Re: Miscompilation of remainder expressions

[Andrew Haley <aph at redhat dot com>]

Gabriel Dos Reis writes:
 > Andrew Haley <aph@redhat.com> writes:
 > 
 > | Michael Veksler writes:
 > |  > Roberto Bagnara wrote:
 > |  > >
 > |  > > Reading the thread "Autoconf manual's coverage of signed integer
 > |  > > overflow & portability" I was horrified to discover about GCC's
 > |  > > miscompilation of the remainder expression that causes INT_MIN % -1
 > |  > > to cause a SIGFPE on CPUs of the i386 family.  Are there plans to
 > |  > > fix this bug (which, to me, looks quite serious)?
 > |  > > All the best,
 > |  > >
 > |  > This problem is quite rare in practice (otherwise there would be
 > |  > much more complaining). As such it may be too expensive,
 > |  > performance-wise, to fix in GCC. It seems as one of those
 > |  > classical things that can be worked-around in the kernel.
 > | 
 > | I guess, or it can be fixed-up in user space.  We already do this for
 > | the Java language.
 > 
 > Andrew pointed me to his implementation.  
 > I, however, have a question: why do we need to mess with signals at all?
 > I mean, could not we generate the following for "%":
 > 
 >     rem a b :=
 >       if abs(b) == 1 
 >          return 0
 >       return <machine-instruction> a b
 > 
 > is there any other corner case that will trigger the trap?

I have tried it, and it is pretty expensive.  It messes up the
pipelines and it hides the % from the optimizer.  IMO we should not do
this by default, since it's such a nitpicky corner case.

Andrew.