On August 25, 2020 3:09:13 PM GMT+02:00, Erick Ochoa
<erick.oc...@theobroma-systems.com> wrote:
>Hi,
>
>I'm trying to understand how the escape analysis in IPA-PTA works. I
>was
>testing a hypothesis where if a structure contains an array of
>characters and this array of characters is passed to fopen, the
>structure and all subfields will escape.
>
>To do this, I made a program that has a global structure variable foo2
>that is has a field passed as an argument to fopen. I also made another
>
>variable foo whose array is initialized by the result of rand.
>
>However, after compiling this program with -flto -flto-partition=none
>-fipa -fdump-ipa-pta -fdump-tree-all-all -Ofast (gcc --version 10.2.0)
>
>E.g.
>
>#include <stdio.h>
>#include <math.h>
>#include <string.h>
>
>struct foo_t {
> char buffer1[100];
> char buffer2[100];
>};
>
>struct foo_t foo;
>struct foo_t foo2;
>
>int
>main(int argc, char** argv)
>{
>
> fopen(foo2.buffer1, "r");
> for (int i = 0; i < 100; i++)
> {
> foo.buffer1[i] = rand();
> }
> int i = rand();
> int retval = foo.buffer1[i % 100];
> return retval;
>}
>
>I see the PTA dump state the following:
>
>ESCAPED = { STRING ESCAPED NONLOCAL foo2 }
>foo = { ESCAPED NONLOCAL }
>foo2 = { ESCAPED NONLOCAL }
>
>which I understand as
>* something externally visible might point to foo2
>* foo2 might point to something externally visible
>* foo might point to something externally visible
Yes. So it's exactly as your hypothesis.
>I have seen that global variables are stored in the .gnu.lto_.decls LTO
>
>file section. In the passes I have worked on I have ignored global
>variables. But can foo and foo2 be marked as escaping because the
>declarations are not streamed in yet? Or is there another reason I am
>not seeing? I am aware of aware of the several TODOs at the beginning
>of
>gcc/tree-ssa-structalias.c but I am unsure if they contribute to these
>variables being marked as escaping. (Maybe TODO 1 and TODO 2?)
Not sure what the problem is. Foo2 escapes because it's address is passed to a
function.
?
Richard.
>Just FYI, I've been reading:
>* Structure Aliasing in GCC
>* Gimple Alias Improvements for GCC 4.5
>* Memory SSA - A Unified Approach for Sparsely Representing Memory
>Operations
>
>Thanks, I appreciate all help!
