Re: [PATCH] libgcc: Fix up __divmodbitint4 [PR114755]

[Richard Biener <rguenther@suse.de>]

On Thu, 18 Apr 2024, Jakub Jelinek wrote:

> Hi!
> 
> The following testcase aborts on aarch64-linux but does not on x86_64-linux.
> In both cases there is UB in the __divmodbitint4 implemenetation.
> When the divisor is negative with most significant limb (even when partial)
> all ones, has at least 2 limbs and the second most significant limb has the
> most significant bit clear, when this number is negated, it will have 0
> in the most significant limb.
> Already in the PR114397 r14-9592 fix I was dealing with such divisors, but
> thought the problem is only if because of that un < vn doesn't imply the
> quotient is 0 and remainder u.
> But as this testcase shows, the problem is with such divisors always.
> What happens is that we use __builtin_clz* on the most significant limb,
> and assume it will not be 0 because that is UB for the builtins.
> Normally the most significant limb of the divisor shouldn't be 0, as
> guaranteed by the bitint_reduce_prec e.g. for the positive numbers, unless
> the divisor is just 0 (but for vn == 1 we have special cases).
> 
> The following patch moves the handling of this corner case a few lines
> earlier before the un < vn check, because adjusting the vn later is harder.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, plus tested with
> make check-gcc -j32 -k GCC_TEST_RUN_EXPENSIVE=1 RUNTESTFLAGS="GCC_TEST_RUN_EXPENSIVE=1 dg.exp='*bitint* pr112673.c builtin-stdc-bit-*.c pr112566-2.c pr112511.c' dg-torture.exp=*bitint* dfp.exp=*bitint*"
> on aarch64-linux, ok for trunk?

OK.

Thanks,
Richard.

