[fpc-devel] Object/Pointer Swap Intrinsic

[J. Gareth Moreton]

Of course, I briefly forgot that UsedRegs and TmpUsedRegs aren't 
objects, but arrays of objects - the same principle still applies though 
to their members.


On 11/12/2019 06:26, J. Gareth Moreton wrote:
> Hi everyone,
>
> I was wondering... is there an intrinsic in Free Pascal that swaps the 
> pointers stored in two object or pointer variables? That is... 
> something like "procedure SwapAddresses(var Obj1: TObject; var Obj2: 
> TObject);" so after it is called, Obj2 now points to Obj1 and vice 
> versa?  Under x86-64, for example, it can be implemented as (Intel 
> notation):
>
> XOR RCX, RDX
> XOR RDX, RCX
> XOR RCX, RDX
>
> Or just
>
> XCHG RCX, RDX
>
> Both methods take just 3 cycles to perform (faster than transferring 
> the object data by several orders of magnitude, plus the pointers 
> don't get dereferenced).  But equivalent Pascal code that typecasts 
> the objects or pointers into PtrUint and then uses the xor operator 
> may not be portable (e.g. on platforms that don't have pointers and 
> treat objects differently).
>
> I ask this because I've noticed a slightly clever speed-up in parts of 
> the peephole optimiser where "TmpUsedRegs" contains exactly the 
> information required to be stored in "UsedRegs" (usually because 
> "UpdateUsedRegs(TmpUsedRegs, tai(p.Next))" was called and then p was 
> freed, hence any required call to "UpdateUsedRegs(tai(p.Next))" will 
> reproduce the same register usage.  By simply swapping the pointers 
> between UsedRegs and TmpUsedRegs, I can immediately treat the contents 
> of TmpUsedRegs as the live data and repurpose UsedRegs as the new 
> temporary register tracking.  Calls to TransferUsedRegs and 
> RestoreUsedRegs are slow.
>
> Gareth aka. Kit
>
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