[CodeGen] isInTailCallPosition didn't consider readnone tailcalls

A readnone tailcall may still have a chain of computation which follows
it that would invalidate a tailcall lowering.  Don't skip the analysis
in such cases.

This fixes PR24613.

llvm-svn: 246304

2 files changed, 12 insertions, 13 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index d3d71182b76..ab6907dcbd9 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3147,7 +3147,8 @@ bool llvm::isSafeToSpeculativelyExecute(const Value *V,
         LI->getPointerOperand(), LI->getAlignment(), DL, CtxI, DT, TLI);
   }
   case Instruction::Call: {
-    if (cast<CallInst>(Inst)->doesNotAccessMemory())
+    auto *CI = cast<CallInst>(Inst);
+    if (CI->doesNotAccessMemory() && !CI->isMustTailCall())
       return true;
     return false; // The called function could have undefined behavior or
                   // side-effects.
diff --git a/llvm/lib/CodeGen/Analysis.cpp b/llvm/lib/CodeGen/Analysis.cpp
index 98d4c8afc7b..33ad88358e5 100644
--- a/llvm/lib/CodeGen/Analysis.cpp
+++ b/llvm/lib/CodeGen/Analysis.cpp
@@ -506,18 +506,16 @@ bool llvm::isInTailCallPosition(ImmutableCallSite CS, const TargetMachine &TM) {
 
   // If I will have a chain, make sure no other instruction that will have a
   // chain interposes between I and the return.
-  if (I->mayHaveSideEffects() || I->mayReadFromMemory() ||
-      !isSafeToSpeculativelyExecute(I))
-    for (BasicBlock::const_iterator BBI = std::prev(ExitBB->end(), 2);; --BBI) {
-      if (&*BBI == I)
-        break;
-      // Debug info intrinsics do not get in the way of tail call optimization.
-      if (isa<DbgInfoIntrinsic>(BBI))
-        continue;
-      if (BBI->mayHaveSideEffects() || BBI->mayReadFromMemory() ||
-          !isSafeToSpeculativelyExecute(BBI))
-        return false;
-    }
+  for (BasicBlock::const_iterator BBI = std::prev(ExitBB->end(), 2);; --BBI) {
+    if (&*BBI == I)
+      break;
+    // Debug info intrinsics do not get in the way of tail call optimization.
+    if (isa<DbgInfoIntrinsic>(BBI))
+      continue;
+    if (BBI->mayHaveSideEffects() || BBI->mayReadFromMemory() ||
+        !isSafeToSpeculativelyExecute(BBI))
+      return false;
+  }
 
   const Function *F = ExitBB->getParent();
   return returnTypeIsEligibleForTailCall(