[CGP] Be less conservative about tail-duplicating a ret to allow tail calls

CGP tail-duplicates rets into blocks that end with a call that feed the ret.
This puts the call in tail position, potentially allowing the DAG builder to
lower it as a tail call. To avoid tail duplication in cases where we won't
form the tail call, CGP tried to predict whether this is going to be possible,
and avoids doing it when lowering as a tail call will definitely fail.
However, it was being too conservative by always throwing away calls to
functions with a signext/zeroext attribute on the return type.

Instead, we can use the same logic the builder uses to determine whether the
attributes work out.

Differential Revision: https://reviews.llvm.org/D24315

llvm-svn: 280894

1 files changed, 28 insertions, 16 deletions

diff --git a/llvm/lib/CodeGen/Analysis.cpp b/llvm/lib/CodeGen/Analysis.cpp
index ebbcaeb..6582d24 100644
--- a/llvm/lib/CodeGen/Analysis.cpp
+++ b/llvm/lib/CodeGen/Analysis.cpp
@@ -525,19 +525,15 @@ bool llvm::isInTailCallPosition(ImmutableCallSite CS, const TargetMachine &TM) {
       F, I, Ret, *TM.getSubtargetImpl(*F)->getTargetLowering());
 }
 
-bool llvm::returnTypeIsEligibleForTailCall(const Function *F,
-                                           const Instruction *I,
-                                           const ReturnInst *Ret,
-                                           const TargetLoweringBase &TLI) {
-  // If the block ends with a void return or unreachable, it doesn't matter
-  // what the call's return type is.
-  if (!Ret || Ret->getNumOperands() == 0) return true;
+bool llvm::attributesPermitTailCall(const Function *F, const Instruction *I,
+                                    const ReturnInst *Ret,
+                                    const TargetLoweringBase &TLI,
+                                    bool *AllowDifferingSizes) {
+  // ADS may be null, so don't write to it directly.
+  bool DummyADS;
+  bool &ADS = AllowDifferingSizes ? *AllowDifferingSizes : DummyADS;
+  ADS = true;
 
-  // If the return value is undef, it doesn't matter what the call's
-  // return type is.
-  if (isa<UndefValue>(Ret->getOperand(0))) return true;
-
-  // Make sure the attributes attached to each return are compatible.
   AttrBuilder CallerAttrs(F->getAttributes(),
                           AttributeSet::ReturnIndex);
   AttrBuilder CalleeAttrs(cast<CallInst>(I)->getAttributes(),
@@ -548,19 +544,18 @@ bool llvm::returnTypeIsEligibleForTailCall(const Function *F,
   CallerAttrs = CallerAttrs.removeAttribute(Attribute::NoAlias);
   CalleeAttrs = CalleeAttrs.removeAttribute(Attribute::NoAlias);
 
-  bool AllowDifferingSizes = true;
   if (CallerAttrs.contains(Attribute::ZExt)) {
     if (!CalleeAttrs.contains(Attribute::ZExt))
       return false;
 
-    AllowDifferingSizes = false;
+    ADS = false;
     CallerAttrs.removeAttribute(Attribute::ZExt);
     CalleeAttrs.removeAttribute(Attribute::ZExt);
   } else if (CallerAttrs.contains(Attribute::SExt)) {
     if (!CalleeAttrs.contains(Attribute::SExt))
       return false;
 
-    AllowDifferingSizes = false;
+    ADS = false;
     CallerAttrs.removeAttribute(Attribute::SExt);
     CalleeAttrs.removeAttribute(Attribute::SExt);
   }
@@ -568,7 +563,24 @@ bool llvm::returnTypeIsEligibleForTailCall(const Function *F,
   // If they're still different, there's some facet we don't understand
   // (currently only "inreg", but in future who knows). It may be OK but the
   // only safe option is to reject the tail call.
-  if (CallerAttrs != CalleeAttrs)
+  return CallerAttrs == CalleeAttrs;
+}
+
+bool llvm::returnTypeIsEligibleForTailCall(const Function *F,
+                                           const Instruction *I,
+                                           const ReturnInst *Ret,
+                                           const TargetLoweringBase &TLI) {
+  // If the block ends with a void return or unreachable, it doesn't matter
+  // what the call's return type is.
+  if (!Ret || Ret->getNumOperands() == 0) return true;
+
+  // If the return value is undef, it doesn't matter what the call's
+  // return type is.
+  if (isa<UndefValue>(Ret->getOperand(0))) return true;
+
+  // Make sure the attributes attached to each return are compatible.
+  bool AllowDifferingSizes;
+  if (!attributesPermitTailCall(F, I, Ret, TLI, &AllowDifferingSizes))
     return false;
 
   const Value *RetVal = Ret->getOperand(0), *CallVal = I;