[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #63 from rguenther at suse dot de --- On Wed, 2 May 2018, aph at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 > > --- Comment #62 from Andrew Haley --- > Just a bit of clarification: > > (In reply

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #62 from Andrew Haley --- Just a bit of clarification: (In reply to James Kuyper Jr. from comment #59) > > > 1) all type-based alias analysis is effectively impossible > > Alias analysis is only affected by the special guarantee

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #61 from Davin McCall --- (In reply to James Kuyper Jr. from comment #59) > (In reply to Davin McCall from comment #56) > > (In reply to James Kuyper Jr. from comment #55) > > > The problem is, you're using a statement that the

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #60 from Andrew Haley --- (In reply to James Kuyper Jr. from comment #51) > (In reply to Andrew Haley from comment #49) > > (In reply to James Kuyper Jr. from comment #46) > > > > The principle of type-based alias analysis is that

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #59 from James Kuyper Jr. --- (In reply to Davin McCall from comment #56) > (In reply to James Kuyper Jr. from comment #55) > > The problem is, you're using a statement that the access must occur via a > > union, with the

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #58 from Davin McCall --- (In reply to Andrew Haley from comment #57) > (In reply to Davin McCall from comment #52) > > (In reply to Andrew Haley from comment #45) > > > (In reply to Davin McCall from comment #44) > > > > The "one

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #57 from Andrew Haley --- (In reply to Davin McCall from comment #52) > (In reply to Andrew Haley from comment #45) > > (In reply to Davin McCall from comment #44) > > > The "one special guarantee" clause appears in the section

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #56 from Davin McCall --- (In reply to James Kuyper Jr. from comment #55) > The problem is, you're using a statement that the access must occur via a > union, with the implication that the code in question does not access the >

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #55 from James Kuyper Jr. --- > ou need, at a minimum, to modify "accesses via" to > "accesses directly via", in order to convey your intended meaning. (In reply to Davin McCall from comment #54) > (In reply to James Kuyper Jr. from

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #54 from Davin McCall --- (In reply to James Kuyper Jr. from comment #53) > [...] However, because those > pointers are passed to f(), which does dereference them, f() does accesses > those members, and it does so via the use of the

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #53 from James Kuyper Jr. --- (In reply to Davin McCall from comment #52) > (In reply to James Kuyper Jr. from comment #48) > > > The "one special guarantee" clause appears in the section describing union > > > member access via the

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #52 from Davin McCall --- (In reply to James Kuyper Jr. from comment #48) > > The "one special guarantee" clause appears in the section describing union > > member access via the "." or "->" operators, implying that it only applies >

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #51 from James Kuyper Jr. --- (In reply to Andrew Haley from comment #49) > (In reply to James Kuyper Jr. from comment #46) > > (In reply to Andrew Haley from comment #42) > > ... > > > In order to use type-based alias analysis in

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #50 from Andrew Haley --- (In reply to Andrew Haley from comment #49) > > Perhaps so, yes, but in practice it'd be pretty hard to do that. > Functions can only be defined in the other scope, Should be "the outer scope" > and

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #49 from Andrew Haley --- (In reply to James Kuyper Jr. from comment #46) > (In reply to Andrew Haley from comment #42) > ... > > In order to use type-based alias analysis in any LTO framework it's > > necessary to save type

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #48 from James Kuyper Jr. --- (In reply to Davin McCall from comment #44) > > Well, perhaps not, but this is the language specification. > > The "one special guarantee" clause appears in the section describing union > member access

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #47 from Andrew Haley --- (In reply to Richard Biener from comment #43) > (In reply to Andrew Haley from comment #42) > > > > So, if any union types with a common initial sequence are declared > > anywhere in a program, then their

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #46 from James Kuyper Jr. --- (In reply to Andrew Haley from comment #42) ... > In order to use type-based alias analysis in any LTO framework it's > necessary to save type information, and this is just more type > information. ...

