[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, 7 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index 3516f4a7b37..20cd9da31fb 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -709,15 +709,21 @@ void AsmPrinter::EmitFunctionHeader() {
   // Emit M NOPs for -fpatchable-function-entry=N,M where M>0. We arbitrarily
   // place prefix data before NOPs.
   unsigned PatchableFunctionPrefix = 0;
+  unsigned PatchableFunctionEntry = 0;
   (void)F.getFnAttribute("patchable-function-prefix")
       .getValueAsString()
       .getAsInteger(10, PatchableFunctionPrefix);
+  (void)F.getFnAttribute("patchable-function-entry")
+      .getValueAsString()
+      .getAsInteger(10, PatchableFunctionEntry);
   if (PatchableFunctionPrefix) {
     CurrentPatchableFunctionEntrySym =
         OutContext.createLinkerPrivateTempSymbol();
     OutStreamer->EmitLabel(CurrentPatchableFunctionEntrySym);
     emitNops(PatchableFunctionPrefix);
-  } else {
+  } else if (PatchableFunctionEntry) {
+    // May be reassigned when emitting the body, to reference the label after
+    // the initial BTI (AArch64) or endbr32/endbr64 (x86).
     CurrentPatchableFunctionEntrySym = CurrentFnBegin;
   }