[PATCH] Remove const_cast for STI when parsing inline asm

[David Peixotto]

  Update comment to remove temporal references.

  Based on feedback from Jim, the comment is updated to be more
  explicit about why we create a new STI object and how it is shared
  with the code emitters.

Hi garious, rengolin, grosbach,

http://llvm-reviews.chandlerc.com/D2709

CHANGE SINCE LAST DIFF
  http://llvm-reviews.chandlerc.com/D2709?vs=6908&id=6913#toc

Files:
  include/llvm/CodeGen/AsmPrinter.h
  lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp
  lib/Target/ARM/ARMAsmPrinter.cpp
  lib/Target/ARM/ARMAsmPrinter.h

Index: include/llvm/CodeGen/AsmPrinter.h
===================================================================
--- include/llvm/CodeGen/AsmPrinter.h
+++ include/llvm/CodeGen/AsmPrinter.h
@@ -475,7 +475,7 @@
     /// \p EndInfo   - the final subtarget info after parsing the inline asm,
     ///                or NULL if the value is unknown.
     virtual void emitInlineAsmEnd(const MCSubtargetInfo &StartInfo,
-                                  MCSubtargetInfo *EndInfo) const;
+                                  const MCSubtargetInfo *EndInfo) const;
 
   private:
     /// Private state for PrintSpecial()
Index: lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp
===================================================================
--- lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp
+++ lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp
@@ -117,14 +117,17 @@
                                                   OutContext, OutStreamer,
                                                   *MAI));
 
-  // Reuse the existing Subtarget, because the AsmParser may need to
-  // modify it.  For example, when switching between ARM and
-  // Thumb mode.
-  MCSubtargetInfo* STI =
-    const_cast<MCSubtargetInfo*>(&TM.getSubtarget<MCSubtargetInfo>());
-
-  // Preserve a copy of the original STI because the parser may modify it.
-  // The target can restore the original state in emitInlineAsmEnd().
+  // Initialize the parser with a fresh subtarget info. It is better to use a
+  // new STI here because the parser may modify it and we do not want those
+  // modifications to persist after parsing the inlineasm. The modifications
+  // made by the parser will be seen by the code emitters because it passes
+  // the current STI down to the EncodeInstruction() method.
+  OwningPtr<MCSubtargetInfo> STI(TM.getTarget().createMCSubtargetInfo(
+      TM.getTargetTriple(), TM.getTargetCPU(), TM.getTargetFeatureString()));
+
+  // Preserve a copy of the original STI because the parser may modify it.  For
+  // example, when switching between arm and thumb mode. If the target needs to
+  // emit code to return to the original state it can do so in emitInlineAsmEnd().
   MCSubtargetInfo STIOrig = *STI;
 
   OwningPtr<MCTargetAsmParser>
@@ -138,7 +141,7 @@
   // Don't implicitly switch to the text section before the asm.
   int Res = Parser->Run(/*NoInitialTextSection*/ true,
                         /*NoFinalize*/ true);
-  emitInlineAsmEnd(STIOrig, STI);
+  emitInlineAsmEnd(STIOrig, STI.get());
   if (Res && !HasDiagHandler)
     report_fatal_error("Error parsing inline asm\n");
 }
@@ -550,4 +553,4 @@
 }
 
 void AsmPrinter::emitInlineAsmEnd(const MCSubtargetInfo &StartInfo,
-                                  MCSubtargetInfo *EndInfo) const {}
+                                  const MCSubtargetInfo *EndInfo) const {}
Index: lib/Target/ARM/ARMAsmPrinter.cpp
===================================================================
--- lib/Target/ARM/ARMAsmPrinter.cpp
+++ lib/Target/ARM/ARMAsmPrinter.cpp
@@ -443,14 +443,12 @@
 }
 
 void ARMAsmPrinter::emitInlineAsmEnd(const MCSubtargetInfo &StartInfo,
-                                     MCSubtargetInfo *EndInfo) const {
+                                     const MCSubtargetInfo *EndInfo) const {
   // If either end mode is unknown (EndInfo == NULL) or different than
   // the start mode, then restore the start mode.
   const bool WasThumb = isThumb(StartInfo);
   if (EndInfo == NULL || WasThumb != isThumb(*EndInfo)) {
     OutStreamer.EmitAssemblerFlag(WasThumb ? MCAF_Code16 : MCAF_Code32);
-    if (EndInfo)
-      EndInfo->ToggleFeature(ARM::ModeThumb);
   }
 }
 
Index: lib/Target/ARM/ARMAsmPrinter.h
===================================================================
--- lib/Target/ARM/ARMAsmPrinter.h
+++ lib/Target/ARM/ARMAsmPrinter.h
@@ -64,7 +64,8 @@
                                      raw_ostream &O) LLVM_OVERRIDE;
 
   virtual void emitInlineAsmEnd(const MCSubtargetInfo &StartInfo,
-                                MCSubtargetInfo *EndInfo) const LLVM_OVERRIDE;
+                                const MCSubtargetInfo *EndInfo) const
+      LLVM_OVERRIDE;
 
   void EmitJumpTable(const MachineInstr *MI);
   void EmitJump2Table(const MachineInstr *MI);
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