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #45 from Andrew Haley --- (In reply to Davin McCall from comment #44) > > Well, perhaps not, but this is the language specification. > > The "one special guarantee" clause appears in the section describing union > member access via

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #44 from Davin McCall --- > Well, perhaps not, but this is the language specification. The "one special guarantee" clause appears in the section describing union member access via the "." or "->" operators, implying that it only

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #43 from Richard Biener --- (In reply to Andrew Haley from comment #42) > On 04/29/2018 05:42 PM, rguenther at suse dot de wrote:> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 > > > > --- Comment #41 from rguenther at suse

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #42 from Andrew Haley --- On 04/29/2018 05:42 PM, rguenther at suse dot de wrote:> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 > > --- Comment #41 from rguenther at suse dot de --- > On April 29, 2018 1:51:58 PM GMT+02:00,

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #41 from rguenther at suse dot de --- On April 29, 2018 1:51:58 PM GMT+02:00, "aph at gcc dot gnu.org" wrote: >https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 > >--- Comment #40 from Andrew Haley ---

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #40 from Andrew Haley --- (In reply to rguent...@suse.de from comment #29) > Note I repeatedly said this part of the standard is just stupid. It makes > most if not all type-based alias analysis useless. I don't think so. It does

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #39 from joseph at codesourcery dot com --- On Thu, 19 Apr 2018, jameskuyper at verizon dot net wrote: > Code which relies upon this feature to implement a C-style approximation to > inheritance has been fairly common, which is

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #38 from rguenther at suse dot de --- On Fri, 20 Apr 2018, msebor at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 > > --- Comment #35 from Martin Sebor --- > Here are the proposed changes: > >

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #37 from David Brown --- (In reply to Martin Sebor from comment #35) > Here are the proposed changes: > > Pointer Provenance: > http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2219.htm#proposed-technical- > corrigendum > > Trap

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #36 from David Brown --- (In reply to Martin Sebor from comment #34) > I think in the use case below: > >struct { int i; char buf[4]; } s, r; >*(float *)s.buf = 1.; >r = s; > > the aggregate copy has to be viewed as a

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #35 from Martin Sebor --- Here are the proposed changes: Pointer Provenance: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2219.htm#proposed-technical-corrigendum Trap Representations:

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #34 from Martin Sebor --- The questions in N2223 and the other documents are there to provide background and justification for the proposed changes (the questions come surveys they sent to various forums). The proposed words are at

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #33 from Richard Biener --- (In reply to Martin Sebor from comment #30) > Richard, I offered to write a proposal (with Clark) to improve the rules. > With the object model proposals already in the pipeline (N2223) this is a > good

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #32 from Richard Biener --- (In reply to James Kuyper Jr. from comment #31) > (In reply to rguent...@suse.de from comment #29) > > On Thu, 19 Apr 2018, jameskuyper at verizon dot net wrote: > ... > > > The relevant wording is

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #31 from James Kuyper Jr. --- (In reply to rguent...@suse.de from comment #29) > On Thu, 19 Apr 2018, jameskuyper at verizon dot net wrote: ... > > The relevant wording is "anywhere that a declaration of the completed type > > of >

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #30 from Martin Sebor --- Richard, I offered to write a proposal (with Clark) to improve the rules. With the object model proposals already in the pipeline (N2223) this is a good time to review them and see if it makes sense to

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #29 from rguenther at suse dot de --- On Thu, 19 Apr 2018, jameskuyper at verizon dot net wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 > > James Kuyper Jr. changed: > >What|Removed

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 James Kuyper Jr. changed: What|Removed |Added CC||jameskuyper at verizon dot net ---

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #27 from rguenther at suse dot de --- On Wed, 27 Sep 2017, david at westcontrol dot com wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 > > David Brown changed: > >What|Removed

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 David Brown changed: What|Removed |Added CC||david at westcontrol dot com --- Comment

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 Davin McCall changed: What|Removed |Added CC||davmac at davmac dot org --- Comment #25

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #24 from rguenther at suse dot de --- On Wed, 2 Nov 2016, txr at alumni dot caltech.edu wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 > > --- Comment #22 from Tim Rentsch --- > [responding to comments from

