[Webkit-unassigned] [Bug 157741] JSC: DFG::SpeculativeJIT::compile special case for MIPS for PutByValWithThis

bugzilla-daemon at webkit.org bugzilla-daemon at webkit.org
Mon May 16 15:40:31 PDT 2016


--- Comment #9 from Guillaume Emont <guijemont at igalia.com> ---
Comment on attachment 279023
  --> https://bugs.webkit.org/attachment.cgi?id=279023

View in context: https://bugs.webkit.org/attachment.cgi?id=279023&action=review

>>>> Source/JavaScriptCore/dfg/DFGSpeculativeJIT32_64.cpp:2947
>>>> +            m_jit.move(baseTag, GPRInfo::argumentGPR3);
>>> This is wrong if basePayload/baseTag are aliased to argument registers w/ each other.
>>> i.e, if basePayload is argumentGPR3 and baseTag is argumentGPR2, this code will do the wrong thing.
>> I thought this was not possible because the argumentGPR's are not in GPRInfo::toRegister() (and not counted in GPRInfo::numberOfRegisters). Including them might be an idea for the future though, as I suspect more registers available might improve performances (and ARM does that so it's probably possible), but I think that's a trickier change, and I'd like to get the build unbroken.
> I don't quite understand your response here. Tag/Payload regs will be allocated by the DFG's register allocator. It will happily use argument registers.
> I don't think this makes the code harder to read. All code we write that sets up a call frame must account for this.

Sorry if that wasn't clear, and there is a possibility that I misunderstand something. What I mean is that DFG::RegisterBank<GPRInfo>::allocate() (see ::allocateInternal()) uses GPRInfo::toRegister() (and GPRInfo::numberOfRegisters as RegisterBank::NUM_REGS) as its source of registers. In the case of MIPS, GPRInfo::toRegister() never returns an argumentGPRx. Therefore baseTag and basePayload cannot be an argumentGPRx register. That's my current understanding at least.

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