Pull in r226664 from upstream llvm trunk (by Tim Northover):

  AArch64: add backend option to reserve x18 (platform register)

  AAPCS64 says that it's up to the platform to specify whether x18 is
  reserved, and a first step on that way is to add a flag controlling
  it.

  From: Andrew Turner <andrew@fubar.geek.nz>

Introduced here: http://svnweb.freebsd.org/changeset/base/277774

Index: lib/Target/AArch64/AArch64RegisterInfo.cpp
===================================================================
--- lib/Target/AArch64/AArch64RegisterInfo.cpp
+++ lib/Target/AArch64/AArch64RegisterInfo.cpp
@@ -33,6 +33,10 @@ using namespace llvm;
 #define GET_REGINFO_TARGET_DESC
 #include "AArch64GenRegisterInfo.inc"
 
+static cl::opt<bool>
+ReserveX18("aarch64-reserve-x18", cl::Hidden,
+          cl::desc("Reserve X18, making it unavailable as GPR"));
+
 AArch64RegisterInfo::AArch64RegisterInfo(const AArch64InstrInfo *tii,
                                          const AArch64Subtarget *sti)
     : AArch64GenRegisterInfo(AArch64::LR), TII(tii), STI(sti) {}
@@ -90,7 +94,7 @@ AArch64RegisterInfo::getReservedRegs(const Machine
     Reserved.set(AArch64::W29);
   }
 
-  if (STI->isTargetDarwin()) {
+  if (STI->isTargetDarwin() || ReserveX18) {
     Reserved.set(AArch64::X18); // Platform register
     Reserved.set(AArch64::W18);
   }
@@ -117,7 +121,7 @@ bool AArch64RegisterInfo::isReservedReg(const Mach
     return true;
   case AArch64::X18:
   case AArch64::W18:
-    return STI->isTargetDarwin();
+    return STI->isTargetDarwin() || ReserveX18;
   case AArch64::FP:
   case AArch64::W29:
     return TFI->hasFP(MF) || STI->isTargetDarwin();
@@ -379,7 +383,7 @@ unsigned AArch64RegisterInfo::getRegPressureLimit(
   case AArch64::GPR64commonRegClassID:
     return 32 - 1                                      // XZR/SP
            - (TFI->hasFP(MF) || STI->isTargetDarwin()) // FP
-           - STI->isTargetDarwin() // X18 reserved as platform register
+           - (STI->isTargetDarwin() || ReserveX18) // X18 reserved as platform register
            - hasBasePointer(MF);   // X19
   case AArch64::FPR8RegClassID:
   case AArch64::FPR16RegClassID:
Index: test/CodeGen/AArch64/arm64-platform-reg.ll
===================================================================
--- test/CodeGen/AArch64/arm64-platform-reg.ll
+++ test/CodeGen/AArch64/arm64-platform-reg.ll
@@ -1,4 +1,5 @@
-; RUN: llc -mtriple=arm64-apple-ios -o - %s | FileCheck %s --check-prefix=CHECK-DARWIN
+; RUN: llc -mtriple=arm64-apple-ios -o - %s | FileCheck %s --check-prefix=CHECK-RESERVE-X18
+; RUN: llc -mtriple=arm64-freebsd-gnu -aarch64-reserve-x18 -o - %s | FileCheck %s --check-prefix=CHECK-RESERVE-X18
 ; RUN: llc -mtriple=arm64-linux-gnu -o - %s | FileCheck %s
 
 ; x18 is reserved as a platform register on Darwin but not on other
@@ -16,11 +17,11 @@ define void @keep_live() {
 ; CHECK: ldr x18
 ; CHECK: str x18
 
-; CHECK-DARWIN-NOT: ldr fp
-; CHECK-DARWIN-NOT: ldr x18
-; CHECK-DARWIN: Spill
-; CHECK-DARWIN-NOT: ldr fp
-; CHECK-DARWIN-NOT: ldr x18
-; CHECK-DARWIN: ret
+; CHECK-RESERVE-X18-NOT: ldr fp
+; CHECK-RESERVE-X18-NOT: ldr x18
+; CHECK-RESERVE-X18: Spill
+; CHECK-RESERVE-X18-NOT: ldr fp
+; CHECK-RESERVE-X18-NOT: ldr x18
+; CHECK-RESERVE-X18: ret
   ret void
 }