[Bug c++/115291] New: armv8-a GCC emits float32x2_t loads from uninitialized stack

[Bug c++/115291] New: armv8-a GCC emits float32x2_t loads from uninitialized stack

[akihiko.odaki at daynix dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115291

            Bug ID: 115291
           Summary: armv8-a GCC emits float32x2_t loads from uninitialized
                    stack
           Product: gcc
           Version: 14.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: akihiko.odaki at daynix dot com
  Target Milestone: ---

Run the following commands:
git clone https://gitlab.com/libeigen/eigen.git -b
38b9cc263bbaeb03ce408a4e26084543a6c0dedb
cat main.cpp <<'EOF'
#include "Eigen/Core"

auto f() -> Eigen::Matrix2f {
  return Eigen::Matrix2f::Zero();
}

auto g() -> Eigen::Vector2f {
  auto v = Eigen::Product(f(), Eigen::Vector2f::Zero());
  return v;
}
EOF

c++ -Ieigen -O1 -DNDEBUG main.cpp -S

Now main.s looks like:
        .arch armv8-a
        .file   "main.cpp"
        .text
        .align  2
        .global _Z1fv
        .type   _Z1fv, %function
_Z1fv:
.LFB13241:
        .cfi_startproc
        mov     x0, x8
        str     wzr, [x8]
        str     wzr, [x8, 4]
        str     wzr, [x8, 8]
        str     wzr, [x8, 12]
        ret
        .cfi_endproc
.LFE13241:
        .size   _Z1fv, .-_Z1fv
        .align  2
        .global _Z1gv
        .type   _Z1gv, %function
_Z1gv:
.LFB13246:
        .cfi_startproc
        sub     sp, sp, #16
        .cfi_def_cfa_offset 16
        mov     x0, x8
        movi    v30.2s, 0
        ldr     d31, [sp]
        fmul    v31.2s, v31.2s, v30.2s
        ldr     d29, [sp, 8]
        fmla    v31.2s, v29.2s, v30.2s
        str     d31, [x8]
        add     sp, sp, 16
        .cfi_def_cfa_offset 0
        ret
        .cfi_endproc
.LFE13246:
        .size   _Z1gv, .-_Z1gv
        .align  2
        .type   _GLOBAL__sub_I__Z1fv, %function
_GLOBAL__sub_I__Z1fv:
.LFB14439:
        .cfi_startproc
        ret
        .cfi_endproc
.LFE14439:
        .size   _GLOBAL__sub_I__Z1fv, .-_GLOBAL__sub_I__Z1fv
        .ident  "GCC: (GNU) 14.1.1 20240507 (Red Hat 14.1.1-1)"
        .section        .note.GNU-stack,"",@progbits

In _Z1gv, "ldr  d31, [sp]" loads values from nowhere.

[Bug c++/115291] armv8-a GCC emits float32x2_t loads from uninitialized stack

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115291

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
  D.198035 = f (); [return slot optimization]
  v.m_lhs = &D.198035;
  MEM[(struct CwiseNullaryOp *)&v + 8B] ={v} {CLOBBER(bob)};
  MEM[(struct scalar_constant_op *)&v + 12B] ={v} {CLOBBER(bob)};
  MEM[(struct scalar_constant_op *)&v + 12B].m_other = 0.0;
  D.198035 ={v} {CLOBBER(eos)};
  _6 = &MEM[(struct Matrix *)_4(D)].D.198029;
  _12 = v.m_lhs;




  auto v = Eigen::Product(f(), Eigen::Vector2f::Zero());

The bug is there is a temporary created for the return value of f but that is
destoried at the end of the full statement but the product does not happen
until afterwards in the use of v happens.

Rewriting the code like:
```
        auto t = f();
  auto v = Eigen::Product(t, Eigen::Vector2f::Zero());
```

Fixes the code.

[Bug c++/115291] armv8-a GCC emits float32x2_t loads from uninitialized stack

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115291

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
See https://libeigen.gitlab.io/docs/TopicPitfalls.html
section "C++11 and the auto keyword" explictly.

"In short: do not use the auto keywords with Eigen's expressions, unless you
are 100% sure about what you are doing. In particular, do not use the auto
keyword as a replacement for a Matrix<> type."

Describes the issue you are having.

[Bug c++/115291] armv8-a GCC emits float32x2_t loads from uninitialized stack

[akihiko.odaki at daynix dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115291

--- Comment #3 from Akihiko Odaki <akihiko.odaki at daynix dot com> ---
You are right. Sorry for bothering and thanks for pointing out the issue in the
code (and even the relevant documentation!)