[AArch64] Fallback to DWARF when trying to emit compact unwind info with multiple CFA offset adjustments

Instead of asserting, fallback to emitting DWARF unwind info when an
attempt is made to output compact unwind info for a function with
multiple adjustments to the CFA offset.

Multiple adjustments of SP are common and with instruction precise
unwind tables these may translate into multiple `.cfi_def_cfa_offset`
directives.

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=1302998

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D121017

Author: momchil-velikov

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

@@ -621,7 +621,8 @@ class DarwinAArch64AsmBackend : public AArch64AsmBackend {
         break;
       }
       case MCCFIInstruction::OpDefCfaOffset: {
-        assert(StackSize == 0 && "We already have the CFA offset!");
+        if (StackSize != 0)
+          return CU::UNWIND_ARM64_MODE_DWARF;
         StackSize = std::abs(Inst.getOffset());
         break;
       }

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

@@ -5,11 +5,21 @@
 
 // CHECK: Contents of __compact_unwind section:
 // CHECK: compact encoding:     0x03000000
+// CHECK: compact encoding:     0x03000000
 
 // CHECK: .eh_frame contents:
 // CHECK: DW_CFA_def_cfa: reg1 +32
 
-_cfi_dwarf:
+//  DW_CFA_def_cfa_offset: +32
+//  DW_CFA_def_cfa_offset: +64
+
+_cfi_dwarf0:
  .cfi_startproc
  .cfi_def_cfa x1, 32;
  .cfi_endproc
+
+_cfi_dwarf1:
+ .cfi_startproc
+ .cfi_def_cfa_offset 32
+ .cfi_def_cfa_offset 64
+ .cfi_endproc