[wasm][arm] Emit MaybeCheckConstPool in the trap code generation

src/compiler/arm/code-generator-arm.cc

 // Copyright 2014 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.
 
 #include "src/compiler/code-generator.h"
 
 #include "src/arm/macro-assembler-arm.h"
 #include "src/assembler-inl.h"
 #include "src/compilation-info.h"
 #include "src/compiler/code-generator-impl.h"
@@ skipping 2117 matching lines @@
         __ EnterFrame(StackFrame::WASM_COMPILED);
       }
       GenerateCallToTrap(trap_id);
       if (frame_elided_) {
         __ set_has_frame(old_has_frame);
       }
     }
 
    private:
     void GenerateCallToTrap(Builtins::Name trap_id) {
+      // Ensure to emit the constant pool first if necessary.
+      __ MaybeCheckConstPool();

[jbramley, 2017/03/07 14:24:08]

What is special about this location? I would have thought that the subsequent
code generation would trigger this check. (The pools aren't blocked.)

If the subsequent code _doesn't_ emit the pool properly, we should work out why
not, because otherwise this crash is probably just a symptom of a different
underlying problem.

       if (trap_id == Builtins::builtin_count) {
         // We cannot test calls to the runtime in cctest/test-run-wasm.
         // Therefore we emit a call to C here instead of a call to the runtime.
         // We use the context register as the scratch register, because we do
         // not have a context here.
         __ PrepareCallCFunction(0, 0, cp);
         __ CallCFunction(
             ExternalReference::wasm_call_trap_callback_for_testing(isolate()),
             0);
         __ LeaveFrame(StackFrame::WASM_COMPILED);
@@ skipping 536 matching lines @@
       padding_size -= v8::internal::Assembler::kInstrSize;
     }
   }
 }
 
 #undef __
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8