[WebAssembly] Change comments on SP writing back (NFC)

Summary: We now write back not to memory but to __stack_pointer global.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D51074

llvm-svn: 340372

llvm/lib/Target/WebAssembly/WebAssemblyFrameLowering.cpp

@@ -99,8 +99,9 @@ bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF,
 }
 
 /// Returns true if the local user-space stack pointer needs to be written back
-/// to memory by this function (this is not meaningful if needsSP is false). If
-/// false, the stack red zone can be used and only a local SP is needed.
+/// to __stack_pointer global by this function (this is not meaningful if
+/// needsSP is false). If false, the stack red zone can be used and only a local
+/// SP is needed.
 bool WebAssemblyFrameLowering::needsSPWriteback(
     const MachineFunction &MF, const MachineFrameInfo &MFI) const {
   assert(needsSP(MF, MFI));

llvm/lib/Target/WebAssembly/WebAssemblyFrameLowering.h

@@ -24,7 +24,8 @@ class MachineFrameInfo;
 class WebAssemblyFrameLowering final : public TargetFrameLowering {
  public:
   /// Size of the red zone for the user stack (leaf functions can use this much
-  /// space below the stack pointer without writing it back to memory).
+  /// space below the stack pointer without writing it back to __stack_pointer
+  /// global).
   // TODO: (ABI) Revisit and decide how large it should be.
   static const size_t RedZoneSize = 128;