[wasm] Reduce size of ProtectedInstructionData

Since code objects cannot grow larger than 2GB anyway, it's enough to
store the instruction offset and landing pad offset as 32-bit values.
This reduces the size of the ProtectedInstructionData struct by 50%.

R=eholk@chromium.org

Bug: v8:5277
Change-Id: I4d2e0dc76b8a853fb50d51d70d5ec4038ee594ac
Reviewed-on: https://chromium-review.googlesource.com/686757
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48192}

src/trap-handler/trap-handler.h

@@ -30,12 +30,13 @@ namespace trap_handler {
 
 struct ProtectedInstructionData {
   // The offset of this instruction from the start of its code object.
-  intptr_t instr_offset;
+  // Wasm code never grows larger than 2GB, so uint32_t is sufficient.
+  uint32_t instr_offset;
 
   // The offset of the landing pad from the start of its code object.
   //
   // TODO(eholk): Using a single landing pad and store parameters here.
-  intptr_t landing_offset;
+  uint32_t landing_offset;
 };
 
 const int kInvalidIndex = -1;

src/wasm/wasm-module.cc

@@ -330,14 +330,17 @@ void UnpackAndRegisterProtectedInstructions(Isolate* isolate,
       continue;
     }
 
-    const intptr_t base = reinterpret_cast<intptr_t>(code->entry());
+    byte* base = code->entry();
 
     const int mode_mask =
         RelocInfo::ModeMask(RelocInfo::WASM_PROTECTED_INSTRUCTION_LANDING);
     for (RelocIterator it(code, mode_mask); !it.done(); it.next()) {
       trap_handler::ProtectedInstructionData data;
-      data.instr_offset = it.rinfo()->data();
-      data.landing_offset = reinterpret_cast<intptr_t>(it.rinfo()->pc()) - base;
+      data.instr_offset = static_cast<uint32_t>(it.rinfo()->data());
+      data.landing_offset = static_cast<uint32_t>(it.rinfo()->pc() - base);
+      // Check that now over-/underflow happened.
+      DCHECK_EQ(it.rinfo()->data(), data.instr_offset);
+      DCHECK_EQ(it.rinfo()->pc() - base, data.landing_offset);
       unpacked.emplace_back(data);
     }
     if (unpacked.empty()) continue;