Re: C as intermediate language, signed integer overflow and -ftrapv

[Richard Biener <richard dot guenther at gmail dot com>]

On Fri, Jul 25, 2014 at 10:43 AM, Thomas Mertes <thomas.mertes@gmx.at> wrote:
> On Thu, Jul 24 at 10:36 PM, Richard Biener <richard.guenther@gmail.com> wrote:
>> Fact is that if somebody is interested in
>> -ftrapv he/she is welcome to contribute patches.  Especially testing
>> coverage is poor.
>
> As I said I have test programs for integer overflow (not written
> in C). Now I have converted one test program to C. This program
> checks if an int64_t overflow raises SIGABRT or SIGILL. The name of
> the program is chkovf64.c and I have uploaded it to
>
>   http://sourceforge.net/projects/seed7/files/
>
> It is licenced with GPL. You can use it to improve the testing
> coverage of gcc. When I compile it with:
>
>   gcc -ftrapv chkovf64.c -o chkovf64
>
> it writes a lot of warnings about "integer overflow in expression".
> Running chkovf64 shows that -ftrapv does not work correct.

That's https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61893 - basically
as soon as we can constant-fold we lose the trap.  Which is probably
not important for practical purposes, but you have a point here.

Note the slight complication with static initializers that _do_ have
to simplify to something (well, maybe not with -ftrapv ...).  Joseph,
I can easily make fold fail if you don't explicitely use a constant
folding API (int_const_binop) and -ftrapv is set.  Is it valid to
reject

static int x = __INT_MAX__ + 1;

with an initializer-not-constant error?

Index: fold-const.c
===================================================================
--- fold-const.c        (revision 212388)
+++ fold-const.c        (working copy)
@@ -1121,7 +1121,12 @@
   STRIP_NOPS (arg2);

   if (TREE_CODE (arg1) == INTEGER_CST)
-    return int_const_binop (code, arg1, arg2);
+    {
+      tree res = int_const_binop (code, arg1, arg2);
+      if (res && TYPE_OVERFLOW_TRAPS (TREE_TYPE (arg1)) && TREE_OVERFLOW (res))
+        return NULL_TREE;
+      return res;
+    }

   if (TREE_CODE (arg1) == REAL_CST)
     {


> It seems that gcc thinks that writing a warning is enough and
> raising a signal at runtime is not necessary. For human
> programmers this makes sense, since they read the warnings and
> correct the code. But for generated C programs this is not the
> right strategy. There are different needs when C is used as
> intermediate language.
>
> Maybe all -ftrapv problems uncovered by chkovf64.c are because
> of this. Unfortunately there are also other test cases where
> a signal is not raised although a signed integer overflow occurred.
> This happens in a much bigger program and until now I was not
> able to create a simple test case from it.

Yes, not all optimizations may be aware of -ftrapv.

Richard.

> I used clang version 3.4-1 to proof that chkovf64.c works correct.
> When I compile it with:
>
>   clang -ftrapv chkovf64.c -o chkovf64
>
> and start chkovf64 afterwards it writes:
>
> Overflow checking of negation works correct.
> Overflow checking of addition works correct.
> Overflow checking of addition assignment works correct.
> Overflow checking of subtraction works correct.
> Overflow checking of subtraction assignment works correct.
> Overflow checking of incr works correct.
> Overflow checking of decr works correct.
> Overflow checking of multiplication works correct.
> Overflow checking of multiplication assignment works correct.
>
> Greetings Thomas Mertes
>
> --
> Seed7 Homepage:  http://seed7.sourceforge.net
> Seed7 - The extensible programming language: User defined statements
> and operators, abstract data types, templates without special
> syntax, OO with interfaces and multiple dispatch, statically typed,
> interpreted or compiled, portable, runs under linux/unix/windows.