Here is the unincluded version of the source:
#include <utility>

template <typename C>
bool __attribute__((noinline))
compare_pairs(const std::pair<int, C> &lhs, const std::pair<int, C> &rhs) {
  return lhs.first == rhs.first && lhs.second == rhs.second;
}

template <typename B> struct Combined {
  std::pair<int, B *> __attribute__((noinline)) get_const() {
    return std::make_pair(123, nullptr);
  }
  bool __attribute__((noinline)) clashy() {
    return compare_pairs(get_const(), get_const());
  }
};

class SomeClass {};
class OtherClass {};

void some_func() {
  Combined<SomeClass> myvar;
  __builtin_printf("%i\n", myvar.clashy());
}

void other_func() {
  Combined<OtherClass> myvar;
  __builtin_printf("%i\n", myvar.clashy());
}

Here is a single file example which just needs -Os -flto -fno-strict-aliasing:

template <class T>
struct pair
{
    int first;
    T second;
};

template <typename C>
[[gnu::optimize("strict-aliasing")]]
bool __attribute__((noinline))
compare_pairs(const pair<C> &lhs, const pair<C> &rhs) {
  return lhs.first == rhs.first && lhs.second == rhs.second;
}

template <typename B> struct Combined {
  pair<B *> 
__attribute__((noinline)) get_const() {
    return pair<B *>{123, nullptr};
  }
[[gnu::optimize("strict-aliasing")]]
  bool 
__attribute__((noinline)) clashy() {
    return compare_pairs(get_const(), get_const());
  }
};

class SomeClass {};
class OtherClass {};

[[gnu::optimize("strict-aliasing")]]
[[gnu::used]]
void some_func() {
  Combined<SomeClass> myvar;
  __builtin_printf("%i\n", myvar.clashy());
}

[[gnu::optimize("strict-aliasing")]]
void other_func() {
  Combined<OtherClass> myvar;
  __builtin_printf("%i\n", myvar.clashy());
}

[[gnu::optimize("O0")]]
int main()
{
  other_func();
}

Created attachment 53799 [details]
Testcase compile with -O2 -fno-strict-aliasing -flto

New self contained testcase.

doesn't reproduce with -fwhole-program instead of -flto or with -flto-partition=max added.  -fno-ipa-modref avoids the issue.

Also related to ICF, where -fno-ipa-icf fixes that as well. Lemme take a look.

Running that under valgrind I see it started with r12-656-ga564da506f52be66:

==13643== Conditional jump or move depends on uninitialised value(s)
==13643==    at 0x401148: bool compare_pairs<SomeClass*>(pair<SomeClass*> const&, pair<SomeClass*> const&) (pr107467-2.C:12)
==13643==    by 0x401176: Combined<OtherClass>::clashy() [clone .isra.0] (pr107467-2.C:23)
==13643==    by 0x4011B2: other_func() (pr107467-2.C:40)
==13643==    by 0x4011ED: main (pr107467-2.C:48)

So I guess it might be something in between DSE and aliasing info investigated by IPA modref.

@Honza: Can you take a look?

mine.

> diff -u a.ltrans0.ltrans.115t.vrp1 a.ltrans0.ltrans.116t.dse2
...
 __attribute__((noinline, optimize ("strict-aliasing")))
 bool clashy.isra ()
 {
@@ -90,8 +64,6 @@
   bool _1;
 
   <bb 2> [local count: 1073741824]:
-  D.5191 = get_const.isra ();
-  D.5190 = get_const.isra ();
   _1 = compare_pairs (&D.5190, &D.5191);

so it's ICFed compare_pairs having modref TBAA info that makes the
stores dead.  I suppose ICF needs to reset / alter the modref summaries?

> 
> so it's ICFed compare_pairs having modref TBAA info that makes the
> stores dead.  I suppose ICF needs to reset / alter the modref summaries?

Well, matching that ICF does should be enough to verify that modref
sumaries are same.  Modref only does what would happen after inlining
and ICF should be safe WRT both modref and inlining.

GCC 12.3 is being released, retargeting bugs to GCC 12.4.

GCC 12.4 is being released, retargeting bugs to GCC 12.5.

This seems to work on trunk now.

(In reply to Sam James from comment #12)
> This seems to work on trunk now.

Fixed on trunk by r15-626-ga5b3721c06646b for PR115110, then see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115110#c8 which refers to r15-512-g9b7cad5884f21c which sounds a lot like the fix for this.

Mine for backporting then (I've verified this fixes the attached single-file testcase).

Fixed for 14.3

We should add the testcase as well (I can do it).

(In reply to Sam James from comment #16)
> We should add the testcase as well (I can do it).

Posted: https://inbox.sourceware.org/gcc-patches/2a3d484399534a1449ac3a29d6089ed62b839a5a.1729885673.git.sam@gentoo.org/

The master branch has been updated by Sam James <sjames@gcc.gnu.org>:

https://gcc.gnu.org/g:4e09ae37dbe0a10f48490214f50ff733cc92280a

commit r15-4723-g4e09ae37dbe0a10f48490214f50ff733cc92280a
Author: Sam James <sam@gentoo.org>
Date:   Mon Oct 21 12:11:42 2024 +0100

    testsuite: add testcase for fixed PR107467
    
    PR107467 ended up being fixed by the fix for PR115110, but let's
    add the testcase on top.
    
    gcc/testsuite/ChangeLog:
            PR tree-optimization/107467
            PR middle-end/115110
    
            * g++.dg/lto/pr107467_0.C: New test.

The releases/gcc-14 branch has been updated by Sam James <sjames@gcc.gnu.org>:

https://gcc.gnu.org/g:2f0d109bd871d11b5b93468f271aa6dc34ef88d8

commit r14-10845-g2f0d109bd871d11b5b93468f271aa6dc34ef88d8
Author: Sam James <sam@gentoo.org>
Date:   Mon Oct 21 12:11:42 2024 +0100

    testsuite: add testcase for fixed PR107467
    
    PR107467 ended up being fixed by the fix for PR115110, but let's
    add the testcase on top.
    
    gcc/testsuite/ChangeLog:
            PR tree-optimization/107467
            PR middle-end/115110
    
            * g++.dg/lto/pr107467_0.C: New test.

The releases/gcc-14 branch has been updated by Sam James <sjames@gcc.gnu.org>:

https://gcc.gnu.org/g:6247aae35faaf04de48c5f3d9c4e4af6f7e3789c

commit r14-10847-g6247aae35faaf04de48c5f3d9c4e4af6f7e3789c
Author: Sam James <sam@gentoo.org>
Date:   Mon Oct 21 12:11:42 2024 +0100

    testsuite: add testcase for fixed PR107467
    
    PR107467 ended up being fixed by the fix for PR115110, but let's
    add the testcase on top.
    
    gcc/testsuite/ChangeLog:
            PR tree-optimization/107467
            PR middle-end/115110
    
            * g++.dg/lto/pr107467_0.C: New test.
    
    (cherry picked from commit 4e09ae37dbe0a10f48490214f50ff733cc92280a)