Re: [PATCH] libgcc: Fix quotient and/or remainder negation in __divmodbitint4 [PR114327]

[Joseph Myers <josmyers@redhat.com>]

On Fri, 15 Mar 2024, Jakub Jelinek wrote:

> Hi!
> 
> While for __mulbitint3 we actually don't negate anything and perform the
> multiplication in unsigned style always, for __divmodbitint4 if the operands
> aren't unsigned and are negative, we negate them first and then try to
> negate them as needed at the end.
> quotient is negated if just one of the operands was negated and the other
> wasn't or vice versa, and remainder is negated if the first operand was
> negated.
> The case which doesn't work correctly is if due to limited range of the
> operands we perform the division/modulo in some smaller number of limbs
> and then extend it to the desired precision of the quotient and/or
> remainder results.  If they aren't negated, the extension is done with
> memset to 0, if they are negated, the extension was done with memset
> to -1.  The problem is that if the quotient or remainder is zero,
> then bitint_negate negates it again to zero (that is ok), but we should
> then extend with memset to 0, not memset to -1.
> 
> The following patch achieves that by letting bitint_negate also check if
> the negated operand is zero and changes the memset argument based on that.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK.

-- 
Joseph S. Myers
josmyers@redhat.com