[AArch64] -fpatchable-function-entry=N,0: place patch label after BTI

Summary:
For -fpatchable-function-entry=N,0 -mbranch-protection=bti, after
9a24488cb67a90f889529987275c5e411ce01dda, we place the NOP sled after
the initial BTI.

```
.Lfunc_begin0:
bti c
nop
nop

.section __patchable_function_entries,"awo",@progbits,f,unique,0
.p2align 3
.xword .Lfunc_begin0
```

This patch adds a label after the initial BTI and changes the __patchable_function_entries entry to reference the label:

```
.Lfunc_begin0:
bti c
.Lpatch0:
nop
nop

.section __patchable_function_entries,"awo",@progbits,f,unique,0
.p2align 3
.xword .Lpatch0
```

This placement is compatible with the resolution in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92424 .

A local linkage function whose address is not taken does not need a BTI.
Placing the patch label after BTI has the advantage that code does not
need to differentiate whether the function has an initial BTI.

Reviewers: mrutland, nickdesaulniers, nsz, ostannard

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73680

(cherry picked from commit 06b8e32d4fd3f634f793e3bc0bc4fdb885e7a3ac)

1 files changed, 2 insertions, 1 deletions

diff --git a/llvm/include/llvm/CodeGen/AsmPrinter.h b/llvm/include/llvm/CodeGen/AsmPrinter.h
index a860ce2773e..c710c5d7055 100644
--- a/llvm/include/llvm/CodeGen/AsmPrinter.h
+++ b/llvm/include/llvm/CodeGen/AsmPrinter.h
@@ -135,7 +135,6 @@ public:
   MapVector<const MCSymbol *, GOTEquivUsePair> GlobalGOTEquivs;
 
 private:
-  MCSymbol *CurrentFnBegin = nullptr;
   MCSymbol *CurrentFnEnd = nullptr;
   MCSymbol *CurExceptionSym = nullptr;
 
@@ -148,6 +147,8 @@ private:
   static char ID;
 
 protected:
+  MCSymbol *CurrentFnBegin = nullptr;
+
   /// Protected struct HandlerInfo and Handlers permit target extended
   /// AsmPrinter adds their own handlers.
   struct HandlerInfo {