Re: Query regarding pointer analysis

[Ian Lance Taylor <iant at google dot com>]

Mukta Joglekar <muktaj@cse.iitb.ac.in> writes:

> We are trying to find how pointer analysis information is being used
> by other optimizations in gcc. We disovered that pointer information
> is stored as a bitmap which is accessed via a macro named
> "SSA_NAME_PTR_INFO". However, when we tried to find where this macro
> is being used, we came up with following observations:
>
> For the example given below:
>
> int main()
> {
>         int *a,*b;
>
>         int p,q,r;
>
>         a = &r;
>
>         *a = p + q;
>
>         b = &p;
>
>         *b = p + q;
>
>         printf("%d,%d",*a,*b);
>
> }
>
> There is scope for copy propagation, and the program does get
> optimized stating that *a should be equal to *b. Pass
> "COPY_PROPAGATION" has a call to SSA_NAME_PTR_INFO wherein it
> duplicates the points-to information in case of copy propagation.
> However, for the example given above, this macro is not being called.
> Here, we realized that the pointer dereferences are already converted
> to scalar variables in the first execution of the pass "CONDITIONAL
> CONSTANT PROPAGATION". However, this pass also does not seem to call
> the macro SSA_NAME_PTR_INFO.
>
> Thus, we are not sure how exactly this conversion takes place.
>
> In general, we need answers for the following questions :
> 1. How are the pointer dereferences converted to scalar variables and
> whether points-to information is being used for this conversion or
> not?
> 2. Which passes use pointer information?
> 3. How does the pointer information exactly help these passes in
> optimizations?
> 4. How do the passes that execute before the pta pass deal with
> pointers? For instance following passes have one execution before
> pass_ipa_pta and another execution after it.
>         pass_ccp
>         pass_build_ealias
>         pass_sra_early
>         pass_fre
>         pass_copy_prop
>         pass_merge_phi
>         pass_cd_dce
>
> Can anyone help us with this issue?


I don't offhand know the answers to your questions.  Still, some notes.

You didn't mention which version of GCC you are using.  This code
changes in every version of GCC.

If you use -fdump-tree-all-vops you will get tree dumps for each pass
that show the tree-dependent aliasing information.  Look for VUSE and
VDEF lines.

The most recent versions of GCC use an aliasing oracle anyhow, in
gcc/tree-ssa-alias.c.  Look at how that code works.

Ian