[llvm] r281006 - [X86] Tighten up a comment which confused x64 ABI terminology.

[David Majnemer via llvm-commits]

Author: majnemer
Date: Thu Sep  8 20:07:01 2016
New Revision: 281006

URL: http://llvm.org/viewvc/llvm-project?rev=281006&view=rev
Log:
[X86] Tighten up a comment which confused x64 ABI terminology.

The x64 ABI has two major function types:
 - frame functions
 - leaf functions

A frame function is one which requires a stack frame.  A leaf function
is one which does not.  A frame function may or may not have a frame
pointer.

A leaf function does not require a stack frame and may never modify SP
except via a return (RET, tail call via JMP).

A frame function which has a frame pointer is permitted to use the LEA
instruction in the epilogue, a frame function without which doesn't
establish a frame pointer must use ADD to adjust the stack pointer epilogue.

Fun fact: Leaf functions don't require a function table entry
(associated PDATA/XDATA).

Modified:
    llvm/trunk/lib/Target/X86/X86FrameLowering.cpp

Modified: llvm/trunk/lib/Target/X86/X86FrameLowering.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86FrameLowering.cpp?rev=281006&r1=281005&r2=281006&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86FrameLowering.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86FrameLowering.cpp Thu Sep  8 20:07:01 2016
@@ -1400,9 +1400,9 @@ void X86FrameLowering::emitPrologue(Mach
 
 bool X86FrameLowering::canUseLEAForSPInEpilogue(
     const MachineFunction &MF) const {
-  // We can't use LEA instructions for adjusting the stack pointer if this is a
-  // leaf function in the Win64 ABI.  Only ADD instructions may be used to
-  // deallocate the stack.
+  // We can't use LEA instructions for adjusting the stack pointer if we don't
+  // have a frame pointer in the Win64 ABI.  Only ADD instructions may be used
+  // to deallocate the stack.
   // This means that we can use LEA for SP in two situations:
   // 1. We *aren't* using the Win64 ABI which means we are free to use LEA.
   // 2. We *have* a frame pointer which means we are permitted to use LEA.