Re: toolchain/60289 (NetBSD 11 gcc mis-optimises some NULL checks)

["Manuel Bouyer via gnats" <gnats-admin%NetBSD.org@localhost>]

The following reply was made to PR toolchain/60289; it has been noted by GNATS.

From: Manuel Bouyer <manuel.bouyer%lip6.fr@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: toolchain-manager%netbsd.org@localhost, netbsd-bugs%netbsd.org@localhost,
        gnats-admin%netbsd.org@localhost, riastradh%NetBSD.org@localhost
Subject: Re: toolchain/60289 (NetBSD 11 gcc mis-optimises some NULL checks)
Date: Sun, 24 May 2026 10:49:30 +0200

 On Sat, May 23, 2026 at 09:59:12PM +0000, riastradh%NetBSD.org@localhost wrote:
 > Synopsis: NetBSD 11 gcc mis-optimises some NULL checks
 > 
 > State-Changed-From-To: open->analyzed
 > State-Changed-By: riastradh%NetBSD.org@localhost
 > State-Changed-When: Sat, 23 May 2026 21:59:11 +0000
 > State-Changed-Why:
 > This actually may be a gcc bug, because we compile the kernel with
 > -fno-delete-null-pointer-checks, which is supposed to prevent gcc from
 > eliminating these checks:
 > 
 > https://nxr.netbsd.org/xref/src/sys/conf/Makefile.kern.inc?r=1.306#108
 > https://gcc.gnu.org/onlinedocs/gcc-14.3.0/gcc/Optimize-Options.html#index-fdelete-null-pointer-checks
 > 
 > It's not NetBSD-specific -- here's a godbolt.org example with 14.3.0,
 > presumably not on NetBSD:
 > 
 > https://godbolt.org/z/aT6xqdMer
 > 
 > .LC0:
 >         .string "A "
 > ...
 >         mov     edi, OFFSET FLAT:.LC0
 >         call    printf
 >         test    BYTE PTR [rbx+16], 1
 >         je      .L9
 > 
 > So I think someone should file a bug with gcc upstream.
 
 I found at last one gcc bug report on this topic:
 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100404
 
 Adding
 	asm("" : "+r"(barp)); 
 just before the !barp test works around the isssue.
 
 but explicitely checking for NULL before &barp->foo is a more correct
 fix, even if it leads to an extra test which is not needed (could an
 overly agressive optimisation leading to slower code be considered as a bug ?)
 
 But,
  -fno-delete-null-pointer-checks doesn't work in this case; this is a real
    problem
  It seems that there's no way to get a warning when this optimisation is
    triggered, which I think is also a problem for developers.
 
 A quick search showed other projects (bind, openjdk) running into this
 
 -- 
 Manuel Bouyer, LIP6, Sorbonne Université.           Manuel.Bouyer%lip6.fr@localhost
      NetBSD: 26 ans d'experience feront toujours la difference
 --