Add VerifiedMemory helper for write-barrier verification.

Add VerifiedMemory helper for write-barrier verification.

Allocates hidden extra memory after VirtualMemory, and
verified writes are duplicated at a fixed offset.

This CL only adds the helper.
Next steps:
1. Use Reserve when reserving semi-spaces and heap pages.
2. Use Write for storing pointers (+ offset in generated code).
3. Use Accept for GC-related object lifecycle.
4. Call Verify regularly.
5. Fix failures (= places where we forget/ignore write barriers).

Committed: https://code.google.com/p/dart/source/detail?r=41399

[koda, 2014-10-21 23:34:57]

Patchset #5 (id:80001) has been deleted

[Ivan Posva, 2014-10-29 05:06:07]

LGTMwC

-Ivan

https://codereview.chromium.org/641243004/diff/100001/runtime/vm/verified_mem...
File runtime/vm/verified_memory.cc (right):

https://codereview.chromium.org/641243004/diff/100001/runtime/vm/verified_mem...
runtime/vm/verified_memory.cc:24: // Commit the offset part of the reservation
(writable, not executable).
This is a little light on comments given what is going on here: You basically
rely on the VirtualMemory class to remember the original reservation size and
thus by committing before truncating you create the shadow are at the
preselected offset.

https://codereview.chromium.org/641243004/diff/100001/runtime/vm/verified_mem...
File runtime/vm/verified_memory.h (right):

https://codereview.chromium.org/641243004/diff/100001/runtime/vm/verified_mem...
runtime/vm/verified_memory.h:32: static VirtualMemory* Reserve(intptr_t size) {
Here and other places in this code:
We had planned to not add any new APIs with byte sizes and instead switch to
word sizes.

Having said that, I am not sure whether this is the right call to do it here as
VerifiedMemory is supposed to be a thin wrapper over VirtualMemory.

[koda, 2014-10-29 19:47:31]

Also upped the default offset to 32 MB for 64 bit (to match default semi space
size).

https://codereview.chromium.org/641243004/diff/100001/runtime/vm/verified_mem...
File runtime/vm/verified_memory.cc (right):

https://codereview.chromium.org/641243004/diff/100001/runtime/vm/verified_mem...
runtime/vm/verified_memory.cc:24: // Commit the offset part of the reservation
(writable, not executable).
On 2014/10/29 05:06:07, Ivan Posva wrote:
> This is a little light on comments given what is going on here: You basically
> rely on the VirtualMemory class to remember the original reservation size and
> thus by committing before truncating you create the shadow are at the
> preselected offset.

Commented here, and also documented Truncate's 'try_unmap' parameter in
virtual_memory.h.

https://codereview.chromium.org/641243004/diff/100001/runtime/vm/verified_mem...
File runtime/vm/verified_memory.h (right):

https://codereview.chromium.org/641243004/diff/100001/runtime/vm/verified_mem...
runtime/vm/verified_memory.h:32: static VirtualMemory* Reserve(intptr_t size) {
On 2014/10/29 05:06:07, Ivan Posva wrote:
> Here and other places in this code:
> We had planned to not add any new APIs with byte sizes and instead switch to
> word sizes.
> 
> Having said that, I am not sure whether this is the right call to do it here
as
> VerifiedMemory is supposed to be a thin wrapper over VirtualMemory.

I agree with your comment. Will leave as is for now. If we want to change it, I
expect very few call sites for these methods.

[koda, 2014-10-29 19:56:17]

Committed patchset #6 (id:120001) manually as r41399 (presubmit successful).