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

[chill]

Summary

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

[chill]

Ping.

[dmgreen]

Looks OK, compared to what the other code does here. LGTM

[t.p.northover]

I don't think this is the ideal fix. We don't do asynchronous unwind on Darwin, and for synchronous cases it looked to me like the final stack update was still correct. So rather than pessimizing lots and lots of functions' unwind tables (a significant size impact), I think it's better to simply disable the assertion: https://reviews.llvm.org/D121116 (I'll update it for the status quo with tests on Monday).

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!");
+        if (StackSize != 0)
+          return CU::UNWIND_ARM64_MODE_DWARF;
         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/MC/AArch64/arm64-compact-unwind-fallback.s

 // 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: 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