Add function attribute "patchable-function-prefix" to support -fpatchable-function-entry=N,M where M>0

Similar to the function attribute `prefix` (prefix data),
"patchable-function-prefix" inserts data (M NOPs) before the function
entry label.

-fpatchable-function-entry=2,1 (1 NOP before entry, 1 NOP after entry)
will look like:

```
  .type	foo,@function
.Ltmp0:               # @foo
  nop
foo:
.Lfunc_begin0:
  # optional `bti c` (AArch64 Branch Target Identification) or
  # `endbr64` (Intel Indirect Branch Tracking)
  nop

  .section  __patchable_function_entries,"awo",@progbits,get,unique,0
  .p2align  3
  .quad .Ltmp0
```

-fpatchable-function-entry=N,0 + -mbranch-protection=bti/-fcf-protection=branch has two reasonable
placements (https://gcc.gnu.org/ml/gcc-patches/2020-01/msg01185.html):

```
(a)         (b)

func:       func:
.Ltmp0:     bti c
  bti c     .Ltmp0:
  nop       nop
```

(a) needs no additional code. If the consensus is to go for (b), we will
need more code in AArch64BranchTargets.cpp / X86IndirectBranchTracking.cpp .

Differential Revision: https://reviews.llvm.org/D73070

(cherry picked from commit 22467e259507f5ead2a87d989251b4c951a587e4)

2 files changed, 2 insertions, 6 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp b/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
index 00e321f9b85..b8953583a31 100644
--- a/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
+++ b/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
@@ -250,8 +250,7 @@ void AArch64AsmPrinter::LowerPATCHABLE_FUNCTION_ENTER(const MachineInstr &MI)
             .getValueAsString()
             .getAsInteger(10, Num))
       return;
-    for (; Num; --Num)
-      EmitToStreamer(*OutStreamer, MCInstBuilder(AArch64::HINT).addImm(0));
+    emitNops(Num);
     return;
   }
 
diff --git a/llvm/lib/Target/ARM/ARMMCInstLower.cpp b/llvm/lib/Target/ARM/ARMMCInstLower.cpp
index c92689f4942..8e01b998d90 100644
--- a/llvm/lib/Target/ARM/ARMMCInstLower.cpp
+++ b/llvm/lib/Target/ARM/ARMMCInstLower.cpp
@@ -207,10 +207,7 @@ void ARMAsmPrinter::EmitSled(const MachineInstr &MI, SledKind Kind)
   EmitToStreamer(*OutStreamer, MCInstBuilder(ARM::Bcc).addImm(20)
     .addImm(ARMCC::AL).addReg(0));
 
-  MCInst Noop;
-  Subtarget->getInstrInfo()->getNoop(Noop);
-  for (int8_t I = 0; I < NoopsInSledCount; I++)
-    OutStreamer->EmitInstruction(Noop, getSubtargetInfo());
+  emitNops(NoopsInSledCount);
 
   OutStreamer->EmitLabel(Target);
   recordSled(CurSled, MI, Kind);