[Wasm] Convert many of the fixed-size values to LEB128.

src/wasm/ast-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_AST_DECODER_H_
 #define V8_WASM_AST_DECODER_H_
 
 #include "src/signature.h"
 #include "src/wasm/decoder.h"
 #include "src/wasm/wasm-opcodes.h"
@@ skipping 79 matching lines @@
     machine_type = MachineType::None();
   }
 };
 
 struct Block;
 struct BreakDepthOperand {
   uint32_t depth;
   Block* target;
   int length;
   inline BreakDepthOperand(Decoder* decoder, const byte* pc) {
-    depth = decoder->checked_read_u8(pc, 1, "break depth");
-    length = 1;
+    depth = decoder->checked_read_u32v(pc, 1, &length, "break depth");
     target = nullptr;
   }
 };
 
 struct BlockCountOperand {
   uint32_t count;
   int length;
   inline BlockCountOperand(Decoder* decoder, const byte* pc) {
-    count = decoder->checked_read_u8(pc, 1, "block count");
-    length = 1;
+    count = decoder->checked_read_u32v(pc, 1, &length, "block count");
   }
 };
 
 struct SignatureIndexOperand {
   uint32_t index;
   FunctionSig* sig;
   int length;
   inline SignatureIndexOperand(Decoder* decoder, const byte* pc) {
     index = decoder->checked_read_u32v(pc, 1, &length, "signature index");
     sig = nullptr;
@@ skipping 17 matching lines @@
   inline ImportIndexOperand(Decoder* decoder, const byte* pc) {
     index = decoder->checked_read_u32v(pc, 1, &length, "import index");
     sig = nullptr;
   }
 };
 
 struct BranchTableOperand {
   uint32_t table_count;
   const byte* table;
   int length;
+  // Cache entry offsets, assuming we are iterating through them in order.
+  int next_entry_index;
+  const byte* next_entry;
   inline BranchTableOperand(Decoder* decoder, const byte* pc) {

[titzer, 2016/03/08 10:06:53]

Maybe we can leave the branch table target as uint16? They encode depth, so I
hope that 64k should be enough for anybody. The design PR doesn't update this
yet, so let's be consistent and leave it.

[binji, 2016/03/08 16:30:15]

:)
Yeah, it does:
https://github.com/WebAssembly/design/pull/592/files#diff-7c076c52f23f4a77097...

-    table_count = decoder->checked_read_u16(pc, 1, "expected #entries");
-    length = 2 + table_count * 2 + 2;
+    int varint_length;
+    table_count =
+        decoder->checked_read_u32v(pc, 1, &varint_length, "expected #entries");
+    length = varint_length;
+    uint32_t table_start = 1 + length;
+    for (uint32_t i = 0; i < table_count; ++i) {
+      decoder->checked_read_u32v(pc, 1 + length, &varint_length);
+      length += varint_length;
+    }
+    // Default entry.
+    decoder->checked_read_u32v(pc, 1 + length, &varint_length);
+    length += varint_length;
 
-    if (decoder->check(pc, 3, table_count * 2 + 2,
-                       "expected <table entries>")) {
-      table = pc + 3;
+    if (decoder->check(pc, 1, length, "expected <table entries>")) {
+      table = pc + table_start;
     } else {
       table = nullptr;
     }
+    next_entry_index = 0;
+    next_entry = table;
   }
-  inline uint16_t read_entry(Decoder* decoder, int i) {
+  inline uint32_t read_entry(Decoder* decoder, int i) {
     DCHECK(i >= 0 && static_cast<uint32_t>(i) <= table_count);
-    return table ? decoder->read_u16(table + i * sizeof(uint16_t)) : 0;
+    if (!table) return 0;
+    if (next_entry_index > i) {
+      // Reset to zero and iterate.
+      next_entry_index = 0;
+      next_entry = table;
+    }
+    uint32_t result;
+    while (next_entry_index <= i) {

[titzer, 2016/03/08 10:06:53]

Linear search; that's no good. Definitely gotta go back to uint16.

[binji, 2016/03/08 16:30:15]

It shouldn't actually end up being linear here. In both uses of read_entry, the
entire table is looped over, in order. So I cache the previous offset. This was
just defensive coding, but I could put a DCHECK here instead to enforce it.

+      int varint_length;
+      result = decoder->checked_read_u32v(next_entry, 0, &varint_length);
+      next_entry_index++;
+      next_entry += varint_length;
+    }
+    return result;
   }
 };
 
 struct MemoryAccessOperand {
   bool aligned;
   uint32_t offset;
   int length;
   inline MemoryAccessOperand(Decoder* decoder, const byte* pc) {
     byte bitfield = decoder->checked_read_u8(pc, 1, "memory access byte");
     aligned = MemoryAccess::AlignmentField::decode(bitfield);
@@ skipping 55 matching lines @@
 int OpcodeLength(const byte* pc, const byte* end);
 
 // Computes the arity (number of sub-nodes) of the opcode at the given address.
 int OpcodeArity(ModuleEnv* module, FunctionSig* sig, const byte* pc,
                 const byte* end);
 }  // namespace wasm
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_WASM_AST_DECODER_H_