atmel-samd: Align stack on 8-byte boundary

[dhalbert]

The stack should be aligned on an 8-byte boundary, according to the ARM procedure calling standard (AAPCS). A 4-byte boundary might work, but the interrupt controller expects an 8-byte-boundary, and compilers will also expect this.

Good discussion of several related points: https://community.arm.com/processors/f/discussions/6344/what-is-the-meaning-of-a-64-bit-aligned-stack-pointer-address

Right now _estack is on a 4-byte, but not an 8-byte boundary, due to the introduction of _bootloader_dbl_tap. This appears to have caused issue #521, which was fixed by PR #548, which avoids using alloca() in mp_import_from(). My guess is that gcc assumes an 8-byte boundary for alloca().

The fix is to adjust all the linker maps in ports/atmel-samd/boards/*.ld.Change:

_estack = ORIGIN(RAM) + LENGTH(RAM) - 4;

to

_estack = ORIGIN(RAM) + LENGTH(RAM) - 8;

The #548 changes should also be undone.

This appears to have caused a problem only on M4, not M0. But the NVIC supposedly expects an 8-byte boundary, so perhaps this should be backported to 2.x as well.

[jerryneedell]

Nice! This makes a lot of sense. I suppose this is better than finding a compiler bug!! At least you can fix this.