Add .pushsection', .popsection', and `.previous' directives to Darwin ASM.

There are situations where inline ASM may want to change the section -- for
instance, to create a variable in the .data section. However, it cannot do this
without (potentially) restoring to the wrong section. E.g.:

  asm volatile (".section __DATA, __data\n\t"
                ".globl _fnord\n\t"
                "_fnord: .quad 1f\n\t"
                ".text\n\t"
                "1:" :::);

This may be wrong if this is inlined into a function that has a "section"
attribute. The user should use `.pushsection' and `.popsection' here instead.

The addition of `.previous' is added for completeness.
<rdar://problem/12048387>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161477 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCParser/DarwinAsmParser.cpp

@@ -50,6 +50,9 @@ public:
     AddDirectiveHandler<&DarwinAsmParser::ParseDirectiveDumpOrLoad>(".dump");
     AddDirectiveHandler<&DarwinAsmParser::ParseDirectiveDumpOrLoad>(".load");
     AddDirectiveHandler<&DarwinAsmParser::ParseDirectiveSection>(".section");
+    AddDirectiveHandler<&DarwinAsmParser::ParseDirectivePushSection>(".pushsection");
+    AddDirectiveHandler<&DarwinAsmParser::ParseDirectivePopSection>(".popsection");
+    AddDirectiveHandler<&DarwinAsmParser::ParseDirectivePrevious>(".previous");
     AddDirectiveHandler<&DarwinAsmParser::ParseDirectiveSecureLogUnique>(
       ".secure_log_unique");
     AddDirectiveHandler<&DarwinAsmParser::ParseDirectiveSecureLogReset>(
@@ -112,6 +115,9 @@ public:
   bool ParseDirectiveDumpOrLoad(StringRef, SMLoc);
   bool ParseDirectiveLsym(StringRef, SMLoc);
   bool ParseDirectiveSection(StringRef, SMLoc);
+  bool ParseDirectivePushSection(StringRef, SMLoc);
+  bool ParseDirectivePopSection(StringRef, SMLoc);
+  bool ParseDirectivePrevious(StringRef, SMLoc);
   bool ParseDirectiveSecureLogReset(StringRef, SMLoc);
   bool ParseDirectiveSecureLogUnique(StringRef, SMLoc);
   bool ParseDirectiveSubsectionsViaSymbols(StringRef, SMLoc);
@@ -297,7 +303,7 @@ public:
 
 };
 
-}
+} // end anonymous namespace
 
 bool DarwinAsmParser::ParseSectionSwitch(const char *Segment,
                                          const char *Section,
@@ -457,6 +463,37 @@ bool DarwinAsmParser::ParseDirectiveSection(StringRef, SMLoc) {
   return false;
 }
 
+/// ParseDirectivePushSection:
+///   ::= .pushsection identifier (',' identifier)*
+bool DarwinAsmParser::ParseDirectivePushSection(StringRef S, SMLoc Loc) {
+  getStreamer().PushSection();
+
+  if (ParseDirectiveSection(S, Loc)) {
+    getStreamer().PopSection();
+    return true;
+  }
+
+  return false;
+}
+
+/// ParseDirectivePopSection:
+///   ::= .popsection
+bool DarwinAsmParser::ParseDirectivePopSection(StringRef, SMLoc) {
+  if (!getStreamer().PopSection())
+    return TokError(".popsection without corresponding .pushsection");
+  return false;
+}
+
+/// ParseDirectivePrevious:
+///   ::= .previous
+bool DarwinAsmParser::ParseDirectivePrevious(StringRef DirName, SMLoc) {
+  const MCSection *PreviousSection = getStreamer().getPreviousSection();
+  if (PreviousSection == NULL)
+      return TokError(".previous without corresponding .section");
+  getStreamer().SwitchSection(PreviousSection);
+  return false;
+}
+
 /// ParseDirectiveSecureLogUnique
 ///  ::= .secure_log_unique ... message ...
 bool DarwinAsmParser::ParseDirectiveSecureLogUnique(StringRef, SMLoc IDLoc) {
@@ -707,4 +744,4 @@ MCAsmParserExtension *createDarwinAsmParser() {
   return new DarwinAsmParser;
 }
 
-}
+} // end llvm namespace

test/MC/MachO/previous.s

@@ -0,0 +1,13 @@
+// RUN: llvm-mc -triple i386-apple-darwin9 %s -o - | FileCheck %s
+
+.text
+// CHECK: .section __TEXT,__text
+
+.data
+// CHECK: .section __DATA,__data
+
+.previous
+// CHECK: .section __TEXT,__text
+
+.previous
+// CHECK: .section __DATA,__data

test/MC/MachO/pushsection.s

@@ -0,0 +1,16 @@
+// RUN: llvm-mc -triple i386-apple-darwin9 %s -o - | FileCheck %s
+
+.text
+// CHECK: .section __TEXT,__text
+
+.pushsection __DATA, __data
+// CHECK: .section __DATA,__data
+
+.pushsection __TEXT, initcode
+// CHECK: .section __TEXT,initcode
+        
+.popsection
+// CHECK: .section __DATA,__data
+        
+.popsection
+// CHECK: .section __TEXT,__text