From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marc Lehmann <pcg@goof.com>
To: egcs@cygnus.com
Subject: Re: [EGCS] Re: double alignment patch for x86
Date: Sun, 17 Aug 1997 21:48:08 -0000
Message-id: <E0x0BO4-0004OH-00.1997-08-17-21-53-20_pgcc_forever_@cerebro>
In-reply-to: 199708171940.PAA05296@jfc.
X-SW-Source: 1997-08/0049.html

John Carr wrote:
>
>byte alignment.  8 byte alignment is desirable (and has always been --
>Sun made a bad choice 10 years ago).  gcc copies misaligned double
>arguments to aligned locations on the stack.  Doing this on x86 might

I thinkt the patch already does this with -mstack-align-double
the (broken) -marg-align-double is just to squeeze out
the last bit of performance ;)

>help performance without breaking compatibility (or it might not: on x86
>there is only a cost if the dynamic value of the pointer is not aligned
>but SPARC requires two instructions instead of one to support possibly
>misaligned pointers).

It takes 2 additional cycles per double access.. this can sum up
to 30% runtime penalty on important algorithms like
matrix multiply...

It was a actually physicist that actually pointed this out to me
(and H.J.Lu for the libc, I believe).

      -----==-
      ----==-- _
      ---==---(_)__  __ ____  __       Marc Lehmann
      --==---/ / _ \/ // /\ \/ /       pcg@goof.com
      -=====/_/_//_/\_,_/ /_/\_\
    The choice of a GNU generation