[MS][NFC] Remove SEH chaininfo assembler macros

[namazso]

Summary

These macros cannot be used to generate valid output and it has been like that since their introduction 10 years ago.

Chaining is to be used for function chunks out of the main function's region, and RUNTIME_FUNCTION ranges should never intersect or overlap. The current implementation *only* allows such use that the generated output contains overlapping ones, which is invalid (and also ignored by windows). Additionally, the current syntax is not suitable for implementing any correct behavior, as the chained function chunk should refer to the info or function it chains to, which is not done in the current syntax. A search on search engines and github show that this macro wasn't used outside of LLVM (which is understandable as it never worked) so I think this warrants the NFC tag.

The tests also exhibit the incorrect behavior, and pass since the offsets are regexed out so overlapping ranges don't fail.

[mstorsjo]

I think this is reasonable - I'm not familiar with the chained stuff, but the arguments sound sensible. Adding @rnk who I think might be more familiar with it (or might know who's the best authority on the matter).

(When uploading diffs, it's preferrable if the diffs are created with lots of extra context, like git diff -U999 or similar.)

[rnk]

I think the intended usage model is supposed to be something like this:

.section .text, ...
.seh_proc foo
  push ...
  push ...
.seh_endprologue
  j...
  ret

.pushsection .text$cold, ...  # switch to new section (discontiguous)
.seh_startchained # implicitly references the open unwind info for foo
.seh_endprologue
  mov ...  # cold code moved out of line to separate section
  mov ...
.seh_endchained # end chained region

.popsection # return to main text section
.seh_endproc # main end label is in main section

The test doesn't do a good job representing proper usage, since it puts everything in the same section (.text) and the chained region has no code in it.

What do you think of the example? Is that a possible way that one could set up correct chained unwind info with the directives as they are? Unfortunately, I'm not familiar with these. As you can see, the compiler doesn't use them, they are only implemented at the assembler level.

[namazso]

In D115918#3226107, @rnk wrote:

What do you think of the example? Is that a possible way that one could set up correct chained unwind info with the directives as they are? Unfortunately, I'm not familiar with these. As you can see, the compiler doesn't use them, they are only implemented at the assembler level.

If you mean how it is right now, no, because that will just error:

<unknown>:0: error: expected relocatable expression
<unknown>:0: error: expected relocatable expression

syntax-wise it might be possible to make it work like this though, although I think a syntax like .seh_proc procname, chainedprocname might be better as it allows you to generate output similar to some that MSVC generates if you want to.

llvm/include/llvm/MC/MCStreamer.h

   /// for the frame.  We cannot use the End marker, as it is not set at the
   /// point of emitting .xdata, in order to indicate that the frame is active.
   virtual void EmitWinCFIFuncletOrFuncEnd(SMLoc Loc = SMLoc());
-  virtual void EmitWinCFIStartChained(SMLoc Loc = SMLoc());
-  virtual void EmitWinCFIEndChained(SMLoc Loc = SMLoc());
   virtual void EmitWinCFIPushReg(MCRegister Register, SMLoc Loc = SMLoc());
   virtual void EmitWinCFISetFrame(MCRegister Register, unsigned Offset,
                                   SMLoc Loc = SMLoc());

llvm/lib/MC/MCAsmStreamer.cpp

   void EmitWinCFIStartProc(const MCSymbol *Symbol, SMLoc Loc) override;
   void EmitWinCFIEndProc(SMLoc Loc) override;
   void EmitWinCFIFuncletOrFuncEnd(SMLoc Loc) override;
-  void EmitWinCFIStartChained(SMLoc Loc) override;
-  void EmitWinCFIEndChained(SMLoc Loc) override;
   void EmitWinCFIPushReg(MCRegister Register, SMLoc Loc) override;
   void EmitWinCFISetFrame(MCRegister Register, unsigned Offset,
                           SMLoc Loc) override;

