[Bug c++/84816] [7.2.0/8.0.1 x86_64] Incorrect code generation if signed overflow

2018-03-11 Thread ebotcazou at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84816

Eric Botcazou  changed:

   What|Removed |Added

 CC||ebotcazou at gcc dot gnu.org

--- Comment #8 from Eric Botcazou  ---
> Yes, but code generation incorrect. It's not runtime yet.

The criterion is as follows: does the code invoke undefined behavior when
executed in the abstract machine specified by the C standard?  If so, then the
compiler is allowed to generate object code that does anything, including
playing some music or erasing the hard drive; here it generates an infinite
loop and that's OK.

[Bug c++/84816] [7.2.0/8.0.1 x86_64] Incorrect code generation if signed overflow

2018-03-10 Thread dlesnikov at hotmail dot com 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84816

--- Comment #7 from Dmitry Lesnikov  ---
(In reply to Andrew Pinski from comment #5)
> (In reply to Dmitry Lesnikov from comment #4)
> > (In reply to Andrew Pinski from comment #1)
> > > signed overflow is undefined behavior at runtime.
> > 
> > for(int i=0; i<10; i++)
> > 
> > this loop is correct.
> 
> But there is an overflow with the variable a when i is 6.

Why does the line
C1 c1;
affect the behavior of the loop?

[Bug c++/84816] [7.2.0/8.0.1 x86_64] Incorrect code generation if signed overflow

2018-03-10 Thread dlesnikov at hotmail dot com 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84816

--- Comment #6 from Dmitry Lesnikov  ---
(In reply to Andrew Pinski from comment #5)
> (In reply to Dmitry Lesnikov from comment #4)
> > (In reply to Andrew Pinski from comment #1)
> > > signed overflow is undefined behavior at runtime.
> > 
> > for(int i=0; i<10; i++)
> > 
> > this loop is correct.
> 
> But there is an overflow with the variable a when i is 6.

Yes, but code generation incorrect. It's not runtime yet.

[Bug c++/84816] [7.2.0/8.0.1 x86_64] Incorrect code generation if signed overflow

2018-03-10 Thread pinskia at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84816

--- Comment #5 from Andrew Pinski  ---
(In reply to Dmitry Lesnikov from comment #4)
> (In reply to Andrew Pinski from comment #1)
> > signed overflow is undefined behavior at runtime.
> 
> for(int i=0; i<10; i++)
> 
> this loop is correct.

But there is an overflow with the variable a when i is 6.

[Bug c++/84816] [7.2.0/8.0.1 x86_64] Incorrect code generation if signed overflow

2018-03-10 Thread dlesnikov at hotmail dot com 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84816

--- Comment #4 from Dmitry Lesnikov  ---
(In reply to Andrew Pinski from comment #1)
> signed overflow is undefined behavior at runtime.

for(int i=0; i<10; i++)

this loop is correct.

[Bug c++/84816] [7.2.0/8.0.1 x86_64] Incorrect code generation if signed overflow

2018-03-10 Thread pinskia at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84816

Andrew Pinski  changed:

   What|Removed |Added

 Status|UNCONFIRMED |RESOLVED
 Resolution|--- |INVALID

--- Comment #3 from Andrew Pinski  ---
Does not matter, the behavior is undefined.

[Bug c++/84816] [7.2.0/8.0.1 x86_64] Incorrect code generation if signed overflow

2018-03-10 Thread dlesnikov at hotmail dot com 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84816

Dmitry Lesnikov  changed:

   What|Removed |Added

 Status|RESOLVED|UNCONFIRMED
 Resolution|INVALID |---

--- Comment #2 from Dmitry Lesnikov  ---
gcc 5.4.1 make correct code.

[Bug c++/84816] [7.2.0/8.0.1 x86_64] Incorrect code generation if signed overflow

2018-03-10 Thread pinskia at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84816

Andrew Pinski  changed:

   What|Removed |Added

 Target|x86_84  |
 Status|UNCONFIRMED |RESOLVED
  Known to work|5.4.1   |
   Host|Ubuntu 16.04 x86_84 |
 Resolution|--- |INVALID
  Known to fail|7.2.0, 8.0.1|

--- Comment #1 from Andrew Pinski  ---
signed overflow is undefined behavior at runtime.