[sdcc-devel] [ sdcc-Feature Requests-979838 ] Register parameter passing

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Feature Requests item #979838, was opened at 2004-06-25 18:26
Message generated for change (Comment added) made by bernhardheld
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Category: None
Group: None
Status: Open
Priority: 5
Submitted By: Stephen (easyplan)
Assigned to: Nobody/Anonymous (nobody)
Summary: Register parameter passing

Initial Comment:
First, well done on a great product, to all those who 
have worked on the compiler. 

I have used Keil and HiTech compilers for an 8052, and 
both use the registers to pass parameters to functions. 
My project which compiles using the Keil compiler is 96 
bytes short of data when compiled using SDCC because 
most of the parameters are passed in data space. 

Have you considered using the registers to pass 
parameters as part of the optimisation of data space? 
or allowing this as an option?

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>Comment By: Bernhard Held (bernhardheld)
Date: 2004-06-28 09:00

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At the moment the register allocator is not clever enough to 
efficiently deal with parameters in registers. 
In general the benefit from register parameters is often 
overestimated. It certainly makes sense on CPUs with plenty 
of large registers. On a 8051 tiny functions will look much 
better. But in larger or complex functions you'll often find that 
the registers must be saved immediately after the entrance 
code. In many cases it's better to let the register allocator 
decide which variables should be kept in registers. It would be 
a big performance drawback, if e.g. an automatic variable 
used as a loop counter is located on the stack, because the 
parameters eat up all the register space.

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Comment By: Stephen (easyplan)
Date: 2004-06-26 10:56

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I didn't know about that option.
Helps with data optimisation, but I see the libraries need to 
be re-compiled. 
Would it not make sense to use the active register bank to 
pass some of the parameters because often the parameters 
are being used in the registers. They are copied into bank 1 
to be passed as parameters, and then the called function 
moves them back into registers to use. 

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Comment By: Bernhard Held (bernhardheld)
Date: 2004-06-25 18:31

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What about --parms-in-bank1?

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