[lld] Add double-braces for std::array to suppress warnings. NFC.

[Higuoxing]

Summary

This patch helps suppress warnings when building lld.
According to https://en.cppreference.com/w/cpp/container/array, double-braces required in C++11 prior to the CWG 1270 revision
(not needed in C++11 after the revision and in C++14 and beyond)

/Users/me/Workspace/llvm/llvm-project/lld/ELF/Arch/MSP430.cpp:44:16: warning: suggest braces around initialization of subobject [-Wmissing-braces]
  TrapInstr = {0x43, 0x43, 0x43, 0x43};
               ^~~~~~~~~~~~~~~~~~~~~~
               {                     }
1 warning generated.
[4/9] Building CXX object tools/lld/ELF/CMakeFiles/lldELF.dir/Arch/ARM.cpp.o
/Users/me/Workspace/llvm/llvm-project/lld/ELF/Arch/ARM.cpp:63:16: warning: suggest braces around initialization of subobject [-Wmissing-braces]
  TrapInstr = {0xd4, 0xd4, 0xd4, 0xd4};
               ^~~~~~~~~~~~~~~~~~~~~~
               {                     }
1 warning generated.
[5/9] Building CXX object tools/lld/ELF/CMakeFiles/lldELF.dir/Arch/X86.cpp.o
/Users/me/Workspace/llvm/llvm-project/lld/ELF/Arch/X86.cpp:62:16: warning: suggest braces around initialization of subobject [-Wmissing-braces]
  TrapInstr = {0xcc, 0xcc, 0xcc, 0xcc}; // 0xcc = INT3
               ^~~~~~~~~~~~~~~~~~~~~~
               {                     }
1 warning generated.
[6/9] Building CXX object tools/lld/ELF/CMakeFiles/lldELF.dir/Arch/X86_64.cpp.o
/Users/me/Workspace/llvm/llvm-project/lld/ELF/Arch/X86_64.cpp:65:16: warning: suggest braces around initialization of subobject [-Wmissing-braces]
  TrapInstr = {0xcc, 0xcc, 0xcc, 0xcc}; // 0xcc = INT3
               ^~~~~~~~~~~~~~~~~~~~~~
               {                     }
1 warning generated.
[7/9] Building CXX object tools/lld/ELF/CMakeFiles/lldELF.dir/OutputSections.cpp.o
/Users/me/Workspace/llvm/llvm-project/lld/ELF/OutputSections.cpp:412:11: warning: suggest braces around initialization of subobject [-Wmissing-braces]
  return {0, 0, 0, 0};
          ^~~~~~~~~~
          {         }
1 warning generated.

[MaskRay]

rC314838 (included in clang 6) suppressed -Wmissing-braces (enabled by -Wmost and -Wall) warning for the idiomatic brace elision.

http://lists.llvm.org/pipermail/llvm-dev/2019-April/131486.html llvm will migrate to C++14 soon... (though clang < 6 will continue warning the case; I'm not sure if we want to make code less idiomatic just to suppress warnings for old clang; gcc -Wall doesn't enable -Wmissing-braces)

[Higuoxing]

In D60646#1465405, @MaskRay wrote:

rC314838 (included in clang 6) suppressed -Wmissing-braces (enabled by -Wmost and -Wall) warning for the idiomatic brace elision.

http://lists.llvm.org/pipermail/llvm-dev/2019-April/131486.html llvm will migrate to C++14 soon... (though clang < 6 will continue warning the case; I'm not sure if we want to make code less idiomatic just to suppress warnings for old clang; gcc -Wall doesn't enable -Wmissing-braces)

Thanks a lot for pointing this out. Actually, I'd prefer not using double-braces. Just a few days ago, I saw a commit that added double-braces to suppress this warning in another project, so I put it here for discussion.

[ruiu]

Yeah, if recent versions of clang and gcc are happy with the code, I wouldn't add the extra braces as Fangrui suggested. Thank you for your effort to fix warnings, though. It's important to keep our code warning-free.

lld/ELF/Arch/ARM.cpp

ARM::ARM() {
   TlsGotRel = R_ARM_TLS_TPOFF32;
   TlsModuleIndexRel = R_ARM_TLS_DTPMOD32;
   TlsOffsetRel = R_ARM_TLS_DTPOFF32;
   GotBaseSymInGotPlt = false;
   GotEntrySize = 4;
   GotPltEntrySize = 4;
   PltEntrySize = 16;
   PltHeaderSize = 32;
-  TrapInstr = {0xd4, 0xd4, 0xd4, 0xd4};
+  TrapInstr = {{0xd4, 0xd4, 0xd4, 0xd4}};
   NeedsThunks = true;
 }
 
 uint32_t ARM::calcEFlags() const {
   // The ABIFloatType is used by loaders to detect the floating point calling
   // convention.
   uint32_t ABIFloatType = 0;
   if (Config->ARMVFPArgs == ARMVFPArgKind::Base ||

lld/ELF/Arch/MSP430.cpp

public:
   RelExpr getRelExpr(RelType Type, const Symbol &S,
                      const uint8_t *Loc) const override;
   void relocateOne(uint8_t *Loc, RelType Type, uint64_t Val) const override;
 };
 } // namespace
 
