Emit unwind information in .debug_frame section when .cfi_sections .debug_frame intrinsic is used

[rastogishubham]

Summary

The

.cfi_sections .debug_frame

intrinsic is used to emit .debug_frame section. This directive tells the assembler to write out a section of debug frame data. ARM64 however, is a platform where eh_frame is not needed for unwind information. Unfortunately, that means that even when the

.cfi_sections .debug_frame

intrinsic is used, the compiler skips emitting the CIE's and FDE's in the debug_frame section. This patch address that issue by making sure that the emission of CIE's and FDE's are only skipped if the unwind information does not require a debug_frame section and is a platform where the eh_frame can be skipped

[rastogishubham]

Add some comments

[aprantl]

llvm/lib/MC/MCDwarf.cpp

1910

I still find this sentence confusing.

Does this convey what you want to say?

Don't generate eh_frame if it's not needed. It's not needed if either eh_frame isn't requested or if we can use compact unwind instead.

[rastogishubham]

I addressed the review feedback about clarifying the comment I had written for when to skip emitting the FDEs and CIEs

[aprantl]

llvm/test/DebugInfo/AArch64/debugframeinfo.s

7

You only want top test that some debug_frame contents is emitted, right?
Assuming that there is another test that already tests whether we are emitting .debug_frame correctly, it should be sufficient to test for a nonempty .debug_frame section here.
All the hardcoded offsets and numbers here are not relevant to you patch and I'm slightly worried that they would change frequently with other unrelated compiler changes?

[rastogishubham]

Addressed review feedback about test just checking for if debug_frame section is non-empty

[aprantl]

Let's give this a try.

llvm/lib/MC/MCDwarf.cpp

       }
       NeedsEHFrameSection |=
         Frame.CompactUnwindEncoding ==
           MOFI->getCompactUnwindDwarfEHFrameOnly();
       Emitter.EmitCompactUnwind(Frame);
     }
   }
 
-  if (!NeedsEHFrameSection) return;
+  // Compact unwind information can be emitted in the eh_frame section or the
+  // debug_frame section. Skip emitting FDEs and CIEs when the compact unwind
+  // doesn't need an eh_frame section and the emission location is the eh_frame
+  // section.
+  if (!NeedsEHFrameSection && IsEH) return;
 
   MCSection &Section =
       IsEH ? *const_cast<MCObjectFileInfo *>(MOFI)->getEHFrameSection()
            : *MOFI->getDwarfFrameSection();
 
   Streamer.switchSection(&Section);
   MCSymbol *SectionStart = Context.createTempSymbol();
   Streamer.emitLabel(SectionStart);

   llvm::stable_sort(FrameArrayX,
                     [](const MCDwarfFrameInfo &X, const MCDwarfFrameInfo &Y) {
                       return CIEKey(X) < CIEKey(Y);
                     });
   for (auto I = FrameArrayX.begin(), E = FrameArrayX.end(); I != E;) {
     const MCDwarfFrameInfo &Frame = *I;
     ++I;
     if (CanOmitDwarf && Frame.CompactUnwindEncoding !=
-          MOFI->getCompactUnwindDwarfEHFrameOnly())
-      // Don't generate an EH frame if we don't need one. I.e., it's taken care
-      // of by the compact unwind encoding.
+          MOFI->getCompactUnwindDwarfEHFrameOnly() && IsEH)
+      // Don't generate an EH frame if we don't need one and location of unwind
+      // information is the eh_frame section. I.e., it's taken care of by the
+      // compact unwind encoding.
       continue;
 
     CIEKey Key(Frame);
     const MCSymbol *&CIEStart = IsEH ? CIEStarts[Key] : DummyDebugKey;
     if (!CIEStart)
       CIEStart = &Emitter.EmitCIE(Frame);
 
     Emitter.EmitFDE(*CIEStart, Frame, I == E, *SectionStart);

llvm/test/DebugInfo/AArch64/debugframeinfo.s

+# RUN: llvm-mc -filetype=obj --triple=arm64-apple-darwin22.1.0 %s -o %t.o
+# RUN: llvm-dwarfdump -debug-frame %t.o | FileCheck %s
+
+# CHECK: .debug_frame contents:
+# CHECK-EMPTY: 
+# CHECK-NEXT: 00000000 00000014 ffffffff CIE
+# CHECK-NEXT: Format:                DWARF32
+# CHECK-NEXT: Version:               4
+# CHECK-NEXT: Augmentation:          ""
+# CHECK-NEXT: Address size:          8
+# CHECK-NEXT: Segment desc size:     0
+# CHECK-NEXT: Code alignment factor: 1
+# CHECK-NEXT: Data alignment factor: -8
+# CHECK-NEXT: Return address column: 30
+# CHECK-EMPTY: 
+# CHECK-NEXT: DW_CFA_def_cfa: WSP +0
+# CHECK-NEXT: DW_CFA_nop:
+# CHECK-NEXT: DW_CFA_nop:
+# CHECK-NEXT: DW_CFA_nop:
+# CHECK-NEXT: DW_CFA_nop:
+# CHECK-NEXT: DW_CFA_nop:
+# CHECK-NEXT: DW_CFA_nop:
+# CHECK-EMPTY: 
+# CHECK-NEXT: CFA=WSP
+# CHECK-EMPTY: 
+# CHECK-NEXT: 00000018 00000014 00000000 FDE cie=00000000 pc=00000000...00000000
+# CHECK-NEXT: Format:       DWARF32
+# CHECK-EMPTY: 
+# CHECK-NEXT: 0x0: CFA=WSP
+
+# CHECK: .eh_frame contents:
+# CHECK-EMPTY:
+# CHECK-EMPTY:
+
+        .cfi_sections .debug_frame
+        .cfi_startproc
+        .cfi_personality 0x9b, g
+        .cfi_lsda 0x1b, h
+        .cfi_endproc
+        .global g
+g:
+        .global h
+h: