[wasm] Implement decoding of i32v values

src/wasm/decoder.h

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_WASM_DECODER_H_
 #define V8_WASM_DECODER_H_
 
 #include <memory>
 
 #include "src/base/compiler-specific.h"
@@ skipping 155 matching lines @@
           name ? name : "uint32_t");
     if (checkAvailable(4)) {
       uint32_t val = read_u32(pc_);
       TRACE("%02x %02x %02x %02x = %u\n", pc_[0], pc_[1], pc_[2], pc_[3], val);
       pc_ += 4;
       return val;
     }
     return traceOffEnd<uint32_t>();
   }
 
-  // Reads a LEB128 variable-length 32-bit integer and advances {pc_}.
+  // Reads a LEB128 variable-length unsigned 32-bit integer and advances {pc_}.
   uint32_t consume_u32v(const char* name = nullptr) {
-    TRACE("  +%d  %-20s: ", static_cast<int>(pc_ - start_),
-          name ? name : "varint");
-    if (checkAvailable(1)) {
-      const byte* pos = pc_;
-      const byte* end = pc_ + 5;
-      if (end > limit_) end = limit_;
+    return consume_leb<uint32_t, false>(name);
+  }
 
-      uint32_t result = 0;
-      int shift = 0;
-      byte b = 0;
-      while (pc_ < end) {
-        b = *pc_++;
-        TRACE("%02x ", b);
-        result = result | ((b & 0x7F) << shift);
-        if ((b & 0x80) == 0) break;
-        shift += 7;
-      }
-
-      int length = static_cast<int>(pc_ - pos);
-      if (pc_ == end && (b & 0x80)) {
-        error(pc_ - 1, "varint too large");
-      } else if (length == 0) {
-        error(pc_, "varint of length 0");
-      } else {
-        TRACE("= %u\n", result);
-      }
-      return result;
-    }
-    return traceOffEnd<uint32_t>();
+  // Reads a LEB128 variable-length signed 32-bit integer and advances {pc_}.
+  int32_t consume_i32v(const char* name = nullptr) {

[ahaas, 2016/10/11 07:34:23]

Can you add tests for this function? 

+    return consume_leb<int32_t, true>(name);
   }
 
   // Consume {size} bytes and send them to the bit bucket, advancing {pc_}.
-  void consume_bytes(int size) {
-    TRACE("  +%d  %-20s: %d bytes\n", static_cast<int>(pc_ - start_), "skip",
-          size);
-    if (checkAvailable(size)) {
-      pc_ += size;
-    } else {
-      pc_ = limit_;
-    }
-  }
-
-  // Consume {size} bytes and send them to the bit bucket, advancing {pc_}.
   void consume_bytes(uint32_t size, const char* name = "skip") {
     TRACE("  +%d  %-20s: %d bytes\n", static_cast<int>(pc_ - start_), name,
           size);
     if (checkAvailable(size)) {
       pc_ += size;
     } else {
       pc_ = limit_;
     }
   }
 
@@ skipping 145 matching lines @@
         error(base, ptr, "extra bits in varint");
         return 0;
       }
       if ((b & 0x80) != 0) {
         error(base, ptr, "%s", msg);
         return 0;
       }
     }
     return result;
   }
+
+  template <typename IntType, bool is_signed>

[ahaas, 2016/10/11 07:34:23]

Can you measure a performance difference between is_signed being a template or
is_signed being a normal parameter?

+  IntType consume_leb(const char* name = nullptr) {
+    TRACE("  +%d  %-20s: ", static_cast<int>(pc_ - start_),
+          name ? name : "varint");
+    if (checkAvailable(1)) {
+      constexpr int kMaxLength = (sizeof(IntType) * 8 + 6) / 7;
+      const byte* pos = pc_;
+      const byte* end = pc_ + kMaxLength;
+      if (end > limit_) end = limit_;
+
+      IntType result = 0;
+      int shift = 0;
+      byte b = 0;
+      while (pc_ < end) {
+        b = *pc_++;
+        TRACE("%02x ", b);
+        result = result | (static_cast<IntType>(b & 0x7F) << shift);
+        shift += 7;
+        if ((b & 0x80) == 0) break;
+      }
+
+      int length = static_cast<int>(pc_ - pos);
+      if (pc_ == end && (b & 0x80)) {
+        error(pc_ - 1, "varint too large");
+      } else if (length == 0) {
+        error(pc_, "varint of length 0");
+      } else if (is_signed) {
+        if (length > 0 && length < kMaxLength) {
+          int sign_ext_shift = 8 * sizeof(IntType) - shift;
+          // Perform sign extension.
+          result = (result << sign_ext_shift) >> sign_ext_shift;
+        }
+        TRACE("= %" PRIi64 "\n", static_cast<int64_t>(result));
+      } else {
+        TRACE("= %" PRIu64 "\n", static_cast<uint64_t>(result));
+      }
+      return result;
+    }
+    return traceOffEnd<uint32_t>();
+  }
 };
 
 #undef TRACE
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_WASM_DECODER_H_