On Thu, Jun 20, 2013 at 2:41 PM, Thomas Petazzoni thomas.petazzoni@free-electrons.com wrote:

Hello,

I'm facing a bizarre problem with an armeb toolchain built by Buildroot. I'm also posting this to the crossgcc@ list since there are some gcc/binutils experts out there.

First, a little bit of background. ARM Big Endian comes into two variants:

• BE32, which was used up to ARMv5, where both the instructions and the data are Big Endian.

• BE8, which is used since ARMv6, where the instructions remain little-endian and only the data are big-endian.

See http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0338g/ch06s05... for some details about this.

So, I've built an ARMv7 Cortex-A8 toolchain, with the armeb architecture selected. The CROSS-gcc -v shows that it was configured as follows:

    --target=armeb-buildroot-linux-uclibcgnueabi
    --with-abi=aapcs-linux
    --with-arch=armv7-a
    --with-tune=cortex-a8

Then, I wrote a simple program that contains some data and instructions, built it under several conditions, and observed with hexdump whether the data and code was little-endian or big-endian.

And the results are somewhat surprising: when I explicitly pass -mbig-endian, I get the proper behavior (BE8 code with code in little endian and data in big endian), but when I don't pass any flags to the compiler, I get an incorrect behavior: both the code and data are big endian, as if the BE8 wasn't used (and readelf confirms that it wasn't used). See below the detailed results.

Note that the compiler is supposed to automatically use BE8 on ARMv6/ARMv7 and BE32 on ARMv5 and earlier cores.

The data is DEADBEEF, and the instruction is E52DB004.

Flags used Observed data Observed code Comment ======================= =============== =============== =========================================

-mlittle-endian EFBEADDE 04B02DE5 Code and data in LE -> OK -mbig-endian DEADBEEF 04B02DE5 Code LE, data BE, binary marked BE8 -> OK no flags DEADBEEF E52DB004 Data BE (ok!), code BE (*NOT* ok) -> NOK -march=armv5t -mbig-endian DEADBEEF E52DB004 Code and data in BE, on ARMv5 -> OK -march=armv5t DEADBEEF E52DB004 Code and data in BE, on ARMv5 -> OK

As can be seen in this table:

(*) On ARMv5, regardless of whether -mbig-endian is passed or not, the code produced is correct (both code and data are big endian, which is correct for ARMv5 where the big endian mode is BE32)

(*) On ARMv7 however, the code is different whether -mbig-endian is passed or not, even though an "armeb-linux" compiler is supposed to generate big endian code by default. When no flags is passed, both the data *and* code are big-endian (so it's BE32 like on ARMv5), but passing -mbig-endian makes the thing behave properly (code is little-endian, data is big-endian).

I'm using binutils 2.23.2 and gcc 4.7.3.

Any ideas?

Hi Thomas,

I added linaro-toolchain to CC as there may be someone there who knows the answer.