[Bug c/103725] New: warning: assuming signed overflow does not occur when simplifying conditional to constant

[Bug c/103725] New: warning: assuming signed overflow does not occur when simplifying conditional to constant

[f.heckenbach@fh-soft.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103725

            Bug ID: 103725
           Summary: warning: assuming signed overflow does not occur when
                    simplifying conditional to constant
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: f.heckenbach@fh-soft.de
  Target Milestone: ---

% cat test.c  
int d = 0, b = 8, a[8][8];

struct S
{
  int c;
};

void foo (int *c)
{
  if (b >= 0)
    {
      if (*c) d = *c;
      if (*c < 0) d = *c;
    }
}

int main ()
{
  struct S i = { 0 };
  for (int e = 0; e < 11; e++, i.c++)
    {
      d = a[i.c >= b ? i.c - b : i.c][i.c + 1 >= b ? i.c + 1 - b : i.c + 1];
      foo (&i.c);
    }
}
% gcc -O2 -Wstrict-overflow test.c 
test.c: In function 'main':
test.c:22:66: warning: assuming signed overflow does not occur when simplifying
conditional to constant [-Wstrict-overflow]
   22 |       d = a[i.c >= b ? i.c - b : i.c][i.c + 1 >= b ? i.c + 1 - b : i.c
+ 1];
      |                                      
~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~

This is not the same as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103724 (in
fact, that was a collateral damage while bisecting this one), though they may
be internally related since this warning also depends only many circumstances,
plus a few more (like, there must be a struct, a plain integer won't cause the
warning; and e and i.c must be distinct although they count in lockstep).

To a user, the warning is worrying since none of the visible conditionals
should be able to be simplified to a constant.

What seems to be happening, as far as I can tell from looking at the generated
code, is that GCC internally splits cases and then simplifies conditions in one
of them (since i.c >= b implies i.c + 1 >= b, unless signed overflow which
seems to explain the cause of the warning).

But the user normally doesn't see this and gets the impression one of their
conditions is wrongly simplified. This, and the fact that the warning doesn't
seem very robust in the first place (i.e. depends on many circumstances), seems
to make the whole "-Wstrict-overflow" option a bit less than useful, IMHO.

I get another spurious warning like this in a more complex piece of my code,
and it also seems to depend on many circumstances, so I can't easily provide a
reproducible example, but it may be a similar situation like this one in the
end. (And these were the only ones I got, i.e. it didn't actually help me find
a real bug in my code.)

Since it also doesn't seem possible to suppress the warning locally with a
pragma (since it's generated at a late stage, I suppose), I think I'll have to
disable it again. (I had re-enabled it after I heard
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70455 was fixed. It may work
better now, but still too mysterious to actually help me.)

[Bug tree-optimization/103725] warning: assuming signed overflow does not occur when simplifying conditional to constant

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103725

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
#1  0x00000000018fa008 in simplify_using_ranges::vrp_evaluate_conditional
(this=0x7fffffffd510, code=LE_EXPR, op0=<ssa_name 0x7ffff73ac048 2>,
op1=<ssa_name 0x7ffff73ac168 3>, stmt=<gimple_cond 0x7ffff7391d20>) at
/home/apinski/src/upstream-gcc/gcc/gcc/vr-values.c:2383
#2  0x00000000015e5791 in dom_jt_simplifier::simplify (this=0x7fffffffd920,
stmt=<gimple_cond 0x7ffff73c6460>, within_stmt=<gimple_cond 0x7ffff7391d20>) at
/home/apinski/src/upstream-gcc/gcc/gcc/tree-ssa-dom.c:689
#3  0x0000000001756fb4 in jump_threader::simplify_control_stmt_condition_1
(this=0x7fffffffd940, e=<edge 0x7ffff73af090 (4 -> 5)>, stmt=<gimple_cond
0x7ffff7391d20>, op0=<ssa_name 0x7ffff73ac048 2>, cond_code=LE_EXPR,
op1=<ssa_name 0x7ffff73ac168 3>, limit=4)
    at /home/apinski/src/upstream-gcc/gcc/gcc/tree-ssa-threadedge.c:637
#4  0x0000000001756574 in jump_threader::simplify_control_stmt_condition
(this=0x7fffffffd940, e=<edge 0x7ffff73af090 (4 -> 5)>, stmt=<gimple_cond
0x7ffff7391d20>) at
/home/apinski/src/upstream-gcc/gcc/gcc/tree-ssa-threadedge.c:385
#5  0x0000000001757b2a in jump_threader::thread_through_normal_block
(this=0x7fffffffd940, Python Exception <class 'gdb.error'> There is no member
or method named m_vecpfx.:
path=0x4282240, e=<edge 0x7ffff73af090 (4 -> 5)>, visited=0x7fffffffd760) at
/home/apinski/src/upstream-gcc/gcc/gcc/tree-ssa-threadedge.c:951
#6  0x00000000017580c5 in jump_threader::thread_across_edge
(this=0x7fffffffd940, e=<edge 0x7ffff73af090 (4 -> 5)>) at
/home/apinski/src/upstream-gcc/gcc/gcc/tree-ssa-threadedge.c:1080
#7  0x00000000017585b8 in jump_threader::thread_outgoing_edges
(this=0x7fffffffd940, bb=<basic_block 0x7ffff724e680 (4)>) at
/home/apinski/src/upstream-gcc/gcc/gcc/tree-ssa-threadedge.c:1206
#8  0x00000000015e7a3d in dom_opt_dom_walker::after_dom_children
(this=0x7fffffffd9b0, bb=<basic_block 0x7ffff724e680 (4)>) at
/home/apinski/src/upstream-gcc/gcc/gcc/tree-ssa-dom.c:1579
#9  0x0000000002841322 in dom_walker::walk (this=0x7fffffffd9b0,
bb=<basic_block 0x7ffff724e680 (4)>) at
/home/apinski/src/upstream-gcc/gcc/gcc/domwalk.c:352

[Bug tree-optimization/103725] [9/10/11/12 Regression] warning: assuming signed overflow does not occur when simplifying conditional to constant

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103725

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |7.1.0, 7.5.0
      Known to fail|                            |10.1.0, 8.1.0
   Last reconfirmed|                            |2021-12-18
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
            Summary|warning: assuming signed    |[9/10/11/12 Regression]
                   |overflow does not occur     |warning: assuming signed
                   |when simplifying            |overflow does not occur
                   |conditional to constant     |when simplifying
                   |                            |conditional to constant

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---


int t = i.c >= b ? i.c - b : i.c;
int t1 = i.c + 1 >= b ? i.c + 1 - b : i.c + 1;

I think the warning is happening when jump threading the above in DOM.
I don't think the warning makes sense really because we are inside a loop after
all.

[Bug tree-optimization/103725] [9/10/11/12 Regression] warning: assuming signed overflow does not occur when simplifying conditional to constant

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103725

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |9.5

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
We've mostly pruned all the singed overflow diagnostics when touching stuff but
some are still there ...

[Bug tree-optimization/103725] [9/10/11/12 Regression] warning: assuming signed overflow does not occur when simplifying conditional to constant

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103725

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug tree-optimization/103725] [10/11/12/13 Regression] warning: assuming signed overflow does not occur when simplifying conditional to constant

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103725

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.5                         |10.4

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 9 branch is being closed

[Bug tree-optimization/103725] [10/11/12/13 Regression] warning: assuming signed overflow does not occur when simplifying conditional to constant

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103725

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.4                        |10.5

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 10.4 is being released, retargeting bugs to GCC 10.5.

[Bug tree-optimization/103725] [10/11/12/13 Regression] warning: assuming signed overflow does not occur when simplifying conditional to constant

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103725

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |needs-bisection

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Seems to be fixed on the trunk ....

[Bug tree-optimization/103725] [11/12/13/14 Regression] warning: assuming signed overflow does not occur when simplifying conditional to constant

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103725

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.5                        |11.5

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 10 branch is being closed.

[Bug tree-optimization/103725] [11/12 Regression] warning: assuming signed overflow does not occur when simplifying conditional to constant

[law at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103725

Jeffrey A. Law <law at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu.org
            Summary|[11/12/13/14 Regression]    |[11/12 Regression] warning:
                   |warning: assuming signed    |assuming signed overflow
                   |overflow does not occur     |does not occur when
                   |when simplifying            |simplifying conditional to
                   |conditional to constant     |constant

--- Comment #8 from Jeffrey A. Law <law at gcc dot gnu.org> ---
Per c#6.