Re: -ftrivial-auto-var-init in userland?

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

> Date: Mon, 20 Jul 2026 16:37:51 -0700
> From: Jason Thorpe <thorpej%me.com@localhost>
> 
> > On Jul 20, 2026, at 2:34 PM, Taylor R Campbell <riastradh%NetBSD.org@localhost> wrote:
> > 
> > I propose to enable -ftrivial-auto-var-init for all libraries and
> > utilities in userland by default (under gcc and clang), and
> > specifically to zero-initialize local variables in release builds and
> > to nonzero-initialize them in current builds.
> 
> I think I’d like to see an evaluation of potential code-size
> increase that could impact more constrained platforms.

Looks like overall 1-5% code expansion, in a GENERIC kernel and libc
on sparc and sun2.  Curiously, on sparc, -ftrivial-auto-var-init=zero
expanded the code _more_ than -ftrivial-auto-var-init=pattern!

We could obviously make this configurable in mk.conf and have the
default different by port too.


* Kernel

$ size sun2/compile/GENERIC.*/netbsd
   text	   data	    bss	    dec	    hex	filename
1382055	  77428	  78160	1537643	 17766b	sun2/compile/GENERIC.baseline/netbsd
1404951	  77428	  78160	1560539	 17cfdb	sun2/compile/GENERIC.zeroinit/netbsd
1413333	  77428	  78160	1568921	 17f099	sun2/compile/GENERIC.patterninit/netbsd

-> zeroinit: 1.67% code expansion over baseline
-> patterninit: 2.26% code expansion over baseline

$ size sparc/compile/GENERIC.*/netbsd
   text	   data	    bss	    dec	    hex	filename
5751146	 158640	 135720	6045506	 5c3f42	sparc/compile/GENERIC.baseline/netbsd
5832326	 158640	 135720	6126686	 5d7c5e	sparc/compile/GENERIC.zeroinit/netbsd
5830910	 158640	 135720	6125270	 5d76d6	sparc/compile/GENERIC.patterninit/netbsd

-> zeroinit: 1.41% code expansion over baseline
-> patterninit: 1.39% code expansion over baseline

* Userland

$ size obj.sun2/lib/libc.baseline/libc.a | awk '{t+=$1}END{print t}'
1123171
$ size obj.sun2/lib/libc.zeroinit/libc.a | awk '{t+=$1}END{print t}'
1155625
$ size obj.sun2/lib/libc.patterninit/libc.a | awk '{t+=$1}END{print t}'
1171435

-> zeroinit: 2.89% code expansion over baseline
-> patterninit: 4.28% code expansion over baseline

$ size obj.sparc/lib/libc.*/libc.so
   text	   data	    bss	    dec	    hex	filename
2227198	  59704	 158600	2445502	 2550be	obj.sparc/lib/libc.baseline/libc.so
2263742	  59704	 158600	2482046	 25df7e	obj.sparc/lib/libc.zeroinit/libc.so
2252938	  59704	 158600	2471242	 25b54a	obj.sparc/lib/libc.patterninit/libc.so

-> zeroinit: 1.64% code expansion over baseline
-> patterninit: 1.16% code expansion over baseline