[fpc-pascal] Register Allocation on x86_64

Jonas Maebe jonas at freepascal.org
Mon Jan 8 08:49:19 CET 2018

On 07/01/18 23:24, Martok wrote:
> Am 05.01.2018 um 18:03 schrieb Jonas Maebe:
>> regvars have always been disabled for routines that contain assembler
>> blocks (on all architectures). Sometimes the compiler temporarily uses
>> registers for other purposes over a longer period, which is why marking
>> the used registers is still required.
> I can see that this is safer if the assembler blocks aren't properly annotated,
> but if they are, there shouldn't be any problem?

Part of it is probably legacy, and part of it is the philosophy that the 
compiler does not try to understand assembly blocks and hence cannot 
make any assumptions about what is and what is not safe regarding local 
variables in the same routine. Unlike in e.g. gcc, there are no 
annotations for assembly routines that they expect certain variables to 
be in memory and/or registers, that the block may touch arbitrary memory 
locations, etc. Hence, the compiler is as conservative as possible.

It is probably possible to safely increase the code quality of routines 
containing assembly blocks, but that would require a proper examination 
of what optimizations are safe under which conditions (and it would lead 
to the need to add support for extra annotations for assembly blocks).

> There seem to be a lot of rather non-obvious limitations around inline assembler
> and assembler routines. Are they documented somewhere central?

Afaik there are only two limitations:
* routines containing assembly blocks (including pure assembly routines) 
cannot be inlined
* regular procedures that contain assembly blocks will never use regvars


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