[ELF] splitStrings: replace entSize==1 special case with manual loop unswitch. NFC

My x86-64 lld executable is actually smaller.

lld/ELF/InputSection.cpp

@@ -1358,16 +1358,12 @@ void EhInputSection::split(ArrayRef<RelTy> rels) {
 }
 
 static size_t findNull(StringRef s, size_t entSize) {
-  // Optimize the common case.
-  if (entSize == 1)
-    return s.find(0);
-
   for (unsigned i = 0, n = s.size(); i != n; i += entSize) {
     const char *b = s.begin() + i;
     if (std::all_of(b, b + entSize, [](char c) { return c == 0; }))
       return i;
   }
-  return StringRef::npos;
+  llvm_unreachable("");
 }
 
 SyntheticSection *MergeInputSection::getParent() const {
@@ -1376,20 +1372,24 @@ SyntheticSection *MergeInputSection::getParent() const {
 
 // Split SHF_STRINGS section. Such section is a sequence of
 // null-terminated strings.
-void MergeInputSection::splitStrings(ArrayRef<uint8_t> data, size_t entSize) {
-  size_t off = 0;
+void MergeInputSection::splitStrings(StringRef s, size_t entSize) {
   const bool live = !(flags & SHF_ALLOC) || !config->gcSections;
-  StringRef s = toStringRef(data);
-
-  while (!s.empty()) {
-    size_t end = findNull(s, entSize);
-    if (end == StringRef::npos)
-      fatal(toString(this) + ": string is not null terminated");
-    size_t size = end + entSize;
-
-    pieces.emplace_back(off, xxHash64(s.substr(0, size)), live);
-    s = s.substr(size);
-    off += size;
+  const char *p = s.data(), *end = s.data() + s.size();
+  if (!std::all_of(end - entSize, end, [](char c) { return c == 0; }))
+    fatal(toString(this) + ": string is not null terminated");
+  if (entSize == 1) {
+    // Optimize the common case.
+    do {
+      size_t size = strlen(p) + 1;
+      pieces.emplace_back(p - s.begin(), xxHash64(StringRef(p, size)), live);
+      p += size;
+    } while (p != end);
+  } else {
+    do {
+      size_t size = findNull(StringRef(p, end - p), entSize) + entSize;
+      pieces.emplace_back(p - s.begin(), xxHash64(StringRef(p, size)), live);
+      p += size;
+    } while (p != end);
   }
 }
 
@@ -1428,7 +1428,7 @@ void MergeInputSection::splitIntoPieces() {
   assert(pieces.empty());
 
   if (flags & SHF_STRINGS)
-    splitStrings(data(), entsize);
+    splitStrings(toStringRef(data()), entsize);
   else
     splitNonStrings(data(), entsize);
 }

lld/ELF/InputSection.h

@@ -293,7 +293,7 @@ public:
   SyntheticSection *getParent() const;
 
 private:
-  void splitStrings(ArrayRef<uint8_t> a, size_t size);
+  void splitStrings(StringRef s, size_t size);
   void splitNonStrings(ArrayRef<uint8_t> a, size_t size);
 };