[VE][NFC] Clean stack frame description

[kaz7]

Summary

Move stack frame description from VESubtarget.cpp to VEFrameLowering.cpp
and add detail.

llvm/lib/Target/VE/VEFrameLowering.cpp

 //===-- VEFrameLowering.cpp - VE Frame Information ------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file contains the VE implementation of TargetFrameLowering class.
 //
+// On VE, stack frames are structured as follows:
+//
+// The stack grows downward.
+//
+// All of the individual frame areas on the frame below are optional, i.e. it's
+// possible to create a function so that the particular area isn't present
+// in the frame.
+//
+// At function entry, the "frame" looks as follows:
+//
+// |                                              | Higher address
+// |----------------------------------------------|
+// | Parameter area for this function             |
+// |----------------------------------------------|
+// | Register save area (RSA) for this function   |
+// |----------------------------------------------|
+// | Return address for this function             |
+// |----------------------------------------------|
+// | Frame pointer for this function              |
+// |----------------------------------------------| <- sp
+// |                                              | Lower address
+//
+// VE doesn't use on demand stack allocation, so user code generated by LLVM
+// needs to call VEOS to allocate stack frame.  VE's ABI want to reduce the
+// number of VEOS calls, so ABI requires to allocate not only RSA (in general
+// CSR, callee saved register) area but also call frame at the prologue of
+// caller function.
+//
+// After the prologue has run, the frame has the following general structure.
+// Note that technically the last frame area (VLAs) doesn't get created until
+// in the main function body, after the prologue is run. However, it's depicted
+// here for completeness.
+//
+// |                                              | Higher address
+// |----------------------------------------------|
+// | Parameter area for this function             |
+// |----------------------------------------------|
+// | Register save area (RSA) for this function   |
+// |----------------------------------------------|
+// | Return address for this function             |
+// |----------------------------------------------|
+// | Frame pointer for this function              |
+// |----------------------------------------------| <- fp(=old sp)
+// |.empty.space.to.make.part.below.aligned.in....|
+// |.case.it.needs.more.than.the.standard.16-byte.| (size of this area is
+// |.alignment....................................|  unknown at compile time)
+// |----------------------------------------------|
+// | Local variables of fixed size including spill|
+// | slots                                        |
+// |----------------------------------------------| <- bp(not defined by ABI,
+// |.variable-sized.local.variables.(VLAs)........|       LLVM chooses SX17)
+// |..............................................| (size of this area is
+// |..............................................|  unknown at compile time)
+// |----------------------------------------------| <- stack top (returned by
+// | Parameter area for callee                    |               alloca)
+// |----------------------------------------------|
+// | Register save area (RSA) for callee          |
+// |----------------------------------------------|
+// | Return address for callee                    |
+// |----------------------------------------------|
+// | Frame pointer for callee                     |
+// |----------------------------------------------| <- sp
+// |                                              | Lower address
+//
+// To access the data in a frame, at-compile time, a constant offset must be
+// computable from one of the pointers (fp, bp, sp) to access it. The size
+// of the areas with a dotted background cannot be computed at compile-time
+// if they are present, making it required to have all three of fp, bp and
+// sp to be set up to be able to access all contents in the frame areas,
+// assuming all of the frame areas are non-empty.
+//
+// For most functions, some of the frame areas are empty. For those functions,
+// it may not be necessary to set up fp or bp:
+// * A base pointer is definitely needed when there are both VLAs and local
+//   variables with more-than-default alignment requirements.
+// * A frame pointer is definitely needed when there are local variables with
+//   more-than-default alignment requirements.
+//
+// In addition, VE ABI defines RSA frame, return address, and frame pointer
+// as follows:
+//
+// |----------------------------------------------| <- sp+176
+// | %s18...%s33                                  |
+// |----------------------------------------------| <- sp+48
+// | Linkage area register (%s17)                 |
+// |----------------------------------------------| <- sp+40
+// | Procedure linkage table register (%plt=%s16) |
+// |----------------------------------------------| <- sp+32
+// | Global offset table register (%got=%s15)     |
+// |----------------------------------------------| <- sp+24
+// | Thread pointer register (%tp=%s14)           |
+// |----------------------------------------------| <- sp+16
+// | Return address                               |
+// |----------------------------------------------| <- sp+8
+// | Frame pointer                                |
+// |----------------------------------------------| <- sp+0
+//
+// NOTE: This description is based on VE ABI and description in
+//       AArch64FrameLowering.cpp.  Thanks a lot.
 //===----------------------------------------------------------------------===//
 
 #include "VEFrameLowering.h"
 #include "VEInstrInfo.h"
 #include "VEMachineFunctionInfo.h"
 #include "VESubtarget.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"

llvm/lib/Target/VE/VESubtarget.cpp

 
 VESubtarget::VESubtarget(const Triple &TT, const std::string &CPU,
                          const std::string &FS, const TargetMachine &TM)
     : VEGenSubtargetInfo(TT, CPU, /*TuneCPU=*/CPU, FS), TargetTriple(TT),
       InstrInfo(initializeSubtargetDependencies(CPU, FS)), TLInfo(TM, *this),
       FrameLowering(*this) {}
 
 uint64_t VESubtarget::getAdjustedFrameSize(uint64_t FrameSize) const {
-
-  // VE stack frame:
-  //
-  //         +----------------------------------------+
-  //         | Locals and temporaries                 |
-  //         +----------------------------------------+
-  //         | Parameter area for callee              |
-  // 176(fp) |                                        |
-  //         +----------------------------------------+
-  //         | Register save area (RSA) for callee    |
-  //         |                                        |
-  //  16(fp) |                         20 * 8 bytes   |
-  //         +----------------------------------------+
-  //   8(fp) | Return address                         |
-  //         +----------------------------------------+
-  //   0(fp) | Frame pointer of caller                |
-  // --------+----------------------------------------+--------
-  //         | Locals and temporaries for callee      |
-  //         +----------------------------------------+
-  //         | Parameter area for callee of callee    |
-  //         +----------------------------------------+
-  //  16(sp) | RSA for callee of callee               |
-  //         +----------------------------------------+
-  //   8(sp) | Return address                         |
-  //         +----------------------------------------+
-  //   0(sp) | Frame pointer of callee                |
-  //         +----------------------------------------+
-
-  // RSA frame:
-  //         +----------------------------------------------+
-  // 168(fp) | %s33                                         |
-  //         +----------------------------------------------+
-  //         | %s19...%s32                                  |
-  //         +----------------------------------------------+
-  //  48(fp) | %s18                                         |
-  //         +----------------------------------------------+
-  //  40(fp) | Linkage area register (%s17)                 |
-  //         +----------------------------------------------+
-  //  32(fp) | Procedure linkage table register (%plt=%s16) |
-  //         +----------------------------------------------+
-  //  24(fp) | Global offset table register (%got=%s15)     |
-  //         +----------------------------------------------+
-  //  16(fp) | Thread pointer register (%tp=%s14)           |
-  //         +----------------------------------------------+
+  // Calculate adjusted frame size by adding the size of RSA frame,
+  // return address, and frame poitner as described in VEFrameLowering.cpp.
 
   FrameSize += 176;                   // For RSA, RA, and FP.
   FrameSize = alignTo(FrameSize, 16); // Requires 16 bytes alignment.
 
   return FrameSize;
 }
 
 bool VESubtarget::enableMachineScheduler() const { return true; }