[Webkit-unassigned] [Bug 238008] New: [RISC-V] Support the C (compressed instructions) standard extension

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=238008

            Bug ID: 238008
           Summary: [RISC-V] Support the C (compressed instructions)
                    standard extension
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: New Bugs
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: zan at falconsigh.net
            Blocks: 237775

The initial RISCV64 support is based on RV64I and RV32I instruction sets which impose instruction values that are 32 bits in size. Memory usage efficiency would improve with the adoption of compressed instructions as specified in the C standard extension, enabling generation of instructions that are only 16 bits wide. The spec is quite bullish:

```
Typically, 50%–60%
of the RISC-V instructions in a program can be replaced with RVC instructions, resulting in a
25%–30% code-size reduction.
```

The 32-bit instruction size was taken for granted with good effect in implementing linking and repatching on JIT code, so decoupling these assumptions will be some work.

Also, the extension is just that -- an extension. We need a solid way of detecting it at runtime and adjusting accordingly.


Referenced Bugs:

https://bugs.webkit.org/show_bug.cgi?id=237775
[Bug 237775] RISC-V support in JavaScriptCore
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