInst>(DestBB->getFirstNonPHIOrDbgOrLifetime()))
     return nullptr;
@@ -206,14 +202,14 @@ llvm::SplitCriticalEdge(Instruction *TI, unsigned SuccNum,
   BranchInst *NewBI = BranchInst::Create(DestBB, NewBB);
   NewBI->setDebugLoc(TI->getDebugLoc());
 
-  // Branch to the new block, breaking the edge.
-  TI->setSuccessor(SuccNum, NewBB);
-
   // Insert the block into the function... right after the block TI lives in.
   Function &F = *TIBB->getParent();
   Function::iterator FBBI = TIBB->getIterator();
   F.getBasicBlockList().insert(++FBBI, NewBB);
 
+  // Branch to the new block, breaking the edge.
+  TI->setSuccessor(SuccNum, NewBB);
+
   // If there are any PHI nodes in DestBB, we need to update them so that they
   // merge incoming values from NewBB instead of from TIBB.
   {

llvm/test/Transforms/CallSiteSplitting/callsite-split-callbr.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -callsite-splitting -S -o - < %s | FileCheck %s
+
+; Check that we can split the critical edge between Top and CallSiteBB, and
+; rewrite the first callbr's indirect destination correctly.
+
+define void @caller() {
+; CHECK-LABEL: @caller(
+; CHECK-NEXT:  Top:
+; CHECK-NEXT:    callbr void asm sideeffect "", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@caller, [[TOP_SPLIT:%.*]]))
+; CHECK-NEXT:    to label [[NEXTCOND:%.*]] [label %Top.split]
+; CHECK-LABEL: Top.split:
+; CHECK-NEXT:    call void @callee(i1 false)
+; CHECK-NEXT:    br label [[CALLSITEBB:%.*]]
+; CHECK-LABEL: NextCond:
+; CHECK-NEXT:    br label [[NEXTCOND_SPLIT:%.*]]
+; CHECK-LABEL: NextCond.split:
+; CHECK-NEXT:    call void @callee(i1 true)
+; CHECK-NEXT:    br label [[CALLSITEBB]]
+; CHECK-LABEL: CallSiteBB:
+; CHECK-NEXT:    [[PHI:%.*]] = phi i1 [ false, [[TOP_SPLIT]] ], [ true, [[NEXTCOND_SPLIT]] ]
+; CHECK-NEXT:    callbr void asm sideeffect "", "r,X,~{dirflag},~{fpsr},~{flags}"(i1 [[PHI]], i8* blockaddress(@caller, [[END2:%.*]]))
+; CHECK-NEXT:    to label [[END:%.*]] [label %End2]
+; CHECK-LABEL: End:
+; CHECK-NEXT:    ret void
+; CHECK-LABEL: End2:
+; CHECK-NEXT:    ret void
+;
+Top:
+  callbr void asm sideeffect "", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@caller, %CallSiteBB))
+  to label %NextCond [label %CallSiteBB]
+
+NextCond:
+  br label %CallSiteBB
+
+CallSiteBB:
+  %phi = phi i1 [0, %Top],[1, %NextCond]
+  call void @callee(i1 %phi)
+  callbr void asm sideeffect "", "r,X,~{dirflag},~{fpsr},~{flags}"(i1 %phi, i8* blockaddress(@caller, %End2))
+  to label %End [label %End2]
+
+End:
+  ret void
+End2:
+  ret void
+}
+
+define void @callee(i1 %b) {
+; CHECK-LABEL: @callee(
+; CHECK-NEXT:    ret void
+;
+  ret void
+}