[Bug target/59672] New: Add -m16 support for x86

[Bug target/59672] New: Add -m16 support for x86

[hpa at zytor dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59672

            Bug ID: 59672
           Summary: Add -m16 support for x86
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hpa at zytor dot com

It would be a very good thing to have an official way to generate 16-bit
binaries on x86 (i386), even if all that is provided is a command-line flag
that issues a .code16gcc assembly directive first in the generated assembly
file.

In the Linux kernel, we currently have to do a bunch of hacks to make sure that
gcc doesn't issue any instructions *before* the .code16gcc directive:

                   $(call cc-option, -fno-toplevel-reorder,\
                   $(call cc-option, -fno-unit-at-a-time)) \
                   $(call cc-option, -fno-stack-protector) \

... and the list is likely to grow, which is why providing an actually
supported compiler flag for this would be desirable.

[Bug target/59672] Add -m16 support for x86

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59672

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note GCC does not even support real 16bit code for x86.  So pretending GCC's
output is 16bit code is a joke.

Why can't you just write the 16bit binary support in assembly for the kernel?

[Bug target/59672] Add -m16 support for x86

[hpa at zytor dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59672

--- Comment #2 from H. Peter Anvin <hpa at zytor dot com> ---
It is much cleaner to have it in C.  We converted the assembly code to C back
in 2007 and it has been much easier to maintain ever since.  It works fine,
thankyouverymuch; it isn't *optimal* 16-bit code, but it is real and valid
16-bit code and we use it as such.

Sure, optimization would be nice.  Do we care?  Not a lot.

[Bug target/59672] Add -m16 support for x86

[steven at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59672

Steven Bosscher <steven at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |i386-*-*
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-01-04
     Ever confirmed|0                           |1

--- Comment #3 from Steven Bosscher <steven at gcc dot gnu.org> ---
Nice idea. Not likely to be implemented any time soon, because it's quite
complicated to get right.

Takes more than just placing a directive right. With the right set of flags the
compiler would still emit 32bit instructions. Actually disabling 32bits
instructions is a lot more work: add a TARGET_16BITS or an attribute_enabled to
almost all patterns in i386.md and friends. Not helpful for the vast majority
of users, and kernel people will complain more about the inevitable slowdown
(and, as usual, point out the general incompetence of gcc hackers :-)

(Why/when is 16bit code still necessary anyway? Before entering protected
mode?)

[Bug target/59672] Add -m16 support for x86

[hpa at zytor dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59672

--- Comment #4 from H. Peter Anvin <hpa at zytor dot com> ---
You are missing the plain fact that *it is already working*.

.code16gcc is a binutils directive which takes 32-bit code emitted by gcc and
assembles it to produce valid (although suboptimal) 16-bit code.  So it really
*is* just a matter of putting the directive in the right place.

And yes, it is used before entering protected mode.

[Bug target/59672] Add -m16 support for x86

[hpa at zytor dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59672

--- Comment #6 from H. Peter Anvin <hpa at zytor dot com> ---
This could also be implemented in binutils as a --code16gcc option, in which
case gcc users would have to use "-m32 -Wa,--code16gcc".  Ugly but would work.

[Bug target/59672] Add -m16 support for x86

[hjl.tools at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59672

--- Comment #7 from H.J. Lu <hjl.tools at gmail dot com> ---
I put the initial -m16 support on hjl/x86/m16 branch at:

http://gcc.gnu.org/git/?p=gcc.git;a=summary

We need to add some run-time testcases and fix any bugs.

[Bug target/59672] Add -m16 support for x86

[dwmw2 at infradead dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59672

--- Comment #9 from David Woodhouse <dwmw2 at infradead dot org> ---
Thanks. This appears to work for me to build the Linux kernel's 16-bit boot
code with the patch at
http://lkml.kernel.org/r/1389180083-23249-3-git-send-email-David.Woodhouse@intel.com

[Bug target/59672] Add -m16 support for x86

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59672

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |4.9.0

--- Comment #11 from H.J. Lu <hjl.tools at gmail dot com> ---
Fixed for 4.9.0.