[ELF] Use llvm::toLower instead of libc call tolower

[ruiu]

What is the point of doing this?

[MaskRay]

In D52128#1235891, @ruiu wrote:

What is the point of doing this?

lld does not call setlocale() so the locale remains default "C". tolower exhibits different behaviors with some locales, e.g.:

setlocale(LC_CTYPE, "de_DE@euro");
printf("%x\n", tolower(0xb4)); => 0xb8

So it has slightly higher overhead. llvm::toLower is more efficient as it compiles to a compare and a conditional jump, as opposed to a libc call if tolower is used.

This is what gdb uses:

https://sourceware.org/git/?p=binutils-gdb.git;a=blob;hb=160fc977b6f33e5ef51f6c1f3cdcb57965c522c8;f=gdb/minsyms.h#l184

#define SYMBOL_HASH_NEXT(hash, c)			\
  ((hash) * 67 + TOLOWER ((unsigned char) (c)) - 113)

TOLOWER is a macro that uses a lookup table under the hood which is similar to llvm::tolower.

[ruiu]

LGTM

Please add the description to the commit message.

lld/trunk/ELF/SyntheticSections.cpp

 #include "Target.h"
 #include "Writer.h"
 #include "lld/Common/ErrorHandler.h"
 #include "lld/Common/Memory.h"
 #include "lld/Common/Strings.h"
 #include "lld/Common/Threads.h"
 #include "lld/Common/Version.h"
 #include "llvm/ADT/SetOperations.h"
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/BinaryFormat/Dwarf.h"
 #include "llvm/DebugInfo/DWARF/DWARFDebugPubTable.h"
 #include "llvm/Object/Decompressor.h"
 #include "llvm/Object/ELFObjectFile.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/LEB128.h"
 #include "llvm/Support/MD5.h"
 #include "llvm/Support/RandomNumberGenerator.h"

 unsigned PltSection::getPltRelocOff() const {
   return IsIplt ? InX::Plt->getSize() : 0;
 }
 
 // The string hash function for .gdb_index.
 static uint32_t computeGdbHash(StringRef S) {
   uint32_t H = 0;
   for (uint8_t C : S)
-    H = H * 67 + tolower(C) - 113;
+    H = H * 67 + toLower(C) - 113;
   return H;
 }
 
 GdbIndexSection::GdbIndexSection()
     : SyntheticSection(0, SHT_PROGBITS, 1, ".gdb_index") {}
 
 // Returns the desired size of an on-disk hash table for a .gdb_index section.
 // There's a tradeoff between size and collision rate. We aim 75% utilization.