   EmitEOL();
 }
 
-void MCAsmStreamer::EmitWinCFIStartChained(SMLoc Loc) {
-  MCStreamer::EmitWinCFIStartChained(Loc);
-
-  OS << "\t.seh_startchained";
-  EmitEOL();
-}
-
-void MCAsmStreamer::EmitWinCFIEndChained(SMLoc Loc) {
-  MCStreamer::EmitWinCFIEndChained(Loc);
-
-  OS << "\t.seh_endchained";
-  EmitEOL();
-}
-
 void MCAsmStreamer::EmitWinEHHandler(const MCSymbol *Sym, bool Unwind,
                                      bool Except, SMLoc Loc) {
   MCStreamer::EmitWinEHHandler(Sym, Unwind, Except, Loc);

llvm/lib/MC/MCParser/COFFAsmParser.cpp

                                                                 ".seh_endproc");
     addDirectiveHandler<&COFFAsmParser::ParseSEHDirectiveEndFuncletOrFunc>(
                                                                 ".seh_endfunclet");
-    addDirectiveHandler<&COFFAsmParser::ParseSEHDirectiveStartChained>(
-                                                           ".seh_startchained");
-    addDirectiveHandler<&COFFAsmParser::ParseSEHDirectiveEndChained>(
-                                                             ".seh_endchained");
     addDirectiveHandler<&COFFAsmParser::ParseSEHDirectiveHandler>(
                                                                 ".seh_handler");
     addDirectiveHandler<&COFFAsmParser::ParseSEHDirectiveHandlerData>(

   bool ParseSEHDirectiveStartProc(StringRef, SMLoc);
   bool ParseSEHDirectiveEndProc(StringRef, SMLoc);
   bool ParseSEHDirectiveEndFuncletOrFunc(StringRef, SMLoc);
-  bool ParseSEHDirectiveStartChained(StringRef, SMLoc);
-  bool ParseSEHDirectiveEndChained(StringRef, SMLoc);
   bool ParseSEHDirectiveHandler(StringRef, SMLoc);
   bool ParseSEHDirectiveHandlerData(StringRef, SMLoc);
   bool ParseSEHDirectiveAllocStack(StringRef, SMLoc);

   return false;
 }
 
-bool COFFAsmParser::ParseSEHDirectiveStartChained(StringRef, SMLoc Loc) {
-  Lex();
-  getStreamer().EmitWinCFIStartChained(Loc);
-  return false;
-}
-
-bool COFFAsmParser::ParseSEHDirectiveEndChained(StringRef, SMLoc Loc) {
-  Lex();
-  getStreamer().EmitWinCFIEndChained(Loc);
-  return false;
-}
-
 bool COFFAsmParser::ParseSEHDirectiveHandler(StringRef, SMLoc Loc) {
   StringRef SymbolID;
   if (getParser().parseIdentifier(SymbolID))

llvm/lib/MC/MCStreamer.cpp

   CurFrame->FuncletOrFuncEnd = Label;
 }
 
-void MCStreamer::EmitWinCFIStartChained(SMLoc Loc) {
-  WinEH::FrameInfo *CurFrame = EnsureValidWinFrameInfo(Loc);
-  if (!CurFrame)
-    return;
-
-  MCSymbol *StartProc = emitCFILabel();
-
-  WinFrameInfos.emplace_back(std::make_unique<WinEH::FrameInfo>(
-      CurFrame->Function, StartProc, CurFrame));
-  CurrentWinFrameInfo = WinFrameInfos.back().get();
-  CurrentWinFrameInfo->TextSection = getCurrentSectionOnly();
-}
-
-void MCStreamer::EmitWinCFIEndChained(SMLoc Loc) {
-  WinEH::FrameInfo *CurFrame = EnsureValidWinFrameInfo(Loc);
-  if (!CurFrame)
-    return;
-  if (!CurFrame->ChainedParent)
-    return getContext().reportError(
-        Loc, "End of a chained region outside a chained region!");
-
-  MCSymbol *Label = emitCFILabel();
-
-  CurFrame->End = Label;
-  CurrentWinFrameInfo = const_cast<WinEH::FrameInfo *>(CurFrame->ChainedParent);
-}
-
 void MCStreamer::EmitWinEHHandler(const MCSymbol *Sym, bool Unwind, bool Except,
                                   SMLoc Loc) {
   WinEH::FrameInfo *CurFrame = EnsureValidWinFrameInfo(Loc);

llvm/test/MC/AsmParser/directive_seh.s

 # CHECK: .seh_handlerdata
     .long 0
     .text
-    .seh_startchained
-    .seh_endprologue
-    .seh_endchained
 # CHECK: .text
-# CHECK: .seh_startchained
-# CHECK: .seh_endprologue
-# CHECK: .seh_endchained
     lea (%rbx), %rsp
     pop %rbx
     addq $24, %rsp

llvm/test/MC/COFF/seh.s

 // CHECK-NEXT:   }
 // CHECK:        Section {
 // CHECK:          Name: .xdata
-// CHECK:          RawDataSize: 52
-// CHECK:          RelocationCount: 4
+// CHECK:          RawDataSize: 36
+// CHECK:          RelocationCount: 1
 // CHECK:          Characteristics [
 // CHECK-NEXT:       ALIGN_4BYTES
 // CHECK-NEXT:       CNT_INITIALIZED_DATA

