Should we compile Guile with -fno-strict-aliasing?

Should we compile Guile with -fno-strict-aliasing?

[Mark H Weaver]

Hello all,

For many years, Linux (the kernel) has used the -fno-strict-aliasing
compiler option to disable certain tricky optimizations that depend upon
a very strict reading of the aliasing rules of modern C standards.  It
turns out that it's quite difficult to write robust code in the presence
of those optimizations.  I have not researched this issue carefully, but
it seems that several Guile bugs may be related to this problem.

Perhaps we should simply add this compiler flag where its available, at
least in the short term.  What do you think?

Here are some links for more info:

http://lkml.org/lkml/2003/2/26/158
http://mail.python.org/pipermail/python-dev/2003-July/036898.html
http://en.wikipedia.org/wiki/Aliasing_(computing)#Conflicts_with_optimization

     Mark

Re: Should we compile Guile with -fno-strict-aliasing?

[Ludovic Courtès]

Hi Mark,

Mark H Weaver <mhw@netris.org> skribis:

> Perhaps we should simply add this compiler flag where its available, at
> least in the short term.  What do you think?

I’ve been thinking about that too, because on some platforms GCC emits
may-break-strict-aliasing-rules warnings, notably on x86_64-freebsd8.2
with GCC 4.2.1:

--8<---------------cut here---------------start------------->8---
vm.c: In function 'vm_reinstate_partial_continuation':
vm.c:281: warning: dereferencing type-punned pointer will break strict-aliasing rules
In file included from vm-engine.c:127,
                 from vm.c:467:
vm-i-system.c: In function 'vm_regular_engine':
vm-i-system.c:49: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:52: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:53: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:79: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:212: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:225: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:418: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:430: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:445: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:465: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:487: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:528: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:537: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:546: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:556: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:566: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:575: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:923: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:994: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1043: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1073: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1089: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1121: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1131: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1195: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1218: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1241: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1281: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1332: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1427: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1446: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1457: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1470: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1472: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1480: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1513: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1548: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1574: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1600: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1607: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1619: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1627: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1641: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1679: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1704: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1726: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1751: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1783: warning: dereferencing type-punned pointer will break strict-aliasing rules
In file included from vm-engine.c:128,
                 from vm.c:467:
vm-i-scheme.c:192: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:203: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:599: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:624: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:626: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:629: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:692: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:809: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:859: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:861: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:863: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:865: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:867: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:869: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:871: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:873: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1016: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1016: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1018: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1018: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1020: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1020: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1022: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1022: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1024: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1024: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1026: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1026: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1028: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1028: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1030: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1030: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1119: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1121: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1123: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1125: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1128: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1134: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1139: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1141: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1143: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1145: warning: dereferencing type-punned pointer will break strict-aliasing rules
In file included from vm-engine.c:129,
                 from vm.c:467:
vm-i-loader.c:65: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-loader.c:87: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-loader.c:99: warning: dereferencing type-punned pointer will break strict-aliasing rules
In file included from vm-engine.c:127,
                 from vm.c:475:
vm-i-system.c: In function 'vm_debug_engine':
vm-i-system.c:49: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:52: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:53: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:79: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:212: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:225: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:418: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:430: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:445: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:465: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:487: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:528: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:537: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:546: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:556: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:566: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:575: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:923: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:994: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1043: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1073: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1089: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1121: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1131: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1195: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1218: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1241: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1281: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1325: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1332: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1367: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1427: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1446: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1457: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1470: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1472: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1480: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1513: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1548: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1574: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1600: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1607: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1619: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1627: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1641: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1679: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1704: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1726: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1751: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-system.c:1783: warning: dereferencing type-punned pointer will break strict-aliasing rules
In file included from vm-engine.c:128,
                 from vm.c:475:
vm-i-scheme.c:192: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:203: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:599: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:624: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:626: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:629: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:692: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:809: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:859: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:861: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:863: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:865: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:867: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:869: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:871: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:873: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1016: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1016: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1018: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1018: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1020: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1020: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1022: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1022: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1024: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1024: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1026: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1026: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1028: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1028: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1030: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1030: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1119: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1121: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1123: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1125: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1128: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1134: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1139: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1141: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1143: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-scheme.c:1145: warning: dereferencing type-punned pointer will break strict-aliasing rules
In file included from vm-engine.c:129,
                 from vm.c:475:
vm-i-loader.c:65: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-loader.c:87: warning: dereferencing type-punned pointer will break strict-aliasing rules
vm-i-loader.c:99: warning: dereferencing type-punned pointer will break strict-aliasing rules
--8<---------------cut here---------------end--------------->8---

