[Webkit-unassigned] [Bug 230622] Allow DFG to use regCS0 GPR (LLInt metadataTable) on ARMv7

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=230622

Saam Barati <sbarati at apple.com> changed:

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--- Comment #16 from Saam Barati <sbarati at apple.com> ---
Comment on attachment 440492
  --> https://bugs.webkit.org/attachment.cgi?id=440492
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=440492&action=review

r=me

> Source/JavaScriptCore/ChangeLog:3
> +        Allow DFG to use regCS0 GPR (LLInt metadataTable) on ARMv7

nit: this patch is really doing 2 things:
1. Allow DFG to use this register
2. Make sure tail calls properly restore CSRs in DFG.

I'd argue (2) is the really important part.

I'd make the title reflect (1) and (2) perhaps, or at least make mention of (2)

> Source/JavaScriptCore/jit/CallFrameShuffleData.cpp:63
> +        if (saveSlotIndexInCPURegisters < 0)
> +            saveSlotIndexInCPURegisters -= 1; // Round towards -inf

I'm pretty confused as to what this is doing and why it's needed.

> Source/JavaScriptCore/jit/CallFrameShuffleData.cpp:64
> +        VirtualRegister saveSlot { saveSlotIndexInCPURegisters / 2 };

Why /2?

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