Implement the LEB128 decoder for packed relocations more efficiently.

Replace the current two-loop LEB128 decoder in the crazy linker with a
slight more efficient single-loop implementation.

BUG=385553

Review URL: https://codereview.chromium.org/475093002

Cr-Commit-Position: refs/heads/master@{#289870}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289870 0039d316-1c4b-4281-b951-d872f2087c98

2 files changed, 11 insertions, 9 deletions

diff --git a/third_party/android_crazy_linker/README.chromium b/third_party/android_crazy_linker/README.chromium
index 3b36cea..ef5087d 100644
--- a/third_party/android_crazy_linker/README.chromium
+++ b/third_party/android_crazy_linker/README.chromium
@@ -37,3 +37,5 @@ Local Modifications:
 - Add support for x86_64.
 
 - Speculative fix for crbug/397634.
+
+- Implement LEB128 decoding more efficiently.
diff --git a/third_party/android_crazy_linker/src/src/crazy_linker_elf_relocations.cpp b/third_party/android_crazy_linker/src/src/crazy_linker_elf_relocations.cpp
index a5e53fe..33c385a 100644
--- a/third_party/android_crazy_linker/src/src/crazy_linker_elf_relocations.cpp
+++ b/third_party/android_crazy_linker/src/src/crazy_linker_elf_relocations.cpp
@@ -370,17 +370,17 @@ class Leb128Decoder {
       : encoding_(encoding), cursor_(0) { }
 
   uint32_t Dequeue() {
-    size_t extent = cursor_;
-    while (encoding_[extent] >> 7)
-      extent++;
-
     uint32_t value = 0;
-    for (size_t i = extent; i > cursor_; --i) {
-      value = (value << 7) | (encoding_[i] & 127);
-    }
-    value = (value << 7) | (encoding_[cursor_] & 127);
 
-    cursor_ = extent + 1;
+    size_t shift = 0;
+    uint8_t byte;
+
+    do {
+      byte = encoding_[cursor_++];
+      value |= static_cast<uint32_t>(byte & 127) << shift;
+      shift += 7;
+    } while (byte & 128);
+
     return value;
   }