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

src/wasm/ast-decoder.h

Index: src/wasm/ast-decoder.h
diff --git a/src/wasm/ast-decoder.h b/src/wasm/ast-decoder.h
index 386f6f52b2d1456f28cb01232cfcdfaf47e8eaec..8c459963bfbc01110dc0b560ae5d21300ca50aa9 100644
--- a/src/wasm/ast-decoder.h
+++ b/src/wasm/ast-decoder.h
@@ -97,8 +97,7 @@ struct BreakDepthOperand {
   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;
   }
 };
@@ -107,8 +106,7 @@ 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");
   }
 };
 
@@ -146,20 +144,47 @@ 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;
   }
 };