 MSP430::MSP430() {
   // mov.b #0, r3
-  TrapInstr = {0x43, 0x43, 0x43, 0x43};
+  TrapInstr = {{0x43, 0x43, 0x43, 0x43}};
 }
 
 RelExpr MSP430::getRelExpr(RelType Type, const Symbol &S,
                            const uint8_t *Loc) const {
   switch (Type) {
   case R_MSP430_10_PCREL:
   case R_MSP430_16_PCREL:
   case R_MSP430_16_PCREL_BYTE:

lld/ELF/Arch/X86.cpp

X86::X86() {
   RelativeRel = R_386_RELATIVE;
   TlsGotRel = R_386_TLS_TPOFF;
   TlsModuleIndexRel = R_386_TLS_DTPMOD32;
   TlsOffsetRel = R_386_TLS_DTPOFF32;
   GotEntrySize = 4;
   GotPltEntrySize = 4;
   PltEntrySize = 16;
   PltHeaderSize = 16;
-  TrapInstr = {0xcc, 0xcc, 0xcc, 0xcc}; // 0xcc = INT3
+  TrapInstr = {{0xcc, 0xcc, 0xcc, 0xcc}}; // 0xcc = INT3
 
   // Align to the non-PAE large page size (known as a superpage or huge page).
   // FreeBSD automatically promotes large, superpage-aligned allocations.
   DefaultImageBase = 0x400000;
 }
 
 int X86::getTlsGdRelaxSkip(RelType Type) const {
   return 2;

lld/ELF/Arch/X86_64.cpp

X86_64::X86_64() {
   IRelativeRel = R_X86_64_IRELATIVE;
   TlsGotRel = R_X86_64_TPOFF64;
   TlsModuleIndexRel = R_X86_64_DTPMOD64;
   TlsOffsetRel = R_X86_64_DTPOFF64;
   GotEntrySize = 8;
   GotPltEntrySize = 8;
   PltEntrySize = 16;
   PltHeaderSize = 16;
-  TrapInstr = {0xcc, 0xcc, 0xcc, 0xcc}; // 0xcc = INT3
+  TrapInstr = {{0xcc, 0xcc, 0xcc, 0xcc}}; // 0xcc = INT3
 
   // Align to the large page size (known as a superpage or huge page).
   // FreeBSD automatically promotes large, superpage-aligned allocations.
   DefaultImageBase = 0x200000;
 }
 
 int X86_64::getTlsGdRelaxSkip(RelType Type) const { return 2; }
 

lld/ELF/OutputSections.cpp

void OutputSection::sortInitFini() {
   sort([](InputSectionBase *S) { return getPriority(S->Name); });
 }
 
 std::array<uint8_t, 4> OutputSection::getFiller() {
   if (Filler)
     return *Filler;
   if (Flags & SHF_EXECINSTR)
     return Target->TrapInstr;
-  return {0, 0, 0, 0};
+  return {{0, 0, 0, 0}};
 }
 
 template void OutputSection::writeHeaderTo<ELF32LE>(ELF32LE::Shdr *Shdr);
 template void OutputSection::writeHeaderTo<ELF32BE>(ELF32BE::Shdr *Shdr);
 template void OutputSection::writeHeaderTo<ELF64LE>(ELF64LE::Shdr *Shdr);
 template void OutputSection::writeHeaderTo<ELF64BE>(ELF64BE::Shdr *Shdr);
 
 template void OutputSection::writeTo<ELF32LE>(uint8_t *Buf);
 template void OutputSection::writeTo<ELF32BE>(uint8_t *Buf);
 template void OutputSection::writeTo<ELF64LE>(uint8_t *Buf);
 template void OutputSection::writeTo<ELF64BE>(uint8_t *Buf);
 
 template void OutputSection::maybeCompress<ELF32LE>();
 template void OutputSection::maybeCompress<ELF32BE>();
 template void OutputSection::maybeCompress<ELF64LE>();
 template void OutputSection::maybeCompress<ELF64BE>();