[ADT] Fix itostr handling of min int64_t value

[thopre]

Summary

UBSan buildbot caught an undefined behavior in itostr with INT64_MIN.
The negation cannot be represented in the promoted operand (long long).
Negation is well defined on unsigned value though so this commit does
the negation after the static cast.

[lattner]

nice catch, thanks

[jhenderson]

Shouldn't there be some unit testing for this? It seems like a better testing method than relying on an UBSan build bot failure that might not manifest again in the future if some other testing were changed.

[thopre]

In D82200#2105987, @jhenderson wrote:

Shouldn't there be some unit testing for this? It seems like a better testing method than relying on an UBSan build bot failure that might not manifest again in the future if some other testing were changed.

Will do, mind if I add you as reviewer?

[jhenderson]

In D82200#2106412, @thopre wrote:

In D82200#2105987, @jhenderson wrote:

Shouldn't there be some unit testing for this? It seems like a better testing method than relying on an UBSan build bot failure that might not manifest again in the future if some other testing were changed.

Will do, mind if I add you as reviewer?

That's fine by me.

llvm/include/llvm/ADT/StringExtras.h

inline std::string utostr(uint64_t X, bool isNeg = false) {
   }
 
   if (isNeg) *--BufPtr = '-';   // Add negative sign...
   return std::string(BufPtr, std::end(Buffer));
 }
 
 inline std::string itostr(int64_t X) {
   if (X < 0)
-    return utostr(static_cast<uint64_t>(-X), true);
+    return utostr(-static_cast<uint64_t>(X), true);
   else
     return utostr(static_cast<uint64_t>(X));
 }
 
 /// StrInStrNoCase - Portable version of strcasestr.  Locates the first
 /// occurrence of string 's1' in string 's2', ignoring case.  Returns
 /// the offset of s2 in s1 or npos if s2 cannot be found.
 StringRef::size_type StrInStrNoCase(StringRef s1, StringRef s2);