[SystemZ] Bugfix for backchain with packed-stack

[jonpa]

Summary

The incoming back chain slot was implicitly allocated whenever a GPR was saved in SystemZFrameLowering::getRegSpillOffset(), but in cases where no GPRs were saved/restored this did not take effect.

[uweigand]

LGTM, thanks!

llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp

restoreCalleeSavedRegisters(MachineBasicBlock &MBB,
 
   return true;
 }
 
 void SystemZFrameLowering::
 processFunctionBeforeFrameFinalized(MachineFunction &MF,
                                     RegScavenger *RS) const {
   MachineFrameInfo &MFFrame = MF.getFrameInfo();
+  bool BackChain = MF.getFunction().hasFnAttribute("backchain");
 
-  if (!usePackedStack(MF))
-    // Always create the full incoming register save area.
+  if (!usePackedStack(MF) || BackChain)
+    // Create the incoming register save area.
     getOrCreateFramePointerSaveIndex(MF);
 
   // Get the size of our stack frame to be allocated ...
   uint64_t StackSize = (MFFrame.estimateStackSize(MF) +
                         SystemZMC::CallFrameSize);
   // ... and the maximum offset we may need to reach into the
   // caller's frame to access the save area or stack arguments.
   int64_t MaxArgOffset = 0;

llvm/test/CodeGen/SystemZ/frame-25.ll

+; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+;
+; Test that space is allocated for the incoming back chain also in cases
+; where no GPRs are saved / restored.
+
+define void @fun0() #0 {
+; CHECK-LABEL: fun0:
+; CHECK: lgr     %r1, %r15
+; CHECK-NEXT: aghi    %r15, -24
+; CHECK-NEXT: stg     %r1, 152(%r15)
+; CHECK-NEXT: #APP
+; CHECK-NEXT: stcke   160(%r15)
+; CHECK-NEXT: #NO_APP
+; CHECK-NEXT: aghi    %r15, 24
+; CHECK-NEXT: br      %r14
+
+entry:
+  %b = alloca [16 x i8], align 1
+  %0 = getelementptr inbounds [16 x i8], [16 x i8]* %b, i64 0, i64 0
+  call void asm "stcke $0", "=*Q"([16 x i8]* nonnull %b) #2
+  ret void
+}
+
+attributes #0 = { nounwind "packed-stack" "backchain" "use-soft-float"="true" }