[WebAssembly] Don't create a needless .note.GNU-stack section

[sunfish]

Summary

WebAssembly's stack will never be executable by default, so it isn't necessary to declare .note.GNU-stack sections to request a non-executable stack.

[jfb]

lgtm

I'm assuming other backends may be interested in using this too?

[sunfish]

As requested here:

http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160104/323230.html

changed to introduce and use a usesNonexecutableStackSection() hook instead.

[• rafael]

include/llvm/MC/MCAsmInfoELF.h

29

Can this be just a bool flag that is set by the constructor instead of a virtual function?

[sunfish]

Use a bool flag that is set by the constructor instead of a virtual function, as requested by Rafael.

[• rafael]

LGTM

include/llvm/MC/MCAsmInfoELF.h

 #include "llvm/MC/MCAsmInfo.h"
 
 namespace llvm {
 class MCAsmInfoELF : public MCAsmInfo {
   virtual void anchor();
   MCSection *getNonexecutableStackSection(MCContext &Ctx) const final;
 
 protected:
+  /// Targets which have non-executable stacks by default can set this to false
+  /// to disable the special section which requests a non-executable stack.
+  bool UsesNonexecutableStackSection;
+
   MCAsmInfoELF();
 };
 }
 
 #endif

lib/MC/MCAsmInfoELF.cpp

 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCSectionELF.h"
 #include "llvm/Support/ELF.h"
 using namespace llvm;
 
 void MCAsmInfoELF::anchor() { }
 
 MCSection *MCAsmInfoELF::getNonexecutableStackSection(MCContext &Ctx) const {
+  if (!UsesNonexecutableStackSection)
+    return nullptr;
   return Ctx.getELFSection(".note.GNU-stack", ELF::SHT_PROGBITS, 0);
 }
 
 MCAsmInfoELF::MCAsmInfoELF() {
   HasIdentDirective = true;
   WeakRefDirective = "\t.weak\t";
   PrivateGlobalPrefix = ".L";
   PrivateLabelPrefix = ".L";
+  UsesNonexecutableStackSection = true;
 }

lib/Target/WebAssembly/MCTargetDesc/WebAssemblyMCAsmInfo.cpp

WebAssemblyMCAsmInfo::WebAssemblyMCAsmInfo(const Triple &T) {
   LCOMMDirectiveAlignmentType = LCOMM::Log2Alignment;
 
   SupportsDebugInformation = true;
 
   // For now, WebAssembly does not support exceptions.
   ExceptionsType = ExceptionHandling::None;
 
   // TODO: UseIntegratedAssembler?
+
+  // WebAssembly's stack is never executable.
+  UsesNonexecutableStackSection = false;
 }