[PowerPC] fix a bug in TCO eligibility check

If the callee and caller use different calling convensions, we cannot apply TCO if the callee requires arguments on stack; e.g. C calling convention and Fast CC use the same registers for parameter passing, but the stack offset is not necessarily same.

This patch also recommit r319218 "[PowerPC] Allow tail calls of fastcc functions from C CallingConv functions." by @sfertile since the problem reported in r320106 should be fixed.

Differential Revision: https://reviews.llvm.org/D40893

llvm-svn: 321579

1 files changed, 29 insertions, 6 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
index cea59de3e8a..f9de65fcb1d 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -4397,13 +4397,18 @@ hasSameArgumentList(const Function *CallerFn, ImmutableCallSite CS) {
 static bool
 areCallingConvEligibleForTCO_64SVR4(CallingConv::ID CallerCC,
                                     CallingConv::ID CalleeCC) {
-  // Tail or Sibling call optimization (TCO/SCO) needs callee and caller to
-  // have the same calling convention.
-  if (CallerCC != CalleeCC)
+  // Tail calls are possible with fastcc and ccc.
+  auto isTailCallableCC  = [] (CallingConv::ID CC){
+      return  CC == CallingConv::C || CC == CallingConv::Fast;
+  };
+  if (!isTailCallableCC(CallerCC) || !isTailCallableCC(CalleeCC))
     return false;
 
-  // Tail or Sibling calls can be done with fastcc/ccc.
-  return (CallerCC == CallingConv::Fast || CallerCC == CallingConv::C);
+  // We can safely tail call both fastcc and ccc callees from a c calling
+  // convention caller. If the caller is fastcc, we may have less stack space
+  // than a non-fastcc caller with the same signature so disable tail-calls in
+  // that case.
+  return CallerCC == CallingConv::C || CallerCC == CalleeCC;
 }
 
 bool
@@ -4434,10 +4439,28 @@ PPCTargetLowering::IsEligibleForTailCallOptimization_64SVR4(
   // Callee contains any byval parameter is not supported, too.
   // Note: This is a quick work around, because in some cases, e.g.
   // caller's stack size > callee's stack size, we are still able to apply
-  // sibling call optimization. See: https://reviews.llvm.org/D23441#513574
+  // sibling call optimization. For example, gcc is able to do SCO for caller1
+  // in the following example, but not for caller2.
+  //   struct test {
+  //     long int a;
+  //     char ary[56];
+  //   } gTest;
+  //   __attribute__((noinline)) int callee(struct test v, struct test *b) {
+  //     b->a = v.a;
+  //     return 0;
+  //   }
+  //   void caller1(struct test a, struct test c, struct test *b) {
+  //     callee(gTest, b); }
+  //   void caller2(struct test *b) { callee(gTest, b); }
   if (any_of(Outs, [](const ISD::OutputArg& OA) { return OA.Flags.isByVal(); }))
     return false;
 
+  // If callee and caller use different calling conventions, we cannot pass
+  // parameters on stack since offsets for the parameter area may be different.
+  if (Caller.getCallingConv() != CalleeCC &&
+      needStackSlotPassParameters(Subtarget, Outs))
+    return false;
+
   // No TCO/SCO on indirect call because Caller have to restore its TOC
   if (!isFunctionGlobalAddress(Callee) &&
       !isa<ExternalSymbolSDNode>(Callee))