arm cortex a9 cross compiling strange floating point behaviour

Question

I am trying to port a larger application from x86 to arm cortex a9, but I'm getting strange segmentation faults with floating point functions like modf when cross compiling the application, other libc++ functions just seem to handle floats wrong, but don't crash(see below).

So I tried this small test programm, which can trigger the error too. The output of the test programm(see below) should demonstrate my problem.

#include <iostream>
int main(int argc, char *argv[])
{
    double x = 80;
    double y = 0;
    std::cout << x << "\t" << y << std::endl;
    return 0;
}

compiled on arm cortex a9:

@tegra$ g++ -Wall test.cpp -o test_nativ
@tegra$ ./test_nativ 
80      0

cross compiled

@x86$ arm-cortex_a9-linux-gnueabi-g++ test.cpp  -o test_cc
@tegra$ ./test_cc
0       1.47895e-309

cross compiled with '-static' linker option.

@x86$ arm-cortex_a9-linux-gnueabi-g++ -static test.cpp  -o test_cc_static
@tegra$ ./test_cc_static 
80      0

.

@x86$ arm-cortex_a9-linux-gnueabi-objdump -S test_cc
see: http://pastebin.com/3kqHHLgQ

@tegra$ objdump -S test_nativ
see: http://pastebin.com/zK35KL4X

.

To answer some of the comments below:
- Cross compiler is setup for little endian, as is the native compiler on the tegra machine.
- I don't believe its a memory alignment issue, had my share of these while porting to arm and these should send SIGBUS to application or log to syslog, see documentation for /proc/cpu/alignment.

My current workaround is to copy over the crosscompiled toolchain and use it with LD_LIBRARY_PATH ... not nice, but good enough for the time being.




Edit:
Thank you for your Answers.
In the meantime I found out that the linux distribution on the tegra device was compiled with '-mfloat-abi=softfp' though the documentation stated, that a toolchain compiled with '-mfloat-abi=hard' is required.
Changing the toolchain brought the success.

It seems that the difference between hard and softfp can be seen using 'readelf -A' on any system binary:
If the Output contains the line: 'Tag_ABI_VFP_args: VFP registers' it is compiled with '-mfloat-abi=hard'. If this line is missing the binary is most likely compiled with '-mfloat-abi=softfp'.
The line 'Tag_ABI_HardFP_use: SP and DP' does not indicate the compilerflag '-mfloat-abi=hard'.

Answer 1

Looking at the assembly output, we can see a discrepancy in the two files.

In test_nativ:

86ec:       4602            mov     r2, r0
86ee:       460b            mov     r3, r1
86f0:       f241 0044       movw    r0, #4164       ; 0x1044
86f4:       f2c0 0001       movt    r0, #1
86f8:       f7ff ef5c       blx     85b4 <_init+0x20>

This is passing a double in r2:r3, and std::cout in r0.

In test_cc:

86d8:       e28f3068        add     r3, pc, #104    ; 0x68
86dc:       e1c320d0        ldrd    r2, [r3]
86e0:       e14b21f4        strd    r2, [fp, #-20]  ; 0xffffffec
86e4:       e3010040        movw    r0, #4160       ; 0x1040
86e8:       e3400001        movt    r0, #1
86ec:       ed1b0b03        vldr    d0, [fp, #-12]
86f0:       ebffffa5        bl      858c <_init+0x20>

This passes a double in d0 (a VFP register), and std::cout in r0. Observe here that r2:r3 is loaded (by ldrd) with the floating point value that is printed out second, i.e. 0.0. Because the dynamically-linked ostream::operator<<(double val) expects its argument in r2:r3, 0 is printed out first.

I can explain the second weird-looking float too. Here's where the second float is printed:

8708:       e1a03000        mov     r3, r0
870c:       e1a00003        mov     r0, r3
8710:       ed1b0b05        vldr    d0, [fp, #-20]  ; 0xffffffec
8714:       ebffff9c        bl      858c <_init+0x20>

See that r3 is set to r0, the address of cout. From above, r0 = 0x011040. Thus, the register pair r2:r3 becomes 0x0001104000000000, which decodes to 1.478946186471156e-309 as a double.

So the problem is that your desktop GCC's libraries uses VFP/NEON instructions, which are not used by the on-device dynamic libraries. If you use -static, you get the VFP/NEON libraries, and everything works again.

My suggestion would just be to figure out why the device and compiler libraries differ, and get that sorted out.