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Re: [PATCH] Make strlen range computations more conservative


On 08/06/2018 11:45 PM, Richard Biener wrote:
On August 7, 2018 5:38:59 AM GMT+02:00, Martin Sebor <msebor@gmail.com> wrote:
On 08/06/2018 11:40 AM, Jeff Law wrote:
On 08/06/2018 11:15 AM, Martin Sebor wrote:
These examples do not aim to be valid C, they just point out
limitations
of the middle-end design, and a good deal of the problems are due
to trying to do things that are not safe within the boundaries
given
by the middle-end design.
I really think this is important -- and as such I think we need to
move
away from trying to describe scenarios in C because doing so keeps
bringing us back to the "C doesn't allow XYZ" kinds of arguments
when
what we're really discussing are GIMPLE semantic issues.

So examples should be GIMPLE.  You might start with (possibly
invalid) C
code to generate the GIMPLE, but the actual discussion needs to be
looking at GIMPLE.  We might include the C code in case someone
wants to
look at things in a debugger, but bringing the focus to GIMPLE is
really
important here.

I don't understand the goal of this exercise.  Unless the GIMPLE
code is the result of a valid test case (in some language GCC
supports), what does it matter what it looks like?  The basis of
every single transformation done by a compiler is that the source
code is correct.  If it isn't then all bets are off.  I'm no GIMPLE
expert but even I can come up with any number of GIMPLE expressions
that have undefined behavior.  What would that prove?
The GIMPLE IL is less restrictive than the original source language.
The process of translation into GIMPLE and optimization can create
situations in the GIMPLE IL that can't be validly represented in the
original source language.  Subobject crossing being one such case,
there
are certainly others.  We have to handle these scenarios correctly.

Sure, but a valid C test case still needs to exist to show that
such a transformation is possible.  Until someone comes up with
one it's all speculation.

Jakub showed you one wrt CSE of addresses.

Sorry, there have been so many examples I've lost track.  Can
you please copy it here or point to it in the archive?

In any event, I would find it reasonable for the strlen
optimization to be subject to the same constraints as
the aggressive loop optimization.  If there are valid test
cases where the strlen optimization goes beyond that then
let's throttle those.  Doing more than that would be
arbitrary and result in confusing inconsistencies (as
the proposed patch does).  For example, these two equivalent
functions should continue to result in the same optimal code:

  extern char b[2][4];

  void f (int i)
  {
    if (__builtin_strlen (b[i]) >= sizeof b[i])
      __builtin_abort ();
  }

  void g (int i)
  {
    unsigned n = 0;
    while (b[i][n])
      ++n;
    if (n >= sizeof b[i])
      __builtin_abort ();
  }

Martin


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