[fpc-pascal] asm statement

Jonas Maebe jonas.maebe at elis.ugent.be
Tue Jun 12 21:40:00 CEST 2012

On 12 Jun 2012, at 18:41, Rainer Stratmann wrote:

> Is it still necessary to fill in the registers at the end of a asm statement?


> What happens if there is only the end; statement without the used registers?

Then the compiler will assume that the assembler code can change all callee-saved registers for the current target platform, and that it will not modify any caller-saved registers. If you don't know what the callee/caller-saved registers of all of your current and future supported target platforms are (possibly even targets that do not yet exist), or if you simply don't want the compiler to needlessly save and restore all callee-saved registers even when you don't modify them (it would be rather strange to resort to assembler and then force the compiler to generate suboptimal code), then you should specify the registers that are overwritten by your assembler code.

And no, the compiler cannot automagically figure this out for you. It does not contain an abstract interpretation infrastructure for assembler that can determine what registers are changed and not saved/restored by the code, neatly sidestepping the halting problem in the process. Anything less would result in a lot of work with either only suboptimal or unsafe results.


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