arm-none-eabi-gcc 5.3 and LTO, mixing ARM and Thumb instructions

[Christian Bruel]

Hi Gabriel,

On 06/02/2016 01:28 AM, Gabriel Marcano wrote:
> My apologies, I forgot to attach the files.
>
>
>
> ----- Original Message -----
> From: Gabriel Marcano <gabemarcano@yahoo.com>
> To: "gcc-help@gcc.gnu.org" <gcc-help@gcc.gnu.org>
> Sent: Wednesday, June 1, 2016 7:23 PM
> Subject: arm-none-eabi-gcc 5.3 and LTO, mixing ARM and Thumb instructions
>
> Could someone help me understand this behavior with LTO? I am working with an
> ARM cross compiler, arm-none-eabi. I am trying to make the compiler compile two
> separate files in different ARM modes/targets, one of them in ARM, one of them
> in Thumb. When I compile the two source files without -flto, one with -mthumb
> -thumb-interwork, and the other with the same flags just -marm appended as to
> override -mthumb, the resulting binary, when dissasembled with objdump, has code
> in both modes, and it is rather aparent. When I enable -flto, the resulting
> program ends up in a single mode, all of it.
>
> I am attaching a couple of files to this email:
>    arm.c      : The file to be compiled as ARM
>    main.c     : The main function, compiled as Thumb
>    Makefile   : Add -flto to LDFLAGS and CFLAGS to enable lto, remvove it to
>                 disable it.
>    lto.txt    : The arm-none-eabi-objdump dissassembly of the resulting lto
>                 binary.
>    no-lto.txt : the arm-none-eabi-objdump dissassembly of the resulting binary
>                 without lto.
>
> Is there something obvious about the behavior of LTO that I'm missing? Does it
> just generate the final machine code at link time, scrapping the flags used to
> compile object files?
>
> Thanks,
>
> Gabriel E. Marcano

This is correct behavior, but unfortunately, it is broken in 5.3. LTO 
support with target modes intermixing was fixed for GCC 6.1.

Compiled with 6.1, your example correctly shows the jumps from thumb 
main to arm foo with the veneer

Note that you can mix functions compiled with either command line target 
options for attributes targets

Regards

Christian