[NFC][AIX][XCOFF] Fix compile warning on strncpy

[nullptr.cpp]

Summary

GCC warning:

In file included from /usr/include/string.h:495,
                 from /usr/include/c++/9/cstring:42,
                 from /llvm-project/llvm/include/llvm/ADT/Hashing.h:53,
                 from /llvm-project/llvm/include/llvm/ADT/ArrayRef.h:12,
                 from /llvm-project/llvm/include/llvm/MC/MCAsmBackend.h:12,
                 from /llvm-project/llvm/lib/MC/XCOFFObjectWriter.cpp:14:
In function ‘char* strncpy(char*, const char*, size_t)’,
    inlined from ‘{anonymous}::Section::Section(const char*, llvm::XCOFF::SectionTypeFlags, bool, {anonymous}::CsectGroups)’ at /llvm-project/llvm/lib/MC/XCOFFObjectWriter.cpp:146:12:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:34: warning: ‘char* __builtin_strncpy(char*, const char*, long unsigned int)’ specified bound 8 equals destination size [-Wstringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[nullptr.cpp]

rebase

[hubert.reinterpretcast]

llvm/lib/MC/XCOFFObjectWriter.cpp

102

D71119 changed the type of a local variable as a concession to a false-positive compiler warning (the NUL termination is not required for a name with NameSize characters).

Changing the interface of a public member of a class is a more intrusive change. Presumably, we can keep the type of Name here as-is and use memcpy to populate Name after using strncpy in the constructor.

[nullptr.cpp]

Use memcpy

[daltenty]

llvm/lib/MC/XCOFFObjectWriter.cpp

147

It's not clear this is properly null terminated in the other cases. Perhaps we should zero-initialize this field first (especially since we're going to write out the bytes directly)?

[nullptr.cpp]

llvm/lib/MC/XCOFFObjectWriter.cpp

143

@daltenty
Name() here have already zero-initialized the array.

[hubert.reinterpretcast]

llvm/lib/MC/XCOFFObjectWriter.cpp

146

I think N did not need to be a null-terminated string (if 8 characters long) before the change. Would be make sense to change N to be a StringRef and use the length from that? I think that makes it clear that just passing the argument either requires an explicit length or a terminating NUL.

Thanks for adding a check here to catch improper usage.

[nullptr.cpp]

Use StringRef

[hubert.reinterpretcast]

LGTM; thanks!

llvm/lib/MC/XCOFFObjectWriter.cpp

 using CsectGroups = std::deque<CsectGroup *>;
 
 // Represents the data related to a section excluding the csects that make up
 // the raw data of the section. The csects are stored separately as not all
 // sections contain csects, and some sections contain csects which are better
 // stored separately, e.g. the .data section containing read-write, descriptor,
 // TOCBase and TOC-entry csects.
 struct Section {
   char Name[XCOFF::NameSize];
   // The physical/virtual address of the section. For an object file
   // these values are equivalent.
   uint32_t Address;
   uint32_t Size;
   uint32_t FileOffsetToData;
   uint32_t FileOffsetToRelocations;
   uint32_t RelocationCount;
   int32_t Flags;
void reset() {
     Index = UninitializedIndex;
     // Clear any csects we have stored.
     for (auto *Group : Groups)
       Group->clear();
   }
 
   Section(const char *N, XCOFF::SectionTypeFlags Flags, bool IsVirtual,
           CsectGroups Groups)
-      : Address(0), Size(0), FileOffsetToData(0), FileOffsetToRelocations(0),
-        RelocationCount(0), Flags(Flags), Index(UninitializedIndex),
-        IsVirtual(IsVirtual), Groups(Groups) {
-    strncpy(Name, N, XCOFF::NameSize);
+      : Name(), Address(0), Size(0), FileOffsetToData(0),
+        FileOffsetToRelocations(0), RelocationCount(0), Flags(Flags),
+        Index(UninitializedIndex), IsVirtual(IsVirtual), Groups(Groups) {
+    assert(strlen(N) <= XCOFF::NameSize && "section name too long");
+    memcpy(Name, N, strlen(N));
   }
 };
 
 class XCOFFObjectWriter : public MCObjectWriter {
 
   uint32_t SymbolTableEntryCount = 0;
   uint32_t SymbolTableOffset = 0;
   uint16_t SectionCount = 0;