Issue with small struct return values

[Kevin B. Hendricks]

Hi Andy,

Did anyone actually get back to you on this issue?

I think people knew about this inconsistency but did not push matters.
AFAIK, the gcc toolchain for ppc has always ignored that part of the abi.

As you say this would impact everything and require a complete re-compile of
all apps.  It would also be interesting to see whether checking for the size
of returned structures in order to know how to properly return the strucutre
actually impacts performance.  Also code that invokes methods/functions
dynamically used to implement native interfaces and  external object
interfaces would have to be rewritten.  This includes Mozilla, Java JDK,
OpenOffice, etc...

For what it is worth, I have found out that the abis implemented by PPC BSD,
MacOSX, IBM, and PPC Linux all differ in strange ways (even Mac OSX ABI is
not identical to IBM's EABI on which it was based).  So in the long run, I am
not sure that this issue matters.

Please let the linuxppc-dev list know what you find out officially from the
GNU folks (Geoff Keating, Franz Sirl, etc) about this issue.  It would be
interesting to know if we are going through yet again another complete binary
incompatible change!

Kevin


On Friday 19 January 2001 17:14, Andy Johnson wrote:
> Excerpt from the PPC SVR4 ABI:
> ---
> Functions shall return float or double values in f1, with float values
rounded to single
> precision. Functions shall return values of type int, long, enum, short,
and char, or a
> pointer to any type as unsigned or signed integers as appropriate, zero- or
sign-extended
> to 32 bits if necessary, in r3. A structure or union whose size is less
than or equal to 8
> bytes shall be returned in r3 and r4, as if it were first stored in an
8-byte aligned memory
> area and then the low-addressed word were loaded into r3 and the
high-addressed word
> into r4. Bits beyond the last member of the structure or union are not
defined.
> ---
>
> The GCC compiler for powerpc-sysv (or any variant except powerpc-solaris)
is not
> doing this.  Instead, it always passes a pointer to a receiving struct area
in r3, regardless
> of its size.  We are trying to support two different compilers on our
system, one being
> GCC and the other being a commercial compiler (which comes as part of a
larger
> IDE which we would like to offer to our users), which claims to support the
PPC
> EABI.  The commercial compiler is using the r3,r4 convention, which means
that it
> cannot use the div or ldiv library routines which have been built using the
GCC compiler
> (or any other library entry points which return small structs).
>
> I would claim that the GCC compiler is not in conformance with the SVR4
ABI, even
> though it claims to support it.  Have other users encountered this
situation?  Is there
> any thought about bringing GCC in line with the SVR4 ABI requirement?  Among
> other things, it would make the code run faster, especially with
optimization enabled.
>
> Obviously, such a change would require a complete re-build of all binary
files built
> with GCC, regardless of the language being translated, since both caller
sites and
> callee sites involving small structs (<= 8 bytes) would be affected.
>
> -AndyJ
> --
> E. Andrew Johnson               Tel: 978-256-0052 x1482
> Mercury Computer Systems        Fax: 978-244-0520
> 199 Riverneck Road              andyj at mc.com
> Chelmsford, MA 01824-2820       http://www.ma.ultranet.com/~andyjnsn
>
>

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