On Sat, Oct 29, 2016 at 05:47:54AM +0000, Bernd Edlinger wrote:
> On 10/28/16 16:05, Bernd Edlinger wrote:
> > On 10/27/16 22:23, Joseph Myers wrote:
> >> On Thu, 27 Oct 2016, Bernd Edlinger wrote:
> >>
> >>> Hi,
> >>>
> >>> by code reading I became aware that libgcc can call count_leading_zeros
> >>> in certain cases which can give undefined results. This happens on
> >>> signed int128 -> float or double conversions, when the int128 is in
> >>> the range
> >>> INT64_MAX+1 to UINT64_MAX.
> >>
> >> I'd expect testcases added to the testsuite that exercise this case at
> >> runtime, if not already present.
> >>
> >
> > Yes, thanks. I somehow expected there were already test cases,
> > somewhere, but now when you ask that, I begin to doubt as well...
> >
> > I will try to add an asm("int 3") and see if that gets hit at all.
> >
>
> The breakpoint got hit only once, in the libgo testsuite: runtime/pprof.
>
> I see there are some int to float conversion tests at
> gcc.dg/torture/fp-int-convert*.c, where it is easy to add a
> test case that hits the breakpoint too.
>
> However the test case does not fail before the patch,
> it is just slightly undefined behavior, that is not
> causing problems (at least for x86_64).
>
> Find attached a new patch with test case.
These new test cases look like they are going to be out of exponent range
for _Float16 - so the testcases will fail for a target which tests either
of:
gcc.dg/torture/fp-int-convert-float16.c
gcc.dg/torture/fp-int-convert-float16-timode.c
Should they have an M_OK1 check?
Thanks,
James
>
>
> Boot-strapped on x86_64-pc-linux-gnu.
> Is it OK for trunk?
>
>
> Thanks
> Bernd.
> 2016-10-27 Bernd Edlinger <[email protected]>
>
> PR libgcc/78067
> * libgcc2.c (__floatdisf, __floatdidf): Avoid undefined results from
> count_leading_zeros.
>
> testsuite:
> 2016-10-27 Bernd Edlinger <[email protected]>
>
> PR libgcc/78067
> * gcc.dg/torture/fp-int-convert.h: Add more conversion tests.
>
>
> Index: libgcc2.c
> ===================================================================
> --- libgcc2.c (revision 241400)
> +++ libgcc2.c (working copy)
> @@ -1643,6 +1643,11 @@
> hi = -(UWtype) hi;
>
> UWtype count, shift;
> +#if !defined (COUNT_LEADING_ZEROS_0) || COUNT_LEADING_ZEROS_0 != W_TYPE_SIZE
> + if (hi == 0)
> + count = W_TYPE_SIZE;
> + else
> +#endif
> count_leading_zeros (count, hi);
>
> /* No leading bits means u == minimum. */
> Index: gcc/testsuite/gcc.dg/torture/fp-int-convert.h
> ===================================================================
> --- gcc/testsuite/gcc.dg/torture/fp-int-convert.h (revision 241647)
> +++ gcc/testsuite/gcc.dg/torture/fp-int-convert.h (working copy)
> @@ -53,6 +53,8 @@ do {
> \
> TEST_I_F_VAL (U, F, HVAL1U (P, U), P_OK (P, U)); \
> TEST_I_F_VAL (U, F, HVAL1U (P, U) + 1, P_OK (P, U)); \
> TEST_I_F_VAL (U, F, HVAL1U (P, U) - 1, P_OK (P, U)); \
> + TEST_I_F_VAL (I, F, WVAL0S (I), 1); \
> + TEST_I_F_VAL (I, F, -WVAL0S (I), 1); \
> } while (0)
>
> #define P_OK(P, T) ((P) >= sizeof(T) * CHAR_BIT)
> @@ -74,6 +76,7 @@ do {
> \
> ? (S)1 \
> : (((S)1 << (sizeof(S) * CHAR_BIT - 2)) \
> + ((S)3 << (sizeof(S) * CHAR_BIT - 2 - P))))
> +#define WVAL0S(S) (S)((S)1 << (sizeof(S) * CHAR_BIT / 2 - 1))
>
> #define TEST_I_F_VAL(IT, FT, VAL, PREC_OK) \
> do { \
