[SystemZ][z/OS] Update lowerCall (#68259)

This PR moves some calculation out of `LowerCall` and into
`SystemZXPLINKFrameLowering::processFunctionBeforeFrameFinalized`.
We need to make this change because LowerCall isn't invoked for
functions that don't have function calls, and it is required for some
tooling to work correctly. A function that does not make any calls is
required to allocate 32 bytes for the parameter area required by the
ABI. However, we allocate 64 bytes because this additional space is
utilized by certain tools, like the debugger.

Co-authored-by: Yusra Syeda <[email protected]>

Author: ysyeda

llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp

@@ -1423,6 +1423,18 @@ void SystemZXPLINKFrameLowering::processFunctionBeforeFrameFinalized(
 
   // Setup stack frame offset
   MFFrame.setOffsetAdjustment(Regs.getStackPointerBias());
+
+  // Nothing to do for leaf functions.
+  uint64_t StackSize = MFFrame.estimateStackSize(MF);
+  if (StackSize == 0 && MFFrame.getCalleeSavedInfo().empty())
+    return;
+
+  // Although the XPLINK specifications for AMODE64 state that minimum size
+  // of the param area is minimum 32 bytes and no rounding is otherwise
+  // specified, we round this area in 64 bytes increments to be compatible
+  // with existing compilers.
+  MFFrame.setMaxCallFrameSize(
+      std::max(64U, (unsigned)alignTo(MFFrame.getMaxCallFrameSize(), 64)));
 }
 
 // Determines the size of the frame, and creates the deferred spill objects.

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

@@ -1816,13 +1816,6 @@ SystemZTargetLowering::LowerCall(CallLoweringInfo &CLI,
   // Get a count of how many bytes are to be pushed on the stack.
   unsigned NumBytes = ArgCCInfo.getStackSize();
 
-  if (Subtarget.isTargetXPLINK64())
-    // Although the XPLINK specifications for AMODE64 state that minimum size
-    // of the param area is minimum 32 bytes and no rounding is otherwise
-    // specified, we round this area in 64 bytes increments to be compatible
-    // with existing compilers.
-    NumBytes = std::max(64U, (unsigned)alignTo(NumBytes, 64));
-
   // Mark the start of the call.
   if (!IsTailCall)
     Chain = DAG.getCALLSEQ_START(Chain, NumBytes, 0, DL);