linker emits blx instruction for arm7tdmi mcu in some condition

[Sergey A. Borshch]

if call to sscanf() exist somewhere in source code, even if function with this call removed by linker --gc-sections, linker emits unsupported by arm7tdmi core blx instruction to call arm mode functions from thumb code.
Here is simple test code to reproduce bug (file attached):

#include <string.h>
int _start() // _start() instead of main() just to remove all startup code from disassembly
{
    return memcmp((void *)0, (void *) 100, 15); // call to any arm-mode function
}

#include <stdio.h>
void dummy ()
{
    char Buffer[2];
// sscanf(Buffer, " "); <- uncomment this line to trigger bug
}

commands to compile:
arm-none-eabi-gcc -mcpu=arm7tdmi -mthumb-interwork -mthumb -Os -ffunction-sections -c test.c -o test.o
arm-none-eabi-gcc -mcpu=arm7tdmi -mthumb-interwork --specs=nano.specs -nostartfiles -Wl,--gc-sections test.o -o test.elf
arm-none-eabi-objdump -h -S test.elf > test.lss

if line with sscanf() call commented out, linker emits correct call to __memcmp_from_thumb:
00008000 <main>:
    8000: b510 push {r4, lr}
    8002: 220f movs r2, #15
    8004: 2164 movs r1, #100 ; 0x64
    8006: 2000 movs r0, #0
    8008: f000 f822 bl 8050 <__memcmp_from_thumb>

if line with sscanf() call uncommented, linker emits blx instruction, which is not supported by arm7tdmi core:
00008000 <main>:
    8000: b510 push {r4, lr}
    8002: 220f movs r2, #15
    8004: 2164 movs r1, #100 ; 0x64
    8006: 2000 movs r0, #0
    8008: f000 e80a blx 8020 <memcmp>

bug exist since gcc-arm-none-eabi-5_2-2015q4-20151219
Host OS - Linux Mint 17.3, tested on gcc-arm-none-eabi-5_2-2015q4-20151219-linux.tar.bz2, gcc-arm-none-eabi-5_3-2016q1-20160330-linux.tar.bz2.

[Thomas Preud'homme (thomas-preudhomme)]

Hi Sergey,

Thanks for your bug report.

This seems to be related to newlib-nano in some way. When linking with straight newlib I get the expected bl. Only when linking with newlib-nano did I get the bug. We'll keep you updated of the progress on this bug.

By the way, I noticed you use --specs but the documented syntax is -specs. It seems it works as well for now but you might want to use the documented symtax.

Best regards.

[Thomas Preud'homme (thomas-preudhomme)]

Hi Sergey,

it turns out it's a consequence of the bug reported at [1]. It's been fixed in newlib already and the fix will be available in our next release.

[1] https://answers.launchpad.net/gcc-arm-embedded/+question/282991

Best regards.

[Sergey A. Borshch (sb-sf)]

Hi Thomas.

Thanks for a response and pointing to --specs mistype. I'll fix it in my makefiles.