 // CHECK-NEXT:   }
 // CHECK:        Section {
 // CHECK:          Name: .pdata
-// CHECK:          RelocationCount: 9
+// CHECK:          RelocationCount: 6
 // CHECK:          Characteristics [
 // CHECK-NEXT:       ALIGN_4BYTES
 // CHECK-NEXT:       CNT_INITIALIZED_DATA

 // CHECK-NEXT: Relocations [
 // CHECK-NEXT:   Section (4) .xdata {
 // CHECK-NEXT:     0x14 IMAGE_REL_AMD64_ADDR32NB __C_specific_handler
-// CHECK-NEXT:     0x20 IMAGE_REL_AMD64_ADDR32NB .text
-// CHECK-NEXT:     0x24 IMAGE_REL_AMD64_ADDR32NB .text
-// CHECK-NEXT:     0x28 IMAGE_REL_AMD64_ADDR32NB .xdata
 // CHECK-NEXT:   }
 // CHECK-NEXT:   Section (5) .pdata {
 // CHECK-NEXT:     0x0 IMAGE_REL_AMD64_ADDR32NB .text

 // CHECK-NEXT:     0xC IMAGE_REL_AMD64_ADDR32NB .text
 // CHECK-NEXT:     0x10 IMAGE_REL_AMD64_ADDR32NB .text
 // CHECK-NEXT:     0x14 IMAGE_REL_AMD64_ADDR32NB .xdata
-// CHECK-NEXT:     0x18 IMAGE_REL_AMD64_ADDR32NB .text
-// CHECK-NEXT:     0x1C IMAGE_REL_AMD64_ADDR32NB .text
-// CHECK-NEXT:     0x20 IMAGE_REL_AMD64_ADDR32NB .xdata
 // CHECK-NEXT:   }
 // CHECK-NEXT: ]
 

 // CHECK-NEXT:     }
 // CHECK-NEXT:   }
 // CHECK-NEXT:   RuntimeFunction {
-// CHECK-NEXT:     StartAddress: [[CodeSect2:[^ ]+]] [[BeginDisp2:(\+0x[A-F0-9]+)?]]
-// CHECK-NEXT:     EndAddress: [[CodeSect2]] [[BeginDisp2:(\+0x[A-F0-9]+)?]]
-// CHECK-NEXT:     UnwindInfoAddress:
-// CHECK-NEXT:     UnwindInfo {
-// CHECK-NEXT:       Version: 1
-// CHECK-NEXT:       Flags [
-// CHECK-NEXT:         ChainInfo
-// CHECK-NEXT:       ]
-// CHECK-NEXT:       PrologSize: 0
-// CHECK-NEXT:       FrameRegister: -
-// CHECK-NEXT:       FrameOffset: -
-// CHECK-NEXT:       UnwindCodeCount: 0
-// CHECK-NEXT:       UnwindCodes [
-// CHECK-NEXT:       ]
-// CHECK-NEXT:       Chained {
-// CHECK-NEXT:         StartAddress: [[CodeSect1]] [[BeginDisp1]]
-// CHECK-NEXT:         EndAddress: [[CodeSect1]] [[EndDisp1]]
-// CHECK-NEXT:         UnwindInfoAddress:
-// CHECK-NEXT:       }
-// CHECK-NEXT:     }
-// CHECK-NEXT:   }
-// CHECK-NEXT:   RuntimeFunction {
 // CHECK-NEXT:     StartAddress: [[CodeSect3:[^ ]+]] [[BeginDisp3:(\+0x[A-F0-9]+)?]]
 // CHECK-NEXT:     EndAddress: [[CodeSect3]] [[BeginDisp3:(\+0x[A-F0-9]+)?]]
 // CHECK-NEXT:     UnwindInfoAddress:

     .seh_handlerdata
     .long 0
     .text
-    .seh_startchained
-    .seh_endprologue
-    .seh_endchained
     lea (%rbx), %rsp
     pop %rbx
     addq $24, %rsp