Re: Register Files

kendall@centerline.com (Sam Kendall)
Wed, 7 Oct 1992 07:42:00 GMT

          From comp.compilers

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Newsgroups: comp.compilers
From: kendall@centerline.com (Sam Kendall)
Organization: CenterLine Software, Inc.
Date: Wed, 7 Oct 1992 07:42:00 GMT
References: 92-10-013 92-10-024
Keywords: registers, optimize

Steve Hobbs writes:


      A more interesting question is why doesn't the Alpha AXP architecture
      have any instructions that transfer values directly between the split
      register sets.... Transfers between registers banks are not all that
      common and increasing the speed of other operations even a small
      percentage justifies this decision.


Interesting! SPARC also lacks instructions to transfer between integer
and float registers. For the same reason?


Oddly, on SPARC, floating values are passed to functions, and returned
from functions, in integer registers. This means that for a value to
get from a float register in the caller, to a float register in the
callee, requires that the value pass as follows:


function call
float register -> memory -> integer register ---------------->
reg window shift
integer register -> memory -> float register


Was the SPARC calling convention done this way to keep it simple (since
float registers are not windowed), even at the expense of performance,
or is there another explanation?


----
Sam Kendall
CenterLine Software, Inc. (formerly Saber Software)
--


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