I don't think I have the patience to disassemble both kernels and check differences; this would be the starting point to hunt a gcc bug.

Very good point ! How do I test that ? Did you find anything in the initfs that would allow RAM testing in factory tests ?

I'm thinking compiling a kernel with just enough features to boot the device, run the FB and modify the initfs to run as init a modified memtest to output to framebuffer should yield some results. If I can find some spare hours tonight I'll try it, but otherwise it will have to wait until next week.