i386: gcc & asm(): wrong constraint for "mull"

i386: gcc & asm(): wrong constraint for "mull"

[Ulrich Windl]

Hello,

I noticed (with some inspiration from Andy Kleen) that some asm() 
instructions for the ia32 use the "g" constraint for "mull", where my 
Intel 386 Assembly Language Manual suggests the "MUL" instruction needs 
an r/m operand. So I guess the correct constraint is "rm" in gcc, and 
not "g". That change identical assembly output for gcc-2.95.2, but some 
gcc-2.96.x will try a multiplication with an immediate (constant) 
operand for the "g" constarint, and the as will choke on that.
(Redhat 7.0 ships such a version of gcc).

As I won't be online next week, let me say
regards and a good new year to all!

Ulrich

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Re: i386: gcc & asm(): wrong constraint for "mull"

[Jakub Jelinek]

On Fri, Dec 29, 2000 at 10:54:38AM +0100, Ulrich Windl wrote:
> Hello,
> 
> I noticed (with some inspiration from Andy Kleen) that some asm() 
> instructions for the ia32 use the "g" constraint for "mull", where my 
> Intel 386 Assembly Language Manual suggests the "MUL" instruction needs 
> an r/m operand. So I guess the correct constraint is "rm" in gcc, and 
> not "g". That change identical assembly output for gcc-2.95.2, but some 
> gcc-2.96.x will try a multiplication with an immediate (constant) 
> operand for the "g" constarint, and the as will choke on that.
> (Redhat 7.0 ships such a version of gcc).

gcc 2.95.2 md.texi sais:
@cindex @samp{g} in constraint
@item @samp{g}
Any register, memory or immediate integer operand is allowed, except for
registers that are not general registers.

(2.95.2 was chosen to make it clear it is not something new in gcc).
That means gcc is really free to choose which of register, memory or
immediate it puts in and the fact that some gcc version choose one and
others choose other is perfectly correct.
Fix the constraints and be happy (at least during the upcoming millenium) :)

	Jakub
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Re: i386: gcc & asm(): wrong constraint for "mull"

[Ulrich Windl]

On 29 Dec 2000, at 5:17, Jakub Jelinek wrote:

> On Fri, Dec 29, 2000 at 10:54:38AM +0100, Ulrich Windl wrote:
> > Hello,
> > 
> > I noticed (with some inspiration from Andy Kleen) that some asm() 
> > instructions for the ia32 use the "g" constraint for "mull", where my 
> > Intel 386 Assembly Language Manual suggests the "MUL" instruction needs 
> > an r/m operand. So I guess the correct constraint is "rm" in gcc, and 
> > not "g". That change identical assembly output for gcc-2.95.2, but some 
> > gcc-2.96.x will try a multiplication with an immediate (constant) 
> > operand for the "g" constarint, and the as will choke on that.
> > (Redhat 7.0 ships such a version of gcc).
> 
> gcc 2.95.2 md.texi sais:
> @cindex @samp{g} in constraint
> @item @samp{g}
> Any register, memory or immediate integer operand is allowed, except for
> registers that are not general registers.
> 
> (2.95.2 was chosen to make it clear it is not something new in gcc).
> That means gcc is really free to choose which of register, memory or
> immediate it puts in and the fact that some gcc version choose one and
> others choose other is perfectly correct.
> Fix the constraints and be happy (at least during the upcoming millenium) :)

Oh, if it wasn't clear: It's what I wanted to say. As I don't have a 
patch ready for that, maybe start at arch/i386/kernel/time.c; there are 
at least two of these "mull" instructions.

Ulrich

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