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #23 from joseph at codesourcery dot com --- On Wed, 2 Nov 2016, txr at alumni dot caltech.edu wrote: > Seven: Given that the question is now under serious debate, IMO > someone involved with gcc development should take the

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #22 from Tim Rentsch --- [responding to comments from rguent...@suse.de in Comment 20] > GCC already implements this if you specify -fno-strict-aliasing. The main point of my comments is that the ISO C standard requires the

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #21 from Tim Rentsch --- [responding to comments from jos...@codesourcery.com in Comment 19] >> Five: The answer to the question is clearly No. The example code >> is very much on point to the "one special guarantee" clause, and

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #20 from rguenther at suse dot de --- On November 1, 2016 7:16:06 PM GMT+01:00, "txr at alumni dot caltech.edu" wrote: >https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 > >Tim Rentsch changed: > >

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #19 from joseph at codesourcery dot com --- On Tue, 1 Nov 2016, txr at alumni dot caltech.edu wrote: > Five: The answer to the question is clearly No. The example code > is very much on point to the "one special guarantee" clause,

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 Tim Rentsch changed: What|Removed |Added CC||txr at alumni dot caltech.edu --- Comment

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #14 from joseph at codesourcery dot com --- That C++ wording doesn't have any obvious bearing on what "it is permitted" is intended to be an exception to - the general implementation-defined nature of type punning (which I think

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #15 from Jonathan Wakely --- (In reply to Melissa from comment #12) > A C++ conversion of the original example is below. I asked about the word > "read" on the C++ Standard Discussion (std-discussion) mailing list, because > it

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #16 from Jonathan Wakely --- Message [c++std-core-20893] on the C++ core reflector on 2011-12-14 supports the GCC view that a C++ compiler can apply strict aliasing rules to p1->m and p2->m unless the fact they come from the same

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #17 from Martin Sebor --- The C Union Visibility rule was intended to cover that case. The trouble is that the rule tends to be interpreted differently by different people, users and implementers alike: Is it the union object that

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 Melissa changed: What|Removed |Added CC||myriachan at gmail dot com --- Comment #12

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #13 from Melissa --- As for a reason why this should be allowed, all I need is to do is mention struct sockaddr.

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 Marek Polacek mpolacek at gcc dot gnu.org changed: What|Removed |Added Status|UNCONFIRMED |SUSPENDED

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #10 from Martin Sebor msebor at gcc dot gnu.org --- Thanks (again) for your comments, Joseph. I had a chance to discuss this issue with Clark Nelson last week. Clark has worked on improving the aliasing parts of the C specification

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #9 from joseph at codesourcery dot com joseph at codesourcery dot com --- The rule certainly has nothing to do with whether the struct types are defined inside the union definition, or defined outside and then used inside via a tag

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 Richard Biener rguenth at gcc dot gnu.org changed: What|Removed |Added Keywords||alias

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #8 from Martin Sebor msebor at gcc dot gnu.org --- If one of you can explain the problem with it I'm willing to write up a paper and submit it to WG14 and request to have the standard changed.

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 Martin Sebor msebor at gcc dot gnu.org changed: What|Removed |Added Status|RESOLVED|UNCONFIRMED

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #6 from Martin Sebor msebor at gcc dot gnu.org --- I agree it's subtle and could be clearer but I believe the key phrase is a union contains several structures. Here, the term union refers to the type, not the object. This is

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #5 from Andrew Pinski pinskia at gcc dot gnu.org --- (In reply to Martin Sebor from comment #4) Thanks for the pointer! I had looked for a related bug report but couldn't find it. There's an important difference between the test

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 Andrew Pinski pinskia at gcc dot gnu.org changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #3 from Andrew Pinski pinskia at gcc dot gnu.org --- Also if we follow that defect resolution, basically strict aliasing does not mean anything any more and we would have to turn off strict aliasing for all structs.

[Bug c/65892] gcc fails to implement N685 aliasing of union members

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892 --- Comment #1 from Andrew Pinski pinskia at gcc dot gnu.org --- Some folks think that resolution is not fully correct.