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: llvm#72802

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

rdar://111459507

Author: jroelofs

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: