Why do these two trees print differently

Gary Oblock gary@amperecomputing.com
Tue Jan 4 08:59:10 GMT 2022


Richard,

Well during my exploration of pretty printing I spent
a bunch of time looking at mem refs, hence the question.

I understand GIMPLE is not an AST. I worked on a
in house compiler someplace where the only IR was
an AST and it was a really horrible IR so I'm glad GIMPLE
isn't an AST. But the tree expressions seem to be chunks
of AST (with some other stuff thrown in that could be thought of
as node attributes.) Am I wrong to think of them that way?

Thanks,

Gary


________________________________
From: Richard Biener <richard.guenther@gmail.com>
Sent: Monday, January 3, 2022 11:28 PM
To: Gary Oblock <gary@amperecomputing.com>
Cc: gcc@gcc.gnu.org <gcc@gcc.gnu.org>
Subject: Re: Why do these two trees print differently

[EXTERNAL EMAIL NOTICE: This email originated from an external sender. Please be mindful of safe email handling and proprietary information protection practices.]


On Mon, Jan 3, 2022 at 9:16 PM Gary Oblock <gary@amperecomputing.com> wrote:
>
> Richard,
>
> I was able figure it out by looking for "MEM" is
> tree-pretty-print.c. There is