[NFC][PPC][AIX] Add stack frame layout diagram to PPCISelLowering.cpp

Summary:
This NFC patch adds a diagram of the AIX ABI stack frame layout.

Based on https://www.ibm.com/support/knowledgecenter/en/ssw_aix_72/assembler/idalangref_runtime_process.html

Reviewers: sfertile, cebowleratibm, hubert.reinterpretcast, Xiangling_L

Reviewed By: sfertile

Subscribers: wuzish, nemanjai, hiraditya, kbarton, llvm-commits

Tags: #powerpc, #llvm

Differential Revision: https://reviews.llvm.org/D82408

diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
index 28bd806..c5b21b8 100644 (file)
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -7200,6 +7200,46 @@ static unsigned mapArgRegToOffsetAIX(unsigned Reg, const PPCFrameLowering *FL) {
   llvm_unreachable("Only general purpose registers expected.");
 }
 
+//   AIX ABI Stack Frame Layout:
+//
+//   Low Memory +--------------------------------------------+
+//   SP   +---> | Back chain                                 | ---+
+//        |     +--------------------------------------------+    |   
+//        |     | Saved Condition Register                   |    |
+//        |     +--------------------------------------------+    |
+//        |     | Saved Linkage Register                     |    |
+//        |     +--------------------------------------------+    | Linkage Area
+//        |     | Reserved for compilers                     |    |
+//        |     +--------------------------------------------+    |
+//        |     | Reserved for binders                       |    |
+//        |     +--------------------------------------------+    |
+//        |     | Saved TOC pointer                          | ---+
+//        |     +--------------------------------------------+
+//        |     | Parameter save area                        |
+//        |     +--------------------------------------------+
+//        |     | Alloca space                               |
+//        |     +--------------------------------------------+
+//        |     | Local variable space                       |
+//        |     +--------------------------------------------+
+//        |     | Float/int conversion temporary             |
+//        |     +--------------------------------------------+
+//        |     | Save area for AltiVec registers            |
+//        |     +--------------------------------------------+
+//        |     | AltiVec alignment padding                  |
+//        |     +--------------------------------------------+
+//        |     | Save area for VRSAVE register              |
+//        |     +--------------------------------------------+
+//        |     | Save area for General Purpose registers    |
+//        |     +--------------------------------------------+
+//        |     | Save area for Floating Point registers     |
+//        |     +--------------------------------------------+
+//        +---- | Back chain                                 |
+// High Memory  +--------------------------------------------+
+//
+//  Specifications:
+//  AIX 7.2 Assembler Language Reference
+//  Subroutine linkage convention
+
 SDValue PPCTargetLowering::LowerFormalArguments_AIX(
     SDValue Chain, CallingConv::ID CallConv, bool isVarArg,
     const SmallVectorImpl<ISD::InputArg> &Ins, const SDLoc &dl,
@@ -7425,6 +7465,8 @@ SDValue PPCTargetLowering::LowerCall_AIX(
     const SmallVectorImpl<ISD::InputArg> &Ins, const SDLoc &dl,
     SelectionDAG &DAG, SmallVectorImpl<SDValue> &InVals,
     const CallBase *CB) const {
+  // See PPCTargetLowering::LowerFormalArguments_AIX() for a description of the
+  // AIX ABI stack frame layout.
 
   assert((CFlags.CallConv == CallingConv::C ||
           CFlags.CallConv == CallingConv::Cold ||