PATCH: PR target/59672: Add -m16 support for x86

Uros Bizjak
Tue Jan 28 16:02:00 GMT 2014

On Mon, Jan 27, 2014 at 8:44 PM, H.J. Lu <> wrote:

> The .code16gcc directive was added to binutils back in 1999:
> ---
>    '.code16gcc' provides experimental support for generating 16-bit code
> from gcc, and differs from '.code16' in that 'call', 'ret', 'enter',
> 'leave', 'push', 'pop', 'pusha', 'popa', 'pushf', and 'popf'
> instructions default to 32-bit size.  This is so that the stack pointer
> is manipulated in the same way over function calls, allowing access to
> function parameters at the same stack offsets as in 32-bit mode.
> '.code16gcc' also automatically adds address size prefixes where
> necessary to use the 32-bit addressing modes that gcc generates.
> ---
> It encodes 32-bit assembly instructions generated by GCC in 16-bit format
>  so that GCC can be used to generate 16-bit instructions.  To do that, the
>  .code16gcc directive may be placed at the very beginning of the assembly
>  code.  This patch adds -m16 to x86 backend by:
> 1. Add -m16 and make it mutually exclusive with -m32, -m64 and -mx32.
> 2. Treat -m16 like -m32 so that --32 is passed to assembler.
> 3. Output .code16gcc at the very beginning of the assembly code.
> 4. Turn off 64-bit ISA when -m16 is used.
> Tested on Linux/x86 and Linux/x86-64.  OK for trunk?
> Thanks.
> H.J.
> ---
>         PR target/59672
>         * config/i386/gnu-user64.h (SPEC_32): Add "m16|" to "m32".
>         (SPEC_X32): Likewise.
>         (SPEC_64): Likewise.
>         * config/i386/i386.c (ix86_option_override_internal): Turn off
>         for TARGET_16BIT.
>         (x86_file_start): Output .code16gcc for TARGET_16BIT.
>         * config/i386/i386.h (TARGET_16BIT): New macro.
>         (TARGET_16BIT_P): Likewise.
>         * config/i386/i386.opt: Add m16.
>         * doc/invoke.texi: Document -m16.

OK for mainline, needs OK from RMs for a backport.

Please also add the entry to Changes.html, this is user-visible change.


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