> 2024-04-18  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR libgcc/114755
> 	* libgcc2.c (__divmodbitint4): Perform the decrement on negative
> 	v with most significant limb all ones and the second least
> 	significant limb with most significant bit clear always, regardless of
> 	un < vn.
> 
> 	* gcc.dg/torture/bitint-69.c: New test.
> 
> --- libgcc/libgcc2.c.jj	2024-03-21 13:07:43.629886730 +0100
> +++ libgcc/libgcc2.c	2024-04-17 19:00:55.453691368 +0200
> @@ -1705,69 +1705,62 @@ __divmodbitint4 (UBILtype *q, SItype qpr
>    USItype rn = ((USItype) rprec + W_TYPE_SIZE - 1) / W_TYPE_SIZE;
>    USItype up = auprec % W_TYPE_SIZE;
>    USItype vp = avprec % W_TYPE_SIZE;
> +  /* If vprec < 0 and the top limb of v is all ones and the second most
> +     significant limb has most significant bit clear, then just decrease
> +     vn/avprec/vp, because after negation otherwise v2 would have most
> +     significant limb clear.  */
> +  if (vprec < 0
> +      && ((v[BITINT_END (0, vn - 1)] | (vp ? ((UWtype) -1 << vp) : 0))
> +	  == (UWtype) -1)
> +      && vn > 1
> +      && (Wtype) v[BITINT_END (1, vn - 2)] >= 0)
> +    {
> +      vp = 0;
> +      --vn;
> +#if __LIBGCC_BITINT_ORDER__ == __ORDER_BIG_ENDIAN__
> +      ++v;
> +#endif
> +    }
>    if (__builtin_expect (un < vn, 0))
>      {
> -      /* If abs(v) > abs(u), then q is 0 and r is u.
> -	 Unfortunately un < vn doesn't always mean abs(v) > abs(u).
> -	 If uprec > 0 and vprec < 0 and vn == un + 1, if the
> -	 top limb of v is all ones and the second most significant
> -	 limb has most significant bit clear, then just decrease
> -	 vn/avprec/vp and continue, after negation both numbers
> -	 will have the same number of limbs.  */
> -      if (un + 1 == vn
> -	  && uprec >= 0
> -	  && vprec < 0
> -	  && ((v[BITINT_END (0, vn - 1)] | (vp ? ((UWtype) -1 << vp) : 0))
> -	      == (UWtype) -1)
> -	  && (Wtype) v[BITINT_END (1, vn - 2)] >= 0)
> -	{
> -	  vp = 0;
> -	  --vn;
> +      /* q is 0 and r is u.  */
> +      if (q)
> +	__builtin_memset (q, 0, qn * sizeof (UWtype));
> +      if (r == NULL)
> +	return;
>  #if __LIBGCC_BITINT_ORDER__ == __ORDER_BIG_ENDIAN__
> -	  ++v;
> +      r += rn - 1;
> +      u += un - 1;
>  #endif
> +      if (up)
> +	--un;
> +      if (rn < un)
> +	un = rn;
> +      for (rn -= un; un; --un)
> +	{
> +	  *r = *u;
> +	  r += BITINT_INC;
> +	  u += BITINT_INC;
>  	}
> -      else
> +      if (!rn)
> +	return;
> +      if (up)
>  	{
> -	  /* q is 0 and r is u.  */
> -	  if (q)
> -	    __builtin_memset (q, 0, qn * sizeof (UWtype));
> -	  if (r == NULL)
> +	  if (uprec > 0)
> +	    *r = *u & (((UWtype) 1 << up) - 1);
> +	  else
> +	    *r = *u | ((UWtype) -1 << up);
> +	  r += BITINT_INC;
> +	  if (!--rn)
>  	    return;
> -#if __LIBGCC_BITINT_ORDER__ == __ORDER_BIG_ENDIAN__
> -	  r += rn - 1;
> -	  u += un - 1;
> -#endif
> -	  if (up)
> -	    --un;
> -	  if (rn < un)
> -	    un = rn;
> -	  for (rn -= un; un; --un)
> -	    {
> -	      *r = *u;
> -	      r += BITINT_INC;
> -	      u += BITINT_INC;
> -	    }
> -	  if (!rn)
> -	    return;
> -	  if (up)
> -	    {
> -	      if (uprec > 0)
> -		*r = *u & (((UWtype) 1 << up) - 1);
> -	      else
> -		*r = *u | ((UWtype) -1 << up);
> -	      r += BITINT_INC;
> -	      if (!--rn)
> -		return;
> -	    }
> -	  UWtype c = uprec < 0 ? (UWtype) -1 : (UWtype) 0;
> -	  for (; rn; --rn)
> -	    {
> -	      *r = c;
> -	      r += BITINT_INC;
> -	    }
> -	  return;
>  	}
> +      UWtype c = uprec < 0 ? (UWtype) -1 : (UWtype) 0;
> +      for (; rn; --rn)
> +	{
> +	  *r = c;
> +	  r += BITINT_INC;
> +	}
> +      return;
>      }
>    USItype qn2 = un - vn + 1;
>    if (qn >= qn2)
> --- gcc/testsuite/gcc.dg/torture/bitint-69.c.jj	2024-04-17 19:09:34.165521448 +0200
> +++ gcc/testsuite/gcc.dg/torture/bitint-69.c	2024-04-17 19:10:25.343814139 +0200
> @@ -0,0 +1,26 @@
> +/* PR libgcc/114755 */
> +/* { 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
> +_BitInt(65)
> +foo (void)
> +{
> +  _BitInt(255) a = 0x040404040404040404040404wb;
> +  _BitInt(65) b = -0xffffffffffffffffwb;
> +  _BitInt(65) r = a % b;
> +  return r;
> +}
> +#endif
> +
> +int
> +main ()
> +{
> +#if __BITINT_MAXWIDTH__ >= 255
> +  _BitInt(65) x = foo ();
> +  if (x != 0x0404040408080808wb)
> +    __builtin_abort ();
> +#endif
> +}
> 
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)