| Message ID | ZiIMeqO9BNUPw44k@tucnak |
|---|---|
| State | New |
| Headers | show |
| Series | libgcc: Another __divmodbitint4 bug fix [PR114762] | expand |
On Fri, 19 Apr 2024, Jakub Jelinek wrote: > Hi! > > The following testcase is miscompiled because the code to decrement > vn on negative value with all ones in most significant limb (even partial) > and 0 in most significant bit of the second most significant limb doesn't > take into account the case where all bits below the most significant limb > are zero. This has been a problem both in the version before yesterday's > commit where it has been done only if un was one shorter than vn before this > decrement, and is now problem even more often when it is done earlier. > When we decrement vn in such case and negate it, we end up with all 0s in > the v2 value, so have both the problems with UB on __builtin_clz* and the > expectations of the algorithm that the divisor has most significant bit set > after shifting, plus when the decremented vn is 1 it can SIGFPE on division > by zero even when it is not division by zero etc. Other values shouldn't > get 0 in the new most significant limb after negation, because the > bitint_reduce_prec canonicalization should reduce prec if the second most > significant limb is all ones and if that limb is all zeros, if at least > one limb below it is non-zero, carry in will make it non-zero. > > The following patch fixes it by checking if at least one bit below the > most significant limb is non-zero, in that case it decrements, otherwise > it will do nothing (but e.g. for the un < vn case that also means the > divisor is large enough that the result should be q 0 r u). > > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? OK. Richard. > 2024-04-19 Jakub Jelinek <jakub@redhat.com> > > PR libgcc/114762 > * libgcc2.c (__divmodbitint4): Don't decrement vn if all bits > below the most significant limb are zero. > > * gcc.dg/torture/bitint-70.c: New test. > > --- libgcc/libgcc2.c.jj 2024-04-18 09:48:55.172538667 +0200 > +++ libgcc/libgcc2.c 2024-04-18 12:17:28.893616007 +0200 > @@ -1715,11 +1715,18 @@ __divmodbitint4 (UBILtype *q, SItype qpr > && vn > 1 > && (Wtype) v[BITINT_END (1, vn - 2)] >= 0) > { > - vp = 0; > - --vn; > + /* Unless all bits below the most significant limb are zero. */ > + SItype vn2; > + for (vn2 = vn - 2; vn2 >= 0; --vn2) > + if (v[BITINT_END (vn - 1 - vn2, vn2)]) > + { > + vp = 0; > + --vn; > #if __LIBGCC_BITINT_ORDER__ == __ORDER_BIG_ENDIAN__ > - ++v; > + ++v; > #endif > + break; > + } > } > if (__builtin_expect (un < vn, 0)) > { > --- gcc/testsuite/gcc.dg/torture/bitint-70.c.jj 2024-04-18 12:26:09.406383158 +0200 > +++ gcc/testsuite/gcc.dg/torture/bitint-70.c 2024-04-18 12:26:57.253718287 +0200 > @@ -0,0 +1,22 @@ > +/* PR libgcc/114762 */ > +/* { dg-do run { target bitint } } */ > +/* { dg-options "-std=c23" } */ > +/* { dg-skip-if "" { ! run_expensive_tests } { "*" } { "-O0" "-O2" } } */ > +/* { dg-skip-if "" { ! run_expensive_tests } { "-flto" } { "" } } */ > + > +#if __BITINT_MAXWIDTH__ >= 255 > +__attribute__((__noipa__)) signed _BitInt(255) > +foo (signed _BitInt(255) a, signed _BitInt(65) b) > +{ > + return a / b; > +} > +#endif > + > +int > +main () > +{ > +#if __BITINT_MAXWIDTH__ >= 255 > + if (foo (1, -0xffffffffffffffffwb - 1wb)) > + __builtin_abort (); > +#endif > +} > > Jakub > >
--- libgcc/libgcc2.c.jj 2024-04-18 09:48:55.172538667 +0200 +++ libgcc/libgcc2.c 2024-04-18 12:17:28.893616007 +0200 @@ -1715,11 +1715,18 @@ __divmodbitint4 (UBILtype *q, SItype qpr && vn > 1 && (Wtype) v[BITINT_END (1, vn - 2)] >= 0) { - vp = 0; - --vn; + /* Unless all bits below the most significant limb are zero. */ + SItype vn2; + for (vn2 = vn - 2; vn2 >= 0; --vn2) + if (v[BITINT_END (vn - 1 - vn2, vn2)]) + { + vp = 0; + --vn; #if __LIBGCC_BITINT_ORDER__ == __ORDER_BIG_ENDIAN__ - ++v; + ++v; #endif + break; + } } if (__builtin_expect (un < vn, 0)) { --- gcc/testsuite/gcc.dg/torture/bitint-70.c.jj 2024-04-18 12:26:09.406383158 +0200 +++ gcc/testsuite/gcc.dg/torture/bitint-70.c 2024-04-18 12:26:57.253718287 +0200 @@ -0,0 +1,22 @@ +/* PR libgcc/114762 */ +/* { dg-do run { target bitint } } */ +/* { dg-options "-std=c23" } */ +/* { dg-skip-if "" { ! run_expensive_tests } { "*" } { "-O0" "-O2" } } */ +/* { dg-skip-if "" { ! run_expensive_tests } { "-flto" } { "" } } */ + +#if __BITINT_MAXWIDTH__ >= 255 +__attribute__((__noipa__)) signed _BitInt(255) +foo (signed _BitInt(255) a, signed _BitInt(65) b) +{ + return a / b; +} +#endif + +int +main () +{ +#if __BITINT_MAXWIDTH__ >= 255 + if (foo (1, -0xffffffffffffffffwb - 1wb)) + __builtin_abort (); +#endif +}