PPC stack alignment

[rob windgassen]

Depending on the ABI, the stack pointer shall be 16-bytes (Sys V PPC ABI) or 
8-bytes (PPC EABI) aligned.
libgloss/rs6000/crt0.S however has the following code fragment

        /* set up initial stack frame */
        addi    sp,sp,-4       /* make sure we don't overwrite debug mem */

whenever a __stack symbol is defined, e.g. in a linker script, and it is 
aligned properly, the -4 in above code will break the stack pointer 
alignment.
Although nothing seems to break on the platforms I've used, accesses to 
64-bit doubles allocated on the stack may cross cache lines, page 
boundaries and cause unaligned bus accesses, i.e. this can be inefficient.
Is it the responsibility for the linker script to define a __stack symbol in 
a way that corrects for the above -4 or is this a shortcoming of crt0.S ?


-- 
Rob Windgassen
--
When you are looking for a tough bug, you're likely to find a bunch other 
ones.