Author: Robert Hyatt
Date: 19:39:47 02/19/04
Go up one level in this thread
On February 19, 2004 at 16:44:38, Dieter Buerssner wrote:
>On February 19, 2004 at 16:16:23, Robert Hyatt wrote:
>
>>I looked at my old inline asm docs, and I found the following statement,
>>paraphrased:
>>
>>"you don't need to put gcc-allocated registers on the clobber list, it knows you
>>are fooling with them."
>
>That's news to me. I cannot find it in my gcc manual (and I know, that it did
>not work with earlier gcc versions).
>
>Some snippets of my gcc manual:
>
>---
>The compiler cannot check whether the operands have data types that are
>reasonable for the instruction being executed. It does not parse the assembler
>instruction template and does not know what it means or even whether it is valid
>assembler input.
>
>[...]
> Some instructions clobber specific hard registers. To describe this,
^^^^^^^^^^^^^^^
I am keying on that in your quote. "hard register" as opposed to "dynamically
assigned register"... I won't say that is a correct interpretation, of course.
The syntax is not horrible for inline asm, but the documentation leaves a lot to
be desired, clarity-wise...
>write a third colon after the input operands, followed by the names of
>the clobbered hard registers (given as strings).
>[...]
> You may not write a clobber description in a way that overlaps with
>an input or output operand. For example, you may not have an operand
>describing a register class with one member if you mention that register
>in the clobber list. There is no way for you to specify that an input
>operand is modified without also specifying it as an output operand.
>Note that if all the output operands you specify are for this purpose
>(and hence unused), you will then also need to specify `volatile' for
>the `asm' construct, as described below, to prevent GCC from deleting
>the `asm' statement as unused.
>---
>
>To me it looks, as you really have to tell to gcc exactly, which operands to the
>asm are modified. This indeed makes it not really easy, to write correct inline
>assembly for gcc.
>
>I had one project for "big numbers", which I had coded with inline assembly in
>gcc (over a factor of 10 faster, than the C code - mainly because one cannot
>access the carry from C, and the 32*32->64 bit mul, similar for div). At that
>time, gcc allowed to put input operands into the clobber list. Then they changed
>that, and all the asm had to be modified. It was a lot of work. There was no
>option for backwards compability - the code just broke from one version to the
>other (it was correct according to the older documentation, the feature was not
>deprecated at all, there were even examples in the manual, that showed the later
> not allowed input regs in the clobber list).
>
>Regards,
>Dieter
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