https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107839
--- Comment #3 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Richard Biener from comment #2)
> it's loop invariant motion that hoists the v + v compute out of the loop
> and thus outside of its controlling condition. You can see it's careful
> to not introduce undefined overflow that is possibly conditionally
> executed only but it fails to consider the case of 'v' being conditionally
> uninitialized.
>
> It's very difficult to do the right thing here - it might be tempting to
> hoist the compute as
>
> if (c)
> tem = v+v;
> while (1)
> if (c)
> f(tem);
Couldn't the -Wmaybe-uninitialized warning be disabled on hoisted code, so that
the controlling condition wouldn't be needed?
To make sure not to disable potential warnings, the information that v was used
for tem should be kept together with tem in the loop. Something like
((void)v,tem), though GCC doesn't currently warn on that if v is uninitialized
(but that's another issue that should be solved).
However...
> Maybe the simplest thing would be to never hoist v + v, or only
> hoist it when the controlling branch is not loop invariant.
>
> The original testcase is probably more "sensible", does it still have
> a loop invariant controlling condition and a loop invariant computation
> under that control?
In my tmd/binary32/hrcases.c file, there doesn't seem to be a loop invariant,
so I'm wondering what is the real cause. The code looks like the following:
static inline double cldiff (clock_t t1, clock_t t0)
{
return (double) (t1 - t0) / CLOCKS_PER_SEC;
}
and in a function hrsearch() where its mprog argument (named c above) is an
integer that enables progress output when it is nonzero:
if (mprog)
{
mctr = 0;
nctr = 0;
t0 = ti = clock ();
}
do
{
[...]
if (mprog && ++mctr == mprog)
{
mctr = 0;
tj = clock ();
mpfr_fprintf (stderr, "[exponent %ld: %8.2fs %8.2fs %5lu / %lu]\n",
e, cldiff (tj, ti), cldiff (tj, t0), ++nctr, nprog);
ti = tj;
}
[...]
}
while (mpfr_get_exp (x) < e + 2);
The warning I get is
In function ‘cldiff’,
inlined from ‘hrsearch’ at hrcases.c:298:11,
inlined from ‘main’ at hrcases.c:520:9:
hrcases.c:46:23: warning: ‘t0’ may be used uninitialized
[-Wmaybe-uninitialized]
46 | return (double) (t1 - t0) / CLOCKS_PER_SEC;
| ~~~~^~~~~
hrcases.c: In function ‘main’:
hrcases.c:128:11: note: ‘t0’ was declared here
128 | clock_t t0, ti, tj;
| ^~
So the operation on t0 is tj - t0, and as tj is set just before, I don't see
how it can be used in a loop invariant.
This can be simplified as follows:
int f (int);
void g (int mprog)
{
int t0, ti, tj;
if (mprog)
t0 = ti = f(0);
do
if (mprog)
{
tj = f(0);
f(tj - ti);
f(tj - t0);
ti = tj;
}
while (f(0));
}
and I get
tst.c: In function ‘g’:
tst.c:13:9: warning: ‘t0’ may be used uninitialized [-Wmaybe-uninitialized]
13 | f(tj - ti);
| ^~~~~~~~~~
tst.c:4:7: note: ‘t0’ was declared here
4 | int t0, ti, tj;
| ^~
BTW, the warning is incorrect: I can't see t0 in "f(tj - ti);".
