On Tue, 17 Nov 2015, Paolo Bonzini wrote:
> On 17/11/2015 17:02, Joseph Myers wrote:
> > On Tue, 17 Nov 2015, Paolo Bonzini wrote:
> >
> >> * it doesn't promise that GCC will never rely on undefined behavior
> >> rules for signed left shifts
> >
> > I think we should remove the ", but this is subject to change" in
> > implement-c.texi (while replacing it with noting that ubsan will still
> > diagnose such cases, and they will also be diagnosed where constant
> > expressions are required).
>
> ... hmm, are you sure? None of the following warn for me
>
> int x = -1 << 2;
> int y = 1 << 31;
> int z = 2 << 31;
>
> I tried with any combination of -ansi, -pedantic, -std=cXX,
> -fsanitize=undefined.
With a recent trunk build I get:
$ ./build/gcc/xgcc -B./build/gcc/ -S -o /dev/null -pedantic -std=c11 t.c
t.c:1:9: warning: initializer element is not a constant expression [-Wpedantic]
int x = -1 << 2;
^
t.c:2:9: warning: initializer element is not a constant expression [-Wpedantic]
int y = 1 << 31;
^
t.c:3:11: warning: result of '2 << 31' requires 34 bits to represent, but 'int'
only has 32 bits [-Wshift-overflow=]
int z = 2 << 31;
^
t.c:3:9: warning: initializer element is not a constant expression [-Wpedantic]
int z = 2 << 31;
^
(and -pedantic-errors produces errors for the "not a constant expression"
cases, as expected).
--
Joseph S. Myers
jos...@codesourcery.com
