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Date: Wed, 14 Nov 2001 15:26:58 -0800
To: Greg Lehey <grog@FreeBSD.org>
From: Mark Peek <mark@peek.org>
Subject: Re: cvs commit: src/sys/powerpc/powerpc machdep.c
Cc: cvs-committers@FreeBSD.org, cvs-all@FreeBSD.org
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At 9:31 AM +1030 11/15/01, Greg Lehey wrote:
>On Monday, 12 November 2001 at 16:44:21 -0800, Mark Peek wrote:
>>  mp          2001/11/12 16:44:21 PST
>>
>>    Modified files:
>>      sys/powerpc/powerpc  machdep.c
>>    Log:
>>    Don't enable FP in the kernel. It is not needed when -msoft-float is used.
>
>Why is FP needed at all?

Without -msoft-float, gcc uses the FP registers for code optimization 
like 8 byte structure copies. For instance, this contrived example:

struct foo {
     char c[8];
} a, b;

bar()
{
     a = b;
}

produces this:

...
bar:
         lis 9,a@ha
         la 9,a@l(9)
         lis 11,b@ha
         la 11,b@l(11)
         lfd 0,0(11)     <====
         stfd 0,0(9)     <====
         blr
...

and with -msoft-float:

...
bar:
         lis 9,a@ha
         la 9,a@l(9)
         lis 11,b@ha
         la 11,b@l(11)
         lwz 12,4(11)
         lwz 11,0(11)
         stw 11,0(9)
         stw 12,4(9)
         blr
...


Mark

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