[PATCH] handle local aggregate initialization in strlen (PR 83821)

[Jeff Law]

On 01/12/2018 02:30 PM, Martin Sebor wrote:
> A failure in a test for the recently enhanced -Warray-bounds
> warning exposed an unnecessarily broad restriction in the strlen
> pass that prevents it from tracking the length of a member string
> of locally defined and initialized struct:
> 
>   void f (void)
> Â Â {
>     struct { char s[8]; int i } a = { "1234", 5 };
> 
>     if (strlen (a.s) != 4)   // not folded
>       abort ();
> Â Â Â }
> 
> IIUC, the restriction was in place to account for writes into
> an array changing or invalidating the length of a string stored
> in its initial elements.  This would happen if the write either
> changed the string's terminating nul byte, or if it reset one
> of the prior non-nul bytes.
> 
> To reflect just this intent the restriction can be tightened
> up to improve the pass' ability to track even the lengths of
> string members of locally initialized aggregates.  Besides
> leading to better code this change also clears up the test
> failure.
> 
> Tested on x86_64-linux.
> 
> Martin
> 
> 
> gcc-83821.diff
> 
> 
> PR tree-optimization/83821 - local aggregate initialization defeats strlen optimization
> 
> gcc/ChangeLog:
> 
> 	PR tree-optimization/83821
> 	* tree-ssa-strlen.c (maybe_invalidate): Consider the length of
> 	a string when available.
> 	(handle_char_store): Reset calloc statement on a non-nul store.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR tree-optimization/83821
> 	* c-c++-common/Warray-bounds-4.c: Remove XFAIL.
> 	* gcc.dg/strlenopt-43.c: New test.
> 	* gcc.dg/strlenopt-44.c: Same.
> 	* gcc.dg/tree-ssa/calloc-4.c: Same.
I see what you're trying to do.  But I'm really struggling to understand
Marc G's comment "Do not use si->nonzero_chars" since that's precisely
what your patch does.

Your patch seems reasonable on the surface, but I fear there's something
I'm missing.  Can you reach out to Marc G. to see if he recalls the
rational behind the comment.

The comment in its original form was introduced here:

commit 9f15ed6e5c148ded6e7942e75595d91151792c9b
Author: glisse <glisse@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Tue Jun 24 18:50:00 2014 +0000

    2014-06-24  Marc Glisse  <marc.glisse@inria.fr>

            PR tree-optimization/57742
    gcc/
            * tree-ssa-strlen.c (get_string_length): Ignore malloc.
            (handle_builtin_malloc, handle_builtin_memset): New functions.
            (strlen_optimize_stmt): Call them.
            * passes.def: Move strlen after loop+dom but before vrp.
    gcc/testsuite/
            * g++.dg/tree-ssa/calloc.C: New testcase.
            * gcc.dg/tree-ssa/calloc-1.c: Likewise.
            * gcc.dg/tree-ssa/calloc-2.c: Likewise.
            * gcc.dg/strlenopt-9.c: Adapt.


    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@211956
138bc75d-0d04-0410-961f-82ee72b054a4


Jeff