From 7ce61db5caa6858bfa40fd33bcbc44b9123a39cf Mon Sep 17 00:00:00 2001
From: Tom Stellard <tstellar@redhat.com>
Date: Wed, 11 Apr 2018 22:38:11 +0000
Subject: Merging r326404:

------------------------------------------------------------------------
r326404 | rnk | 2018-02-28 17:19:18 -0800 (Wed, 28 Feb 2018) | 14 lines

[IPSCCP] do not break musttail invariant (PR36485)

Do not replace results of `musttail` calls with a constant if the
call itself can't be removed.

Do not zap returns of `musttail` callees, if the call site can't be
removed and replaced with a constant.

Do not zap returns of `musttail`-calling blocks, this breaks
invariant too.

Patch by Fedor Indutny

Differential Revision: https://reviews.llvm.org/D43695
------------------------------------------------------------------------

git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_60@329855 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Scalar/SCCP.cpp | 56 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 55 insertions(+), 1 deletion(-)

diff --git a/lib/Transforms/Scalar/SCCP.cpp b/lib/Transforms/Scalar/SCCP.cpp
index 9dc550ceaeca..69636ed3ac8b 100644
--- a/lib/Transforms/Scalar/SCCP.cpp
+++ b/lib/Transforms/Scalar/SCCP.cpp
@@ -223,6 +223,10 @@ class SCCPSolver : public InstVisitor<SCCPSolver> {
   /// represented here for efficient lookup.
   SmallPtrSet<Function *, 16> MRVFunctionsTracked;
 
+  /// MustTailFunctions - Each function here is a callee of non-removable
+  /// musttail call site.
+  SmallPtrSet<Function *, 16> MustTailCallees;
+
   /// TrackingIncomingArguments - This is the set of functions for whose
   /// arguments we make optimistic assumptions about and try to prove as
   /// constants.
@@ -289,6 +293,18 @@ public:
       TrackedRetVals.insert(std::make_pair(F, LatticeVal()));
   }
 
+  /// AddMustTailCallee - If the SCCP solver finds that this function is called
+  /// from non-removable musttail call site.
+  void AddMustTailCallee(Function *F) {
+    MustTailCallees.insert(F);
+  }
+
+  /// Returns true if the given function is called from non-removable musttail
+  /// call site.
+  bool isMustTailCallee(Function *F) {
+    return MustTailCallees.count(F);
+  }
+
   void AddArgumentTrackedFunction(Function *F) {
     TrackingIncomingArguments.insert(F);
   }
@@ -358,6 +374,12 @@ public:
     return MRVFunctionsTracked;
   }
 
+  /// getMustTailCallees - Get the set of functions which are called
+  /// from non-removable musttail call sites.
+  const SmallPtrSet<Function *, 16> getMustTailCallees() {
+    return MustTailCallees;
+  }
+
   /// markOverdefined - Mark the specified value overdefined.  This
   /// works with both scalars and structs.
   void markOverdefined(Value *V) {
@@ -1672,6 +1694,23 @@ static bool tryToReplaceWithConstant(SCCPSolver &Solver, Value *V) {
           IV.isConstant() ? IV.getConstant() : UndefValue::get(V->getType());
   }
   assert(Const && "Constant is nullptr here!");
+
+  // Replacing `musttail` instructions with constant breaks `musttail` invariant
+  // unless the call itself can be removed
+  CallInst *CI = dyn_cast<CallInst>(V);
+  if (CI && CI->isMustTailCall() && !isInstructionTriviallyDead(CI)) {
+    CallSite CS(CI);
+    Function *F = CS.getCalledFunction();
+
+    // Don't zap returns of the callee
+    if (F)
+      Solver.AddMustTailCallee(F);
+
+    DEBUG(dbgs() << "  Can\'t treat the result of musttail call : " << *CI
+                 << " as a constant\n");
+    return false;
+  }
+
   DEBUG(dbgs() << "  Constant: " << *Const << " = " << *V << '\n');
 
   // Replaces all of the uses of a variable with uses of the constant.
@@ -1802,10 +1841,25 @@ static void findReturnsToZap(Function &F,
   if (!Solver.isArgumentTrackedFunction(&F))
     return;
 
-  for (BasicBlock &BB : F)
+  // There is a non-removable musttail call site of this function. Zapping
+  // returns is not allowed.
+  if (Solver.isMustTailCallee(&F)) {
+    DEBUG(dbgs() << "Can't zap returns of the function : " << F.getName()
+                 << " due to present musttail call of it\n");
+    return;
+  }
+
+  for (BasicBlock &BB : F) {
+    if (CallInst *CI = BB.getTerminatingMustTailCall()) {
+      DEBUG(dbgs() << "Can't zap return of the block due to present "
+                   << "musttail call : " << *CI << "\n");
+      return;
+    }
+
     if (auto *RI = dyn_cast<ReturnInst>(BB.getTerminator()))
       if (!isa<UndefValue>(RI->getOperand(0)))
         ReturnsToZap.push_back(RI);
+  }
 }
 
 static bool runIPSCCP(Module &M, const DataLayout &DL,
-- 
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