r180603 - C++1y: support simple variable assignments in constexpr functions.

[Jordan Rose]

On May 20, 2013, at 6:59 , Hans Wennborg <hans at chromium.org> wrote:

> Hi Richard,
> 
> On Fri, Apr 26, 2013 at 3:36 PM, Richard Smith
> <richard-llvm at metafoo.co.uk> wrote:
>> Author: rsmith
>> Date: Fri Apr 26 09:36:30 2013
>> New Revision: 180603
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=180603&view=rev
>> Log:
>> C++1y: support simple variable assignments in constexpr functions.
> 
> This seems to have caused -Winteger-overflow to become much more aggressive.
> 
> I don't know what in your code causes this, but I bisected it down to
> this revision.
> 
> The following code didn't use to warn, but now does (only in C++11 mode though):
> 
> #include <time.h>
> #include <limits>
> 
> void f() {
>  long long x;
>  x = std::numeric_limits<time_t>::min() * 1000;
> }
> 
> /tmp/a.cc:6:42: warning: overflow in expression; result is 0 with type
> 'long' [-Winteger-overflow]
>  x = std::numeric_limits<time_t>::min() * 1000;
>                                         ^
> 
> This is causing problems in Chromium where we have such code, but we
> know we won't hit it on platforms where time_t is 64-bit.
> 
> Was the warning behaviour change intentional?

That looks like a real warning to me:
- if time_t is signed 32-bit and int is 32-bit, the min value will be -2^31, and multiplying by 1000 will result in negative overflow
- if time_t is signed 64-bit, the min value will be -2^63, and multiplying by 1000 will result in negative overflow

The correct way to handle the 32-bit case would be to use "1000LL", so that the calculation is done in long-long space (64 bits) instead of int-space (presumably 32 bits). The 64-bit case will never be right unless your long long is 128 bits.

But that "result is 0" seems to imply that time_t is unsigned, which is strange.

In general any improvements to constexpr will up the aggressiveness of warnings based on expression values, since the compiler can calculate more values at compile time. I think this is generally a good thing.

Jordan