Reland "[MC][AsmParser] Diagnose improperly nested .cfi frames"

This showed up when simplifying some large testcase, where the cfi directives
became out of sync with the proc's they enclose.

Now restricted to platforms that support .subsections_via_symbols.

This reverts commit 797b68c.

Fixes: #72802

Differential revision: https://reviews.llvm.org/D153167

rdar://111459507

lld/test/COFF/gc-dwarf-eh.s

@@ -13,9 +13,9 @@
  .def _main; .scl 2; .type 32; .endef
  .section .text,"xr",one_only,_main
  .globl _main
+_main:
  .cfi_startproc
  .cfi_personality 0, ___gxx_personality_v0
-_main:
  xorl %eax, %eax
  ret
  .cfi_endproc
@@ -29,8 +29,8 @@ ___gxx_personality_v0:
  .def _unused; .scl 2; .type 32; .endef
  .section .text,"xr",one_only,_unused
  .globl _unused
+_unused:
  .cfi_startproc
  .cfi_personality 0, ___gxx_personality_v0
-_unused:
  ret
  .cfi_endproc

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -125,6 +125,7 @@ class AsmParser : public MCAsmParser {
  void *SavedDiagContext;
  std::unique_ptr<MCAsmParserExtension> PlatformParser;
  SMLoc StartTokLoc;
+ std::optional<SMLoc> CFIStartProcLoc;
 
  /// This is the current buffer index we're lexing from as managed by the
  /// SourceMgr object.
@@ -1949,6 +1950,11 @@ bool AsmParser::parseStatement(ParseStatementInfo &Info,
  Lex();
  }
 
+ if (MAI.hasSubsectionsViaSymbols() && CFIStartProcLoc && Sym->isExternal())
+ return Error(StartTokLoc, "non-private labels cannot appear between "
+ ".cfi_startproc / .cfi_endproc pairs") &&
+ Error(*CFIStartProcLoc, "previous .cfi_startproc was here");
+
  if (discardLTOSymbol(IDVal))
  return false;
 
@@ -4193,6 +4199,8 @@ bool AsmParser::parseDirectiveCFISections() {
 /// parseDirectiveCFIStartProc
 /// ::= .cfi_startproc [simple]
 bool AsmParser::parseDirectiveCFIStartProc() {
+ CFIStartProcLoc = StartTokLoc;
+
  StringRef Simple;
  if (!parseOptionalToken(AsmToken::EndOfStatement)) {
  if (check(parseIdentifier(Simple) || Simple != "simple",
@@ -4213,8 +4221,11 @@ bool AsmParser::parseDirectiveCFIStartProc() {
 /// parseDirectiveCFIEndProc
 /// ::= .cfi_endproc
 bool AsmParser::parseDirectiveCFIEndProc() {
+ CFIStartProcLoc = std::nullopt;
+
  if (parseEOL())
  return true;
+
  getStreamer().emitCFIEndProc();
  return false;
 }

llvm/test/MC/AArch64/cfi-bad-nesting-darwin.s

@@ -0,0 +1,24 @@
+; RUN: not llvm-mc -triple arm64-apple-darwin %s -filetype=obj -o /dev/null 2>&1 | FileCheck %s --check-prefix=DARWIN
+
+; REQUIRES: aarch64-registered-target
+
+ .section __TEXT,locomotive,regular,pure_instructions
+
+ .globl _locomotive
+ .p2align 2
+_locomotive:
+ .cfi_startproc
+ ret
+
+ ; It is invalid to have a non-private label between .cfi_startproc and
+ ; .cfi_endproc on MachO platforms.
+ .section __TEXT,__text,regular,pure_instructions
+ .globl _caboose
+ .p2align 2
+_caboose:
+; DARWIN: [[#@LINE-1]]:1: error: non-private labels cannot appear between .cfi_startproc / .cfi_endproc pairs
+; DARWIN: [[#@LINE-10]]:2: error: previous .cfi_startproc was here
+ ret
+ .cfi_endproc
+
+.subsections_via_symbols

llvm/test/MC/AArch64/cfi-bad-nesting-elf.s

@@ -0,0 +1,20 @@
+# RUN: llvm-mc -triple arm64-pc-linux-gnu %s -filetype=obj -o /dev/null 2>&1 | FileCheck %s --allow-empty --check-prefix=ELF
+# RUN: llvm-mc -triple arm64-windows-gnu %s -filetype=obj -o /dev/null 2>&1 | FileCheck %s --allow-empty --check-prefix=ELF
+
+# REQUIRES: aarch64-registered-target
+
+ .globl _locomotive
+ .p2align 2
+_locomotive:
+ .cfi_startproc
+ ret
+
+ .globl _caboose
+ .p2align 2
+_caboose:
+ ret
+ .cfi_endproc
+
+# Check that the diagnostic does not fire on ELF, nor COFF platforms, which do
+# not support subsections_via_symbols. See also: cfi-bad-nesting-darwin.s
+# ELF-NOT: error: