The regression test gcc-torture-execute-20050613-1.c fails for mcs51 when using --stack-auto. To reproduce, remove the #ifndef __SDCC_mcs51. The code seems rather standard, so I give this priority 7 for bad code being generated silently.
P.S.: The same test file contains a struct with a flexible array, but it is unrelated to the mcs51 failure (see bug #3579067 for a discussion of the flexible array issue).