[GVN] do not repeat PRE on failure to split critical edge

Fixes an infinite loop encountered in GVN.

GVN will delay PRE if it encounters critical edges, attempt to split
them later via calls to SplitCriticalEdge(), then restart.

The caller of GVN::splitCriticalEdges() assumed a return value of true
meant that critical edges were split, that the IR had changed, and that
PRE should be re-attempted, upon which we loop infinitely.

This was exposed after D88438, by compiling the Linux kernel for s390,
but the test case is reproducible on x86.

Fixes: ClangBuiltLinux/linux#1261

Reviewed By: void

Differential Revision: https://reviews.llvm.org/D94996

Author: nickdesaulniers

llvm/lib/Transforms/Scalar/GVN.cpp

@@ -2673,9 +2673,11 @@ BasicBlock *GVN::splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ) {
   BasicBlock *BB = SplitCriticalEdge(
       Pred, Succ,
       CriticalEdgeSplittingOptions(DT, LI, MSSAU).unsetPreserveLoopSimplify());
-  if (MD)
-    MD->invalidateCachedPredecessors();
-  InvalidBlockRPONumbers = true;
+  if (BB) {
+    if (MD)
+      MD->invalidateCachedPredecessors();
+    InvalidBlockRPONumbers = true;
+  }
   return BB;
 }
 
@@ -2684,14 +2686,20 @@ BasicBlock *GVN::splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ) {
 bool GVN::splitCriticalEdges() {
   if (toSplit.empty())
     return false;
+
+  bool Changed = false;
   do {
     std::pair<Instruction *, unsigned> Edge = toSplit.pop_back_val();
-    SplitCriticalEdge(Edge.first, Edge.second,
-                      CriticalEdgeSplittingOptions(DT, LI, MSSAU));
+    Changed |= SplitCriticalEdge(Edge.first, Edge.second,
+                                 CriticalEdgeSplittingOptions(DT, LI, MSSAU)) !=
+               nullptr;
   } while (!toSplit.empty());
-  if (MD) MD->invalidateCachedPredecessors();
-  InvalidBlockRPONumbers = true;
-  return true;
+  if (Changed) {
+    if (MD)
+      MD->invalidateCachedPredecessors();
+    InvalidBlockRPONumbers = true;
+  }
+  return Changed;
 }
 
 /// Executes one iteration of GVN

llvm/test/Transforms/GVN/critical-edge-split-failure.ll

@@ -0,0 +1,49 @@
+; RUN: opt -gvn -S -o - %s | FileCheck %s
+; RUN: opt -passes=gvn -S -o - %s | FileCheck %s
+
+%struct.sk_buff = type opaque
+
+@l2tp_recv_dequeue_session = external dso_local local_unnamed_addr global i32, align 4
+@l2tp_recv_dequeue_skb = external dso_local local_unnamed_addr global %struct.sk_buff*, align 8
+@l2tp_recv_dequeue_session_2 = external dso_local local_unnamed_addr global i32, align 4
+@l2tp_recv_dequeue_session_0 = external dso_local local_unnamed_addr global i32, align 4
+
+declare void @llvm.assume(i1 noundef)
+
+define dso_local void @l2tp_recv_dequeue() local_unnamed_addr {
+entry:
+  %0 = load i32, i32* @l2tp_recv_dequeue_session, align 4
+  %conv = sext i32 %0 to i64
+  %1 = inttoptr i64 %conv to %struct.sk_buff*
+  %2 = load i32, i32* @l2tp_recv_dequeue_session_2, align 4
+  %tobool.not = icmp eq i32 %2, 0
+  br label %for.cond
+
+for.cond:                                         ; preds = %if.end, %entry
+  %storemerge = phi %struct.sk_buff* [ %1, %entry ], [ null, %if.end ]
+  store %struct.sk_buff* %storemerge, %struct.sk_buff** @l2tp_recv_dequeue_skb, align 8
+  br i1 %tobool.not, label %if.end, label %if.then
+
+if.then:                                          ; preds = %for.cond
+  %ns = bitcast %struct.sk_buff* %storemerge to i32*
+  %3 = load i32, i32* %ns, align 4
+  store i32 %3, i32* @l2tp_recv_dequeue_session_0, align 4
+; Splitting the critical edge from if.then to if.end will fail, but should not
+; cause an infinite loop in GVN. If we can one day split edges of callbr
+; indirect targets, great!
+; CHECK: callbr void asm sideeffect "", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@l2tp_recv_dequeue, %if.end))
+; CHECK-NEXT: to label %asm.fallthrough.i [label %if.end]
+  callbr void asm sideeffect "", "X,~{dirflag},~{fpsr},~{flags}"(i8* blockaddress(@l2tp_recv_dequeue, %if.end))
+          to label %asm.fallthrough.i [label %if.end]
+
+asm.fallthrough.i:                                ; preds = %if.then
+  br label %if.end
+
+if.end:                                           ; preds = %asm.fallthrough.i, %if.then, %for.cond
+  %ns1 = bitcast %struct.sk_buff* %storemerge to i32*
+  %4 = load i32, i32* %ns1, align 4
+  %tobool2.not = icmp eq i32 %4, 0
+  tail call void @llvm.assume(i1 %tobool2.not)
+  br label %for.cond
+}
+