Re: inline-asm/9637: arguments passed/return to asm block via mmx registers trigger failure

[Philip Graham Willoughby <pgw99@doc.ic.ac.uk>]

The following reply was made to PR inline-asm/9637; it has been noted by GNATS.

From: Philip Graham Willoughby <pgw99@doc.ic.ac.uk>
To: Dara Hazeghi <dhazeghi@yahoo.com>, gcc-gnats@gcc.gnu.org,
	pgw99@doc.ic.ac.uk, gcc-bugs@gcc.gnu.org, gcc-prs@gcc.gnu.org,
	nobody@gcc.gnu.org
Cc:  
Subject: Re: inline-asm/9637: arguments passed/return to asm block via mmx registers trigger failure
Date: Mon, 12 May 2003 13:26:07 +0100

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 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=3Dview%20audit-trail&database=3D=
 gcc&pr=3D9637
 
 On 2003-05-10 14:07:50 +0000, Dara Hazeghi wrote:
 >=20
 > It seems that the crash has been solved, and we now exit gracefully. =20
 > Can the submitter verify if the code in question is in fact faulty? =20
 
 I now have a somewhat better understanding of inline asm than when I submit=
 ted
 this bug.
 
 Here is an explanation of what the operative (well, problematic) lines of t=
 his
 code:
 
 : "=3Dy" (c), "=3Dy" (a), "=3Dy" (b)
 : "0" (c)
 
 are actually supposed to do.
 
 The first line means "Pick three MMX registers.  These will contain the val=
 ues
 of c, a and b at the end of the ASM instructions.  I've called them $y0, %y1
 and %y2 inside the asm block, and I need you to substitute those names with=
  the
 names of the registers you picked."
 
 The second line means "Remember the first register you picked?  Well, I want
 you to put the value of c into it before running the ASM block."
 
 So far this matches what the documentation says.
 
 Now, on to the reasons I think that this code might have failed:
 
 * Reason 1:
 
 The function tries to use a FP value and the MMX registes in the same funct=
 ion,
 and you can't mix FP calls and MMX calls indiscriminately.
 
 - Good Solution:
 Fix gcc to add the appropriate calls to save/restore the FP stack before/af=
 ter
 asm blocks which use MMX registers
 
 - Bad Solution:
 Change the documentation so it says "You cannot mix FP and MMX calls in the
 same function."
 
 * Reason 2:
 
 GCC doesn't actually understand how to pass things to/from MMX registers.
 
 - Good Solution:
 Fix gcc to bring it in line with what the docs say.
 
 - Bad Solution:
 Change the docs to remove references to MMX register constraints to asm.
 
 * Reason 3:
 
 GCC doesn't know how to move a "long long" value into or out of an MMX
 register.
 
 - Good Solution:
 Teach it how - they are the same length, so it ought not to be a problem.
 Alignment _may_ be an issue however.
 
 - Bad Solution:
 Change the docs to say it doesn't work.  That would raise the question of w=
 hat
 in fact gcc can move into an MMX register...
 
 Hope all this helps.  The bottom line is that the code as written should wo=
 rk
 if gcc behaves as documented.  I can't assure you that the asm actually wor=
 ks
 when compiled because I have never persuaded gcc to compile it ;-), plus of
 course there are better instructions to do the same from the SSE instruction
 set.  I'd still like to resolve this as a fallback path for k6 and pre-xp
 athlon processors.
 
 Regards,
 
 Philip Willoughby
 
 Systems Programmer, Department of Computing, Imperial College, London, UK
 --=20
 echo bzidd@nfo.ho.co.se | tr "bizndfohces" "pwgd9ociaku"
 
 Why reinvent the wheel?
 Because we can make it rounder...
 
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