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

[thegameg]

Summary

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;

[pete]

Nice catch. This LGTM.

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

5 ↗

(On Diff #149301)

You should be able to get this with:

llvm-objdump -macho -unwind-info ...

[thegameg]

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

5 ↗

(On Diff #149301)

Thanks for the tip! I used that in the committed version.

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

for (size_t i = 0, e = Instrs.size(); i != e; ++i) {
       const MCCFIInstruction &Inst = Instrs[i];
 
       switch (Inst.getOperation()) {
       default:
         // Cannot handle this directive:  bail out.
         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];
         assert(LRPush.getOperation() == MCCFIInstruction::OpOffset &&
                "Link register not pushed!");
         const MCCFIInstruction &FPPush = Instrs[++i];
         assert(FPPush.getOperation() == MCCFIInstruction::OpOffset &&
                "Frame pointer not pushed!");

llvm/trunk/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: .eh_frame contents:
+// CHECK: DW_CFA_def_cfa: reg1 +32
+
+_cfi_dwarf:
+ .cfi_startproc
+ .cfi_def_cfa x1, 32;
+ .cfi_endproc