Bug 170089 – [jsc][mips] Add missing MacroAssembler functions after r214187

• Status: RESOLVED
• Resolution: FIXED
• Component: JavaScriptCore

[Guillaume Emont]

We need to implement MacroAssemblerMIPS::loadFloat() and storeFloat() with an ImplicitAddress argument.

[Guillaume Emont]

I am currently testing a patch that I will submit for revision once I'm confident it doesn't break anything (likely on Monday).

[Guillaume Emont]

Created attachment 305523 [details]
Patch

The patch. It does not seem to break anything.

[Yusuke Suzuki]

Comment on attachment 305523 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=305523&action=review

r=me with comment.

> Source/JavaScriptCore/assembler/MacroAssemblerMIPS.h:2462
> +    void loadFloat(ImplicitAddress address, FPRegisterID dest)
> +    {
> +        if (address.offset >= -32768 && address.offset <= 32767
> +            && !m_fixedWidth) {
> +            m_assembler.lwc1(dest, address.base, address.offset);
> +        } else {
> +            /*
> +               lui     addrTemp, (offset + 0x8000) >> 16
> +               addu    addrTemp, addrTemp, base
> +               lwc1    dest, (offset & 0xffff)(addrTemp)
> +               */
> +            m_assembler.lui(addrTempRegister, (address.offset + 0x8000) >> 16);
> +            m_assembler.addu(addrTempRegister, addrTempRegister, address.base);
> +            m_assembler.lwc1(dest, addrTempRegister, address.offset);
> +        }
> +    }

Looking the loadDouble code, we can see WTF_MIPS_ISA(1) guard.
Isn't it required here?

> Source/JavaScriptCore/assembler/MacroAssemblerMIPS.h:2617
> +    }

Ditto.

[Adrian Perez]

(In reply to Yusuke Suzuki from comment #3)
> Comment on attachment 305523 [details]
> Patch
> 
> View in context:
> https://bugs.webkit.org/attachment.cgi?id=305523&action=review
> 
> r=me with comment.
> 
> > Source/JavaScriptCore/assembler/MacroAssemblerMIPS.h:2462
> > +    void loadFloat(ImplicitAddress address, FPRegisterID dest)
> > +    {
> > +        if (address.offset >= -32768 && address.offset <= 32767
> > +            && !m_fixedWidth) {
> > +            m_assembler.lwc1(dest, address.base, address.offset);
> > +        } else {
> > +            /*
> > +               lui     addrTemp, (offset + 0x8000) >> 16
> > +               addu    addrTemp, addrTemp, base
> > +               lwc1    dest, (offset & 0xffff)(addrTemp)
> > +               */
> > +            m_assembler.lui(addrTempRegister, (address.offset + 0x8000) >> 16);
> > +            m_assembler.addu(addrTempRegister, addrTempRegister, address.base);
> > +            m_assembler.lwc1(dest, addrTempRegister, address.offset);
> > +        }
> > +    }
> 
> Looking the loadDouble code, we can see WTF_MIPS_ISA(1) guard.
> Isn't it required here?

I think Yusuke is right. FPRegisterIDs refer to 64-bit registers. In
MIPS32/MIPS-II “ldc1” can be used to load a 32-bit value directly,
but in MIPS-I it is needed to use two “lwc1” instructions, one for
each 32-bit half of the 64-bit value.

[Guillaume Emont]

(In reply to Adrian Perez from comment #4)
> (In reply to Yusuke Suzuki from comment #3)
> > Comment on attachment 305523 [details]
> > Patch
> > 
> > View in context:
> > https://bugs.webkit.org/attachment.cgi?id=305523&action=review
> > 
> > r=me with comment.
> > 
> > > Source/JavaScriptCore/assembler/MacroAssemblerMIPS.h:2462
> > > +    void loadFloat(ImplicitAddress address, FPRegisterID dest)
> > > +    {
> > > +        if (address.offset >= -32768 && address.offset <= 32767
> > > +            && !m_fixedWidth) {
> > > +            m_assembler.lwc1(dest, address.base, address.offset);
> > > +        } else {
> > > +            /*
> > > +               lui     addrTemp, (offset + 0x8000) >> 16
> > > +               addu    addrTemp, addrTemp, base
> > > +               lwc1    dest, (offset & 0xffff)(addrTemp)
> > > +               */
> > > +            m_assembler.lui(addrTempRegister, (address.offset + 0x8000) >> 16);
> > > +            m_assembler.addu(addrTempRegister, addrTempRegister, address.base);
> > > +            m_assembler.lwc1(dest, addrTempRegister, address.offset);
> > > +        }
> > > +    }
> > 
> > Looking the loadDouble code, we can see WTF_MIPS_ISA(1) guard.
> > Isn't it required here?
> 
> I think Yusuke is right. FPRegisterIDs refer to 64-bit registers. In
> MIPS32/MIPS-II “ldc1” can be used to load a 32-bit value directly,
> but in MIPS-I it is needed to use two “lwc1” instructions, one for
> each 32-bit half of the 64-bit value.

