[Webkit-unassigned] [Bug 163246] New: B3 needs a special WASMCheckBounds Opcode

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=163246

            Bug ID: 163246
           Summary: B3 needs a special WASMCheckBounds Opcode
    Classification: Unclassified
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: keith_miller at apple.com

In order to make WASM as fast as possible we need to give B3 a through understanding of how WASM works. In particular, B3 needs to understand the memory model of WASM and what optimizations it can make within those constraints. To this end we should add a WASMCheckBounds opcode. This opcode should take the following bits of information: WASMCheckBounds(Value* ptr, Reg sizeRegister, size_t guardedBytes). When emitting a load WASM will first emit a WASMBoundsCheck value passing WASMCheckBounds(WASMUserPtrValue, pinnedSizeRegister, max(0, MMapedMemoryBeyondMaxRequested - WASMLoadOpOffsetImmediate - sizeOfLoadOpcode)). So if a module had a memory max of 1MB and we mapped 1088 KB (64 KB extra) and a WASM I32.Load operation with an offset immediate of 1024 bytes we could emit a WASMCheckBounds(@ptr, %r13, 64 * KB - 1024 - 4).

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