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)

1 files changed, 29 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index e7a7ca79e33..3516f4a7b37 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -706,6 +706,21 @@ 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;
+  (void)F.getFnAttribute("patchable-function-prefix")
+      .getValueAsString()
+      .getAsInteger(10, PatchableFunctionPrefix);
+  if (PatchableFunctionPrefix) {
+    CurrentPatchableFunctionEntrySym =
+        OutContext.createLinkerPrivateTempSymbol();
+    OutStreamer->EmitLabel(CurrentPatchableFunctionEntrySym);
+    emitNops(PatchableFunctionPrefix);
+  } else {
+    CurrentPatchableFunctionEntrySym = CurrentFnBegin;
+  }
+
   // Emit the function descriptor. This is a virtual function to allow targets
   // to emit their specific function descriptor.
   if (MAI->needsFunctionDescriptors())
@@ -1167,7 +1182,7 @@ void AsmPrinter::EmitFunctionBody() {
     // unspecified.
     if (Noop.getOpcode()) {
       OutStreamer->AddComment("avoids zero-length function");
-      OutStreamer->EmitInstruction(Noop, getSubtargetInfo());
+      emitNops(1);
     }
   }
 
@@ -2797,6 +2812,13 @@ void AsmPrinter::printOffset(int64_t Offset, raw_ostream &OS) const {
     OS << Offset;
 }
 
+void AsmPrinter::emitNops(unsigned N) {
+  MCInst Nop;
+  MF->getSubtarget().getInstrInfo()->getNoop(Nop);
+  for (; N; --N)
+    EmitToStreamer(*OutStreamer, Nop);
+}
+
 //===----------------------------------------------------------------------===//
 // Symbol Lowering Routines.
 //===----------------------------------------------------------------------===//
@@ -3199,11 +3221,14 @@ void AsmPrinter::recordSled(MCSymbol *Sled, const MachineInstr &MI,
 
 void AsmPrinter::emitPatchableFunctionEntries() {
   const Function &F = MF->getFunction();
-  unsigned PatchableFunctionEntry = 0;
+  unsigned PatchableFunctionPrefix = 0, PatchableFunctionEntry = 0;
+  (void)F.getFnAttribute("patchable-function-prefix")
+      .getValueAsString()
+      .getAsInteger(10, PatchableFunctionPrefix);
   (void)F.getFnAttribute("patchable-function-entry")
       .getValueAsString()
       .getAsInteger(10, PatchableFunctionEntry);
-  if (!PatchableFunctionEntry)
+  if (!PatchableFunctionPrefix && !PatchableFunctionEntry)
     return;
   const unsigned PointerSize = getPointerSize();
   if (TM.getTargetTriple().isOSBinFormatELF()) {
@@ -3232,7 +3257,7 @@ void AsmPrinter::emitPatchableFunctionEntries() {
           "__patchable_function_entries", ELF::SHT_PROGBITS, Flags));
     }
     EmitAlignment(Align(PointerSize));
-    OutStreamer->EmitSymbolValue(CurrentFnBegin, PointerSize);
+    OutStreamer->EmitSymbolValue(CurrentPatchableFunctionEntrySym, PointerSize);
   }
 }