[ARM] Avoid spilling lr with Thumb1 tail calls.

Normally, if any registers are spilled, we prefer to spill lr on Thumb1
so we can fold the "bx lr" into the "pop".  However, if there are tail
calls involved, restoring lr is expensive, so skip the optimization in
that case.

The spill of r7 in the new test also isn't necessary, but that's
mostly orthogonal to this patch. (It's the same code in
ARMFrameLowering, but it's not related to tail calls.)

Differential Revision: https://reviews.llvm.org/D49459

llvm-svn: 339283

1 files changed, 7 insertions, 2 deletions

diff --git a/llvm/lib/Target/ARM/ARMFrameLowering.cpp b/llvm/lib/Target/ARM/ARMFrameLowering.cpp
index a8c75702d7b..40de21eb5cd 100644
--- a/llvm/lib/Target/ARM/ARMFrameLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMFrameLowering.cpp
@@ -1920,9 +1920,13 @@ void ARMFrameLowering::determineCalleeSaves(MachineFunction &MF,
                         << "\n");
     }
 
+    // Avoid spilling LR in Thumb1 if there's a tail call: it's expensive to
+    // restore LR in that case.
+    bool ExpensiveLRRestore = AFI->isThumb1OnlyFunction() && MFI.hasTailCall();
+
     // If LR is not spilled, but at least one of R4, R5, R6, and R7 is spilled.
     // Spill LR as well so we can fold BX_RET to the registers restore (LDM).
-    if (!LRSpilled && CS1Spilled) {
+    if (!LRSpilled && CS1Spilled && !ExpensiveLRRestore) {
       SavedRegs.set(ARM::LR);
       NumGPRSpills++;
       SmallVectorImpl<unsigned>::iterator LRPos;
@@ -1948,7 +1952,8 @@ void ARMFrameLowering::determineCalleeSaves(MachineFunction &MF,
           // Windows on ARM, accept R11 (frame pointer)
           if (!AFI->isThumbFunction() ||
               (STI.isTargetWindows() && Reg == ARM::R11) ||
-              isARMLowRegister(Reg) || Reg == ARM::LR) {
+              isARMLowRegister(Reg) ||
+              (Reg == ARM::LR && !ExpensiveLRRestore)) {
             SavedRegs.set(Reg);
             LLVM_DEBUG(dbgs() << "Spilling " << printReg(Reg, TRI)
                               << " to make up alignment\n");