tree 6c84ac9aec6a4cd77fe8cc1ca024c785ca863abc
parent b08412723b360cca9322803d965df36c9e4a6ea3
author Tobias Tebbi <tebbi@chromium.org> 1556202142 +0200
committer Commit Bot <commit-bot@chromium.org> 1556204356 +0000

[csa] verify skipped write-barriers in MemoryOptimizer

With very few exceptions, this verifies all skipped write-barriers in
CSA and Torque, showing that the MemoryOptimizer together with some
type information on the stored value are enough to avoid unsafe skipped
write-barriers.

Changes to CSA:
SKIP_WRITE_BARRIER and Store*NoWriteBarrier are verified by the
MemoryOptimizer by default.
Type information about the stored values (TNode<Smi>) is exploited to
safely skip write barriers for stored Smi values.
In some cases, the code is re-structured to make it easier to consume
for the MemoryOptimizer (manual branch and load elimination).

Changes to the MemoryOptimizer:
Improve the MemoryOptimizer to remove write barriers:
- When the store happens to a CSA-generated InnerAllocate, by ignoring
  Bitcasts and additions.
- When the stored value is the HeapConstant of an immortal immovable root.
- When the stored value is a SmiConstant (recognized by BitcastToTaggedSigned).
- Fast C-calls are treated as non-allocating.
- Runtime calls can be white-listed as non-allocating.

Remaining missing cases:
- C++-style iterator loops with inner pointers.
- Inner allocates that are reloaded from a field where they were just stored
  (for example an elements backing store). Load elimination would fix that.
- Safe stored value types that cannot be expressed in CSA (e.g., Smi|Hole).
  We could handle that in Torque.
- Double-aligned allocations, which are not lowered in the MemoryOptimizer
  but in CSA.

Drive-by change: Avoid Smi suffix for StoreFixedArrayElement since this
can be handled by overload resolution (in Torque and C++).

R=jarin@chromium.org
TBR=mvstanton@chromium.org

Change-Id: I0af9b710673f350e0fe81c2e59f37da93c024b7c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1571414
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61016}