musttail: Forward regparms of variadic functions on x86_64

Summary:
If a variadic function body contains a musttail call, then we copy all
of the remaining register parameters into virtual registers in the
function prologue. We track the virtual registers through the function
body, and add them as additional registers to pass to the call. Because
this is all done in virtual registers, the register allocator usually
gives us good code. If the function does a call, however, it will have
to spill and reload all argument registers (ew).

Forwarding regparms on x86_32 is not implemented because most compilers
don't support varargs in 32-bit with regparms.

Reviewers: majnemer

Subscribers: aemerson, llvm-commits

Differential Revision: http://reviews.llvm.org/D5060

llvm-svn: 216780

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp b/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
index c9da6fd83ee..6570e141181 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
@@ -146,6 +146,13 @@ void FunctionLoweringInfo::set(const Function &fn, MachineFunction &mf,
           MF->getFrameInfo()->setHasVAStart(true);
       }
 
+      // If we have a musttail call in a variadic funciton, we need to ensure we
+      // forward implicit register parameters.
+      if (auto *CI = dyn_cast<CallInst>(I)) {
+        if (CI->isMustTailCall() && Fn->isVarArg())
+          MF->getFrameInfo()->setHasMustTailInVarArgFunc(true);
+      }
+
       // Mark values used outside their block as exported, by allocating
       // a virtual register for them.
       if (isUsedOutsideOfDefiningBlock(I))