Fix function pointer tail calls in armv8-M.base

Summary:
The compiler fails with the following error message:

fatal error: error in backend: ran out of registers during
register allocation

Tail call optimization for Armv8-M.base fails to meet all the required
constraints when handling calls to function pointers where the
arguments take up r0-r3. This is because the pointer to the
function to be called can only be stored in r0-r3, but these are
all occupied by arguments. This patch makes sure that tail call
optimization does not try to handle this type of calls.

Reviewers: chill, MatzeB, olista01, rengolin, efriedma

Reviewed By: olista01, efriedma

Subscribers: efriedma, aemerson, javed.absar, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D40706

llvm-svn: 319664

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index d9893db3773..1726d6bcb30 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -2282,6 +2282,13 @@ ARMTargetLowering::IsEligibleForTailCallOptimization(SDValue Callee,
 
   assert(Subtarget->supportsTailCall());
 
+  // Tail calls to function pointers cannot be optimized for Thumb1 if the args
+  // to the call take up r0-r3. The reason is that there are no legal registers
+  // left to hold the pointer to the function to be called.
+  if (Subtarget->isThumb1Only() && Outs.size() >= 4 &&
+      !isa<GlobalAddressSDNode>(Callee.getNode()))
+      return false;
+
   // Look for obvious safe cases to perform tail call optimization that do not
   // require ABI changes. This is what gcc calls sibcall.