Apple: Compile clang frame size with -O0 vs -O2 (kernel)

I have an existing project, which we compile DEBUG for developers (and -O0 so lldb makes sense). But I have one function in particular that balloons in size when -O0 is used:

-O2 -Wframe-larger-than=100warning: stack frame size of 168 bytes in function 'dsl_scan_visitbp'

-O0 -Wframe-larger-than=100warning: stack frame size of 1160 bytes in function 'dsl_scan_visitbp'

and with some recursion, the stack can be very trashed (16K stacks in kernel).

First thing to inspect are any local variables, but I believe there are only two:

        dsl_pool_t *dp = scn->scn_dp;        blkptr_t *bp_toread = NULL;

If you want to see the whole function: https://github.com/openzfs/zfs/blob/master/module/zfs/dsl_scan.c#L1908(Linux sources, but dealing with Apple clang port)

There are a bunch of alwaysinline in that sourcefile, which may also come to play here.

But I am curious why it grows so large with -O0?

Then what to do about it, I can't see any Apple-clang #pragmas to turn "on" optimize in a source file (only turning off optimize) for one function, or one file. If I knew what the cause was, perhaps I can control that specific issue with a different pragma.

Only solution I see right now, is to have dsl_scan.c processed differently in the Makefile, so that only that file always gets -O2. But that is a bit tedious.