Re: Small values in large registers (char,short,...)

Hi jakob!


I have written an lcc centered compiler system for windows 95/NT. You
can see the source code and get the system (for free) at
http://www.remcomp.com/lcc-win32 When I put a char/short value in a
register I will expand or ignore the high end bit depending on the
unsigned/signed attribute of the data. This is easy in x86
architectures since there is an expanding instruction in the
processor:


movsx _char%,eax


will expand the high bit of the character into all unused bits of the
32 bit register eax. Similarly:


movzx _char,%eax


will ignore the high bit and put zeroes in the bits 8-31. In other
architectures you *have* to synthethize those expansions. If not you
will always encounter the problems you mention. To do that it would be
easy to initialize the destination register to -1 in the case of
signed chars or zero the register in the case of unsigned, THEN put
the data in the register. Any other solution will make for a buggy
compiler.You can always try to catch some errors, but you are sure to
miss something sometimes. Besides, the complexity of the compiler will
increase by a factor that is not justified by the very small gains of
performance that you can expect. At today's clock speed, initializing
a register to a costant is an extremely fast operation. If you are
concerned about code size, remember that for initializing a register
to zero an


xor reg,reg


will zero it. If you want to initialize to minus one, you could do:


xor reg,reg
dec reg


In the case you have the decrement operation as is the case in the
x86. I think the sparc processors use the 'small constant' operation,
so a substraction of one could be done without storing a 32 bit
constant.


The alternative, i.e. keeping track of when a register was loaded with
some small costant/data and some bits in the register contain garbage
is enormous in development/debugging time. It is just not worth the
effort.


Yours sincerely
--
Jacob Navia Logiciels/Informatique
41 rue Maurice Ravel Tel (1) 48.23.51.44
93430 Villetaneuse Fax (1) 48.23.95.39
France
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