[PATCH] D40355: [PowerPC] Allow tail calls of fastcc functions from C CallingConv functions.

[Sean Fertile via Phabricator via llvm-commits]

sfertile created this revision.

Currently the tail call checks require the caller and callee to have the same calling convention. This patches relaxes this restriction to allow fastcc callees to be tail-called from ccc callers.


Repository:
  rL LLVM

https://reviews.llvm.org/D40355

Files:
  lib/Target/PowerPC/PPCISelLowering.cpp
  test/CodeGen/PowerPC/duplicate-returns-for-tailcall.ll


Index: test/CodeGen/PowerPC/duplicate-returns-for-tailcall.ll
===================================================================
--- test/CodeGen/PowerPC/duplicate-returns-for-tailcall.ll
+++ test/CodeGen/PowerPC/duplicate-returns-for-tailcall.ll
@@ -42,10 +42,10 @@
 if.then6:                                         ; preds = %if.end4
   %call7 = tail call fastcc signext i32 @call3(i32 signext %a, i32 signext %b, i32 signext %c)
   br label %return
-; No duplication here because the calling convention mismatch means we won't tail-call
+; tail calling a fastcc function from a ccc function is supported.
 ; CHECK_LABEL: if.then13:
-; CHECK:       tail call fastcc signext i32 @call3
-; CHECK-NEXT:  br
+; CHECK:       %[[T2:[a-zA-Z0-9]+]] = tail call fastcc signext i32 @call3
+; CHECK-NEXT:  ret i32 %[[T2]]
 
 return:                                           ; preds = %if.end4, %if.then6, %if.then2, %if.then
   %retval.0 = phi i32 [ %call, %if.then ], [ %call3, %if.then2 ], [ %call7, %if.then6 ], [ %c, %if.end4 ]
Index: lib/Target/PowerPC/PPCISelLowering.cpp
===================================================================
--- lib/Target/PowerPC/PPCISelLowering.cpp
+++ lib/Target/PowerPC/PPCISelLowering.cpp
@@ -4397,13 +4397,18 @@
 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
+  // then a non-fastcc caller with the same signature so disable tail-calls in
+  // that case.
+  return CallerCC == CallingConv::C || CallerCC == CalleeCC;
 }
 
 bool


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D40355.123933.patch
Type: text/x-patch
Size: 2253 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20171122/a95e259b/attachment.bin>