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[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).

llvm-svn: 281006
This commit is contained in:
David Majnemer 2016-09-09 01:07:01 +00:00
parent ceff022800
commit 2c3ea55498
1 changed files with 3 additions and 3 deletions
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6
llvm/lib/Target/X86/X86FrameLowering.cpp
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@ -1400,9 +1400,9 @@ void X86FrameLowering::emitPrologue(MachineFunction &MF,
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.