No, I don't think we need that guard. loadDouble/storeDouble use that because MIPS-I does not contain the ldc1 (resp. sdc1) instruction, and therefore two lwc1 (resp swc1) instructions are used instead for the MIPS-I version. In loadFloat/storeFloat, we do not use ldc1/sdc1, and all the instructions that we use are MIPS-I compatible AFAIK, so there's no need for an alternative version.
You can see that the pre-existing loadFloat/storeFloat with a BaseIndex argument don't have such a guard either, for the same reasons.

Another way to put it: the need for special cases for MIPS-I only exists when dealing with doubles. The fact of using an ImplicitAddress instead of a BaseIndex does not incur this need. 

Another debate is whether we still want to support MIPS-I, I don't know if there still are many devices using that where it makes sense to use webkit, though I could be wrong, because, hey, we have these guards in the code.

[Adrian Perez]

(In reply to Guillaume Emont from comment #5)

> Another way to put it: the need for special cases for MIPS-I only exists
> when dealing with doubles. The fact of using an ImplicitAddress instead of a
> BaseIndex does not incur this need. 

Oh, I was assuming that loadFloat() would emit a load for a 64-bit value.
Thanks for clarifying.

Then you're right (and I should have checked the code before commenting)
and it's an r+me :-)

> Another debate is whether we still want to support MIPS-I, I don't know if
> there still are many devices using that where it makes sense to use webkit,
> though I could be wrong, because, hey, we have these guards in the code.

The cost of maintaining MIPS-I support is quite low:

  ~/devel/WebKit % rg -F 'WTF_MIPS_ISA(1)' Source | wc -l
  8

There's only the code for emitting load/save of 64-bit floating point values
in “MacroAssemblerMIPS.h”, and two one-liners to insert “nop”-delays after
some load instructions.

It does not seem that it is much of a problem to keep the support around,
though it's difficult to gauge how often JSC ends up running on MIPS-I
hardware. As far as I know that would be only the original R2000/R3000
CPUs, and more recently clones (e.g. OpenCores has a synthetizable clean
room design) often used as microcontrollers — so I have no strong opinion
towards keeping the MIPS-I bits.

[Yusuke Suzuki]

Comment on attachment 305523 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=305523&action=review

>>>> Source/JavaScriptCore/assembler/MacroAssemblerMIPS.h:2462
>>>> +    }
>>> 
>>> Looking the loadDouble code, we can see WTF_MIPS_ISA(1) guard.
>>> Isn't it required here?
>> 
>> I think Yusuke is right. FPRegisterIDs refer to 64-bit registers. In
>> MIPS32/MIPS-II “ldc1” can be used to load a 32-bit value directly,
>> but in MIPS-I it is needed to use two “lwc1” instructions, one for
>> each 32-bit half of the 64-bit value.
> 
> No, I don't think we need that guard. loadDouble/storeDouble use that because MIPS-I does not contain the ldc1 (resp. sdc1) instruction, and therefore two lwc1 (resp swc1) instructions are used instead for the MIPS-I version. In loadFloat/storeFloat, we do not use ldc1/sdc1, and all the instructions that we use are MIPS-I compatible AFAIK, so there's no need for an alternative version.
> You can see that the pre-existing loadFloat/storeFloat with a BaseIndex argument don't have such a guard either, for the same reasons.
> 
> Another way to put it: the need for special cases for MIPS-I only exists when dealing with doubles. The fact of using an ImplicitAddress instead of a BaseIndex does not incur this need. 
> 
> Another debate is whether we still want to support MIPS-I, I don't know if there still are many devices using that where it makes sense to use webkit, though I could be wrong, because, hey, we have these guards in the code.

Make sense.

[WebKit Commit Bot]

Comment on attachment 305523 [details]
Patch

Clearing flags on attachment: 305523

Committed r215226: <http://trac.webkit.org/changeset/215226>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.