I’ve just tried on this platform and it seems to fix the threading
issues we were observing (though I won’t claim victory until Hydra
reproduces the success several times in a row ;-)).

It may well be that my recent COMPILER_BARRIER hack can just go away
with this.

I just committed the change.

Thank you!

Ludo’.

Re: Should we compile Guile with -fno-strict-aliasing?

[Andy Wingo]

Hi!

On Fri 27 Jan 2012 20:02, Mark H Weaver <mhw@netris.org> writes:

> For many years, Linux (the kernel) has used the -fno-strict-aliasing
> compiler option to disable certain tricky optimizations that depend upon
> a very strict reading of the aliasing rules of modern C standards.  It
> turns out that it's quite difficult to write robust code in the presence
> of those optimizations.  I have not researched this issue carefully, but
> it seems that several Guile bugs may be related to this problem.
>
> Perhaps we should simply add this compiler flag where its available, at
> least in the short term.  What do you think?

So, we added it, for GCC; cool.  I was wondering though whether we might
be able to get by with something more limited, at least on GCC.  Have
you looked at __attribute__((__may_alias__))?  It does seem like a good
idea to add it to SCM, as we frequently alias SCM and scm_t_bits values
at the very least.  Also adding it to struct scm_vm_frame would also fix
the vm frame issue.

Dunno.  WDYT?

Regards,

Andy
-- 
http://wingolog.org/

Re: Should we compile Guile with -fno-strict-aliasing?

[Ludovic Courtès]

Hi!

Andy Wingo <wingo@pobox.com> skribis:

> On Fri 27 Jan 2012 20:02, Mark H Weaver <mhw@netris.org> writes:
>
>> For many years, Linux (the kernel) has used the -fno-strict-aliasing
>> compiler option to disable certain tricky optimizations that depend upon
>> a very strict reading of the aliasing rules of modern C standards.  It
>> turns out that it's quite difficult to write robust code in the presence
>> of those optimizations.  I have not researched this issue carefully, but
>> it seems that several Guile bugs may be related to this problem.
>>
>> Perhaps we should simply add this compiler flag where its available, at
>> least in the short term.  What do you think?
>
> So, we added it, for GCC; cool.  I was wondering though whether we might
> be able to get by with something more limited, at least on GCC.  Have
> you looked at __attribute__((__may_alias__))?  It does seem like a good
> idea to add it to SCM, as we frequently alias SCM and scm_t_bits values
> at the very least.

Yes, why not, but post-2.0.4 maybe?

> Also adding it to struct scm_vm_frame would also fix the vm frame
> issue.

I’d say “may”, or “might”, rather than “would”.  ;-)

Thanks,
Ludo’.

Re: Should we compile Guile with -fno-strict-aliasing?

[Ludovic Courtès]

Hi!

ludo@gnu.org (Ludovic Courtès) skribis:

[...]

> I’ve just tried on this platform and it seems to fix the threading
> issues we were observing (though I won’t claim victory until Hydra
> reproduces the success several times in a row ;-)).

Well no, this one had nothing to do (see <http://bugs.gnu.org/10641>.)

> It may well be that my recent COMPILER_BARRIER hack can just go away
> with this.

I’ve verified and that asm volatile is not needed on sparc-linux-gnu and
hppa2.0-linux-gnu when compiling with -fno-strict-aliasing, so I’ve
reverted commit 2b264d7e4fb649955ade1814527b6eb6f34f4e18.

Thanks,
Ludo’.