[LEB128] Don't initialize error on success

This change removes an unnecessary branch from a hot path. It's also
questionable API to override any previous error unconditonally.

(cherry picked from commit 2c07181)

Author: adrian-prantl

lld/COFF/Driver.cpp

@@ -1249,7 +1249,7 @@ static void findKeepUniqueSections(COFFLinkerContext &ctx) {
       const uint8_t *cur = contents.begin();
       while (cur != contents.end()) {
         unsigned size;
-        const char *err;
+        const char *err = nullptr;
         uint64_t symIndex = decodeULEB128(cur, &size, contents.end(), &err);
         if (err)
           fatal(toString(obj) + ": could not decode addrsig section: " + err);

lld/ELF/Driver.cpp

@@ -2229,7 +2229,7 @@ static void findKeepUniqueSections(opt::InputArgList &args) {
       const uint8_t *cur = contents.begin();
       while (cur != contents.end()) {
         unsigned size;
-        const char *err;
+        const char *err = nullptr;
         uint64_t symIndex = decodeULEB128(cur, &size, contents.end(), &err);
         if (err)
           fatal(toString(f) + ": could not decode addrsig section: " + err);

llvm/include/llvm/Support/LEB128.h

@@ -125,14 +125,15 @@ inline unsigned encodeULEB128(uint64_t Value, uint8_t *p,
 }
 
 /// Utility function to decode a ULEB128 value.
+///
+/// If \p error is non-null, it will point to a static error message,
+/// if an error occured. It will not be modified on success.
 inline uint64_t decodeULEB128(const uint8_t *p, unsigned *n = nullptr,
                               const uint8_t *end = nullptr,
                               const char **error = nullptr) {
   const uint8_t *orig_p = p;
   uint64_t Value = 0;
   unsigned Shift = 0;
-  if (error)
-    *error = nullptr;
   do {
     if (p == end) {
       if (error)
@@ -158,15 +159,16 @@ inline uint64_t decodeULEB128(const uint8_t *p, unsigned *n = nullptr,
 }
 
 /// Utility function to decode a SLEB128 value.
+///
+/// If \p error is non-null, it will point to a static error message,
+/// if an error occured. It will not be modified on success.
 inline int64_t decodeSLEB128(const uint8_t *p, unsigned *n = nullptr,
                              const uint8_t *end = nullptr,
                              const char **error = nullptr) {
   const uint8_t *orig_p = p;
   int64_t Value = 0;
   unsigned Shift = 0;
   uint8_t Byte;
-  if (error)
-    *error = nullptr;
   do {
     if (p == end) {
       if (error)

llvm/lib/Object/MachOObjectFile.cpp

@@ -2992,7 +2992,7 @@ void ExportEntry::pushNode(uint64_t offset) {
   ErrorAsOutParameter ErrAsOutParam(E);
   const uint8_t *Ptr = Trie.begin() + offset;
   NodeState State(Ptr);
-  const char *error;
+  const char *error = nullptr;
   uint64_t ExportInfoSize = readULEB128(State.Current, &error);
   if (error) {
     *E = malformedError("export info size " + Twine(error) +
@@ -3127,7 +3127,7 @@ void ExportEntry::pushNode(uint64_t offset) {
 
 void ExportEntry::pushDownUntilBottom() {
   ErrorAsOutParameter ErrAsOutParam(E);
-  const char *error;
+  const char *error = nullptr;
   while (Stack.back().NextChildIndex < Stack.back().ChildCount) {
     NodeState &Top = Stack.back();
     CumulativeString.resize(Top.ParentStringLength);

llvm/lib/Support/DataExtractor.cpp

@@ -202,7 +202,7 @@ static T getLEB128(StringRef Data, uint64_t *OffsetPtr, Error *Err,
   if (isError(Err))
     return T();
 
-  const char *error;
+  const char *error = nullptr;
   unsigned bytes_read;
   T result =
       Decoder(Bytes.data() + *OffsetPtr, &bytes_read, Bytes.end(), &error);

llvm/tools/llvm-readobj/COFFDumper.cpp

@@ -2125,7 +2125,7 @@ void COFFDumper::printAddrsig() {
   const uint8_t *End = AddrsigContents.bytes_end();
   while (Cur != End) {
     unsigned Size;
-    const char *Err;
+    const char *Err = nullptr;
     uint64_t SymIndex = decodeULEB128(Cur, &Size, End, &Err);
     if (Err)
       reportError(createError(Err), Obj->getFileName());

llvm/tools/llvm-readobj/ELFDumper.cpp

@@ -5002,7 +5002,7 @@ static Expected<std::vector<uint64_t>> toULEB128Array(ArrayRef<uint8_t> Data) {
   const uint8_t *End = Data.end();
   while (Cur != End) {
     unsigned Size;
-    const char *Err;
+    const char *Err = nullptr;
     Ret.push_back(decodeULEB128(Cur, &Size, End, &Err));
     if (Err)
       return createError(Err);