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Index > Compiler Internals > 64bit code and Inc/Dec instructions

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Hayden



Joined: 06 Oct 2005
Posts: 132
Hayden
When porting 32bit code to 64bit code.. will the compiler automaticly convert
Inc/Dec instructions to Add/Sub instructions? Inc/Dec bytes are used for the REX prefix on 64bit machines and are now obsolite.

Or do we need to define new inc & dec macro's to do the inc/dec convertion. I dont have a 64bit system and havn't been able to any debug testing.

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Post 06 Oct 2005, 07:20
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Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7802
Location: Kraków, Poland
Tomasz Grysztar
INC/DEC still exist in 64-bit code, since only their short forms for registers (opcodes 40h-4Fh) have been used for other purposes, and their long forms (opcodes 0FEh-0FFh) are still available.

Any such "conversion" would not be suitable, since "INC EAX" is not the same as "ADD EAX,1" - the second one affects CF, while first one doesn't. This is important and utilized feature - thus they could not remove the INC/DEC from the instruction set.
Post 06 Oct 2005, 08:16
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