Re: how to find variable related to a virtual ssa name

[Richard Biener <richard.guenther@gmail.com>]

On Wed, May 13, 2020 at 6:03 AM 易会战 <huizhanyi@foxmail.com> wrote:
>
> It seems the function ptr_deref_may_alias_global_p cannot give right result.
> For example,
> int func(int size, int i)
> {
> int * sum;
> sum = malloc()
> ....here some code access sum pointing to memory
> return sum[i]
> }
> ptr_deref_may_alias_global_p tell me it is a local memory access. indeed sum is a local variable, but the pointer point to heap memory.
> In fact there is a similiar function ref_may_alias_global_p, and it give similiar result.

GCC can be clever and notice your malloc() result does not escape the function
which means stores to it are dead once you leave it.  For this reason
it does not
mark the memory global.  So make sure the allocated pointer escapes
and try again.

>
>
> ---Original---
> From: "Richard Biener"<richard.guenther@gmail.com>
> Date: Tue, May 12, 2020 22:20 PM
> To: "易会战"<huizhanyi@foxmail.com>;
> Cc: "gcc"<gcc@gcc.gnu.org>;
> Subject: Re: how to find variable related to a virtual ssa name
>
> On Tue, May 12, 2020 at 4:16 PM 易会战 <huizhanyi@foxmail.com> wrote:
> >
> > thanks a lot. I will check your advice.
> > Can you give some explaination about memory ssa, and how to use it. I check internal, cannot get it. Maybe you know some examples or some more materials.
>
> memory SSA in GCC is simply a SSA chain of all memory statements local
> to a function
> with a _single_ underlying variable (.MEM) and thus only one SSA name
> live at the same
> time.  It can be used to quickly traverse stores via use->def chains
> and loads inbetween
> two stores via immediate uses.
>
> Richard.
>
> > ---Original---
> > From: "Richard Biener"<richard.guenther@gmail.com>
> > Date: Tue, May 12, 2020 22:02 PM
> > To: "易会战"<huizhanyi@foxmail.com>;
> > Cc: "gcc"<gcc@gcc.gnu.org>;
> > Subject: Re: how to find variable related to a virtual ssa name
> >
> > On Tue, May 12, 2020 at 2:44 PM 易会战 via Gcc <gcc@gcc.gnu.org> wrote:
> > >
> > > hi, I am working on gcc ssa name. For each function, we can traverse all defined ssa name by macro FOR_EACH_SSA_NAME. If a ssa name is default definition for a symbol (check SSA_NAME_IS_DEFAULT_DEF) , I can get the symbol by SSA_NAME_VAR. But for a virtual DEFAULT DEF, I cannot get it, SSA_NAME_VAR return a identifier named .MEM. I cannot find which variable related to the default definition. Why and how I should find the related variable?
> > >
> > >
> > > By the way , I give my current work,&nbsp; I wish find a MEM_REF refer to global/heap memory or local stack. I try my best to get a correct memory type. Since MEM_REF have a base address, which is often a ssa name. Athough it is not virtual ssa name. But I find just check ssa name data flow is not enough to get the info.
> > > For example, a malloc function allocate some heap memory and record the address in a global ptr. On gimple ssa IR, the malloc function return a address assigned to a ssa name , then ssa name assign the value to the global ptr. When i check ssa name defined by the global ptr, I donot know if the ptr point to global memory or local memory.
> > > Please see the gimple code:
> > > _2 = malloc()
> > > ptr = _2
> > > _3 = ptr
> > > MEM_REF[BASE _3]
> > > I wish get _3&nbsp; is a address pointing to global memory. But just from _3=ptr, cannot judge it.&nbsp;
> > > I wish memory SSA can help solve the problem.
> >
> > memory SSA will not solve this problem.  You can instead query
> > points-to information
> > on _3 for example by calling ptr_deref_may_alias_global_p (_3) which internally
> > looks at SSA_NAME_PTR_INFO which contains the solution of the
> > points-to computation.
> >
> > Richard.
> >
> > >
> > > Or gcc gives the info at other pass? wish get some advice. Thanks a lot.