For arm64, D18619 introduced the ability to combine bumping the stack pointer upfront in case it needs to be bumped for both the callee-save area as well as the lo\
cal stack area.
That diff already remarks that "This change can cause an increase in instructions", but argues that even when that happens, it should be still be a performance benefit because the number o\
f micro-ops is reduced.
We have observed that this code-size increase can be significant in practice. This diff disables combining stack bumping for methods that are marked as optimize-for-size.
Example of a prologue with the behavior before this diff (combining stack bumping when possible):
sub sp, sp, #0x40 stp d9, d8, [sp, #0x10] stp x20, x19, [sp, #0x20] stp x29, x30, [sp, #0x30] add x29, sp, #0x30 [... compute x8 somehow ...] stp x0, x8, [sp]
And after this diff, if the method is marked as optimize-for-size:
stp d9, d8, [sp, #-0x30]! stp x20, x19, [sp, #0x10] stp x29, x30, [sp, #0x20] add x29, sp, #0x20 [... compute x8 somehow ...] stp x0, x8, [sp, #-0x10]!
Note that without combining the stack bump there are two auto-decrements, nicely folded into the stp instructions, whereas otherwise there is a single sub sp, ... instruction, but not \