[MC] Fallback on DWARF when generating compact unwind on AArch64

Instead of asserting when using the def_cfa directive with a register
different from fp, fallback on DWARF.

Easily triggered with:

.cfi_def_cfa x1, 32;

rdar://40249694

Differential Revision: https://reviews.llvm.org/D47593

llvm-svn: 333667

llvm/lib/Target/AArch64/MCTargetDesc/AArch64AsmBackend.cpp

@@ -455,9 +455,17 @@ public:
         return CU::UNWIND_ARM64_MODE_DWARF;
       case MCCFIInstruction::OpDefCfa: {
         // Defines a frame pointer.
-        assert(getXRegFromWReg(MRI.getLLVMRegNum(Inst.getRegister(), true)) ==
-                   AArch64::FP &&
-               "Invalid frame pointer!");
+        unsigned XReg =
+            getXRegFromWReg(MRI.getLLVMRegNum(Inst.getRegister(), true));
+
+        // Other CFA registers than FP are not supported by compact unwind.
+        // Fallback on DWARF.
+        // FIXME: When opt-remarks are supported in MC, add a remark to notify
+        // the user.
+        if (XReg != AArch64::FP)
+          return CU::UNWIND_ARM64_MODE_DWARF;
+
+        assert(XReg == AArch64::FP && "Invalid frame pointer!");
         assert(i + 2 < e && "Insufficient CFI instructions to define a frame!");
 
         const MCCFIInstruction &LRPush = Instrs[++i];

llvm/test/MC/AArch64/arm64-compact-unwind-fallback.s

@@ -0,0 +1,15 @@
+// RUN: llvm-mc -triple=arm64-apple-ios -filetype=obj %s -o %t
+// RUN: llvm-objdump -macho -unwind-info -dwarf=frames %t | FileCheck %s
+
+// Check that we fallback on DWARF instead of asserting.
+
+// CHECK: Contents of __compact_unwind section:
+// CHECK: compact encoding:     0x03000000
+
+// CHECK: .eh_frame contents:
+// CHECK: DW_CFA_def_cfa: reg1 +32
+
+_cfi_dwarf:
+ .cfi_startproc
+ .cfi_def_cfa x1, 32;
+ .cfi_endproc