[fpc-devel] Object/Pointer Swap Intrinsic

[J. Gareth Moreton]

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