[Bug c/98418] New: Valid integer constant expressions based on expressions that trigger -Wshift-overflow are treated as non-constants

[Bug c/98418] New: Valid integer constant expressions based on expressions that trigger -Wshift-overflow are treated as non-constants

[pskocik at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98418

            Bug ID: 98418
           Summary: Valid integer constant expressions based on
                    expressions that trigger -Wshift-overflow are treated
                    as non-constants
           Product: gcc
           Version: 6.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pskocik at gmail dot com
  Target Milestone: ---

This causes things like:

struct foo { unsigned bit: (0xffffffffll<<40)!=0; };

to elicit a -pedantic warning about the bitfield width not being a proper
integer constant expression, even though it is.

In other contexts, a complete compilation error might ensue:

extern int bar[ (0xffffffffll<<40)!=0 ]; //seen as an invalid VLA


https://gcc.godbolt.org/z/7zfz96

Neither clang nor gcc <= 5 appear to have this bug.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93241 seems related.

[Bug c/98418] Valid integer constant expressions based on expressions that trigger -Wshift-overflow are treated as non-constants

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98418

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Shifting into the sign bit is problematic. I cant remember the exact rules.
Using ull is valid though.

[Bug c/98418] Valid integer constant expressions based on expressions that trigger -Wshift-overflow are treated as non-constants

[pskocik at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98418

--- Comment #2 from pskocik at gmail dot com ---
You're right. The bug was in my code.

struct foo { unsigned bit: (0xffffffffll<<40)!=0; };

is indeed UB due to http://port70.net/~nsz/c/c11/n1570.html#6.5.7p4, but

struct foo { unsigned bit: (0xffffffffull<<40)!=0; };

isn't and GCC accepts it without complaint.

Apologies for the false alarm.

[Bug c/98418] Valid integer constant expressions based on expressions that trigger -Wshift-overflow are treated as non-constants

[pskocik at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98418

pskocik at gmail dot com changed:

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

--- Comment #3 from pskocik at gmail dot com ---
You're right. The bug was in my code.

struct foo { unsigned bit: (0xffffffffll<<40)!=0; };

is indeed UB due to http://port70.net/~nsz/c/c11/n1570.html#6.5.7p4, but

struct foo { unsigned bit: (0xffffffffull<<40)!=0; };

isn't and GCC accepts it without complaint.

Apologies for the false alarm.