AArch64: take compact unwind frame size from last CFI instruction.

[t.p.northover]

Summary

Asynchronous exception support for the prologue means that there can be multiple .cfi_def_cfa_offset instructions in a single function, which tripped up an assertion in the compact unwind generator.

In reality the compact unwind format is far too restrictive to represent asynchronous frames so if we ever wanted that on Darwin we'd fall back to DWARF (possibly keeping compact unwind around for synchronous users). So the compact format should continue to represent the synchronous situation, and the assertion can be removed.

[t.p.northover]

Issue was resolved a different way.

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

for (size_t i = 0, e = Instrs.size(); i != e; ++i) {
                "Pushing invalid registers for frame!");
 
         // Indicate that the function has a frame.
         CompactUnwindEncoding |= CU::UNWIND_ARM64_MODE_FRAME;
         HasFP = true;
         break;
       }
       case MCCFIInstruction::OpDefCfaOffset: {
-        assert(StackSize == 0 && "We already have the CFA offset!");
         StackSize = std::abs(Inst.getOffset());
         break;
       }
       case MCCFIInstruction::OpOffset: {
         // Registers are saved in pairs. We expect there to be two consecutive
         // `.cfi_offset' instructions with the appropriate registers specified.
         unsigned Reg1 = *MRI.getLLVMRegNum(Inst.getRegister(), true);
         if (i + 1 == e)

llvm/test/CodeGen/AArch64/compact-unwind-async.ll

+; RUN: llc -mtriple=arm64-apple-macosx %s -filetype=obj -o - | llvm-objdump --unwind-info - | FileCheck %s
+
+; CHECK: Contents of __compact_unwind section
+; CHECK: compact encoding: 0x02021010
+
+; 0x02|021|010 => frameless, stack size 0x21 * 16 = 528, x27 & x28 saved
+
+define void @func() {
+  alloca i8, i32 512
+  ret void
+}