[Bug c/101913] New: -Wstrict-overflow -O3 false alarm on tzdb localtime.c

[Bug c/101913] New: -Wstrict-overflow -O3 false alarm on tzdb localtime.c

[eggert at cs dot ucla.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101913

            Bug ID: 101913
           Summary: -Wstrict-overflow -O3 false alarm on tzdb localtime.c
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eggert at cs dot ucla.edu
  Target Milestone: ---

Created attachment 51304
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51304&action=edit
-Wstrict-overflow -O3 false alarms

I ran into this problem when compiling tzdb localtime.c with gcc (GCC) 11.2.1
20210728 (Red Hat 11.2.1-1) on x86-64. To reproduce, compile the attached
program v.i with:

gcc -Wstrict-overflow -O3 -fsanitize=undefined -S v.i

The output is:

v.i: In function ‘tzloadbody’:
v.i:22:1: warning: assuming signed overflow does not occur when simplifying
con\
ditional to constant [-Wstrict-overflow]
   22 | tzloadbody (struct state *sp, int leapcnt0, int timecnt0)
      | ^~~~~~~~~~
v.i:29:25: warning: assuming signed overflow does not occur when simplifying
co\
nditional to constant [-Wstrict-overflow]
   29 |       sp->types[i] = at <= 0xffffffffu;
      |                      ~~~^~~~~~~~~~~~~~

1. These diagnostics make no sense, since none of the underlined code involves
any operations that can overflow.

2. I looked through the code carefully, and none of the arithmetic operations
can possibly overflow. In (result << 8), 'result' is at most 2**32 - 1 and this
fits comfortably into 'long'. -1L << 63 is LONG_MIN. "++i" is executed only
when i is less than some other int. timecnt++ and leapcnt++ are executed at
most INT_MAX times. prevcorr + 1 is evaluated only when prevcorr < corr, and
prevcorr - 1 is evaluated only when prevcorr > corr.

3. The diagnostics vanish if you change MYSTERY from 5 to 4 in the first line.
This suggests that GCC is somehow getting confused about whether 'long' has 8
bytes (which it does on this platform) or 4 bytes.

[Bug middle-end/101913] -Wstrict-overflow -O3 false alarm on tzdb localtime.c

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101913

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
>-1L << 63 is LONG_MIN
No it is undefined and has an overflow bit on it.
You want (long)(-1UL << 63) for it be correct.
But the warning is still there.

I thought -fsanitize=undefined enabled -fwrapv too ...

[Bug middle-end/101913] -Wstrict-overflow -O3 false alarm on tzdb localtime.c

[eggert at cs dot ucla.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101913

eggert at cs dot ucla.edu changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #51304|0                           |1
        is obsolete|                            |

--- Comment #2 from eggert at cs dot ucla.edu ---
Created attachment 51305
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51305&action=edit
-Wstrict-overflow -O3 false alarm example (v2)

This updates the example by replacing -1L<<63 with -1-9223372036854775807L, to
avoid any undefined behavior in evaluating that constant expression.

[Bug middle-end/101913] -Wstrict-overflow -O3 false alarm on tzdb localtime.c

[eggert at cs dot ucla.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101913

--- Comment #3 from eggert at cs dot ucla.edu ---
(In reply to Andrew Pinski from comment #1)
> >-1L << 63 is LONG_MIN
> No it is undefined and has an overflow bit on it.
> You want (long)(-1UL << 63) for it be correct.
> But the warning is still there.

I updated the attachment to avoid any undefined behavior, by replacing -1L<<63
with -1-9223372036854775807L. As you say, the bug is still there.

> I thought -fsanitize=undefined enabled -fwrapv too ...

It doesn't. (Perhaps you meant -ftrapv?)

[Bug middle-end/101913] -Wstrict-overflow -O3 false alarm on tzdb localtime.c

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101913

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

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

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Looks to be fixed on the trunk.

[Bug middle-end/101913] -Wstrict-overflow -O3 false alarm on tzdb localtime.c

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101913

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|needs-bisection             |
                 CC|                            |aldyh at gcc dot gnu.org,
                   |                            |marxin at gcc dot gnu.org

--- Comment #5 from Martin Liška <marxin at gcc dot gnu.org> ---
Fixed with r12-3903-g0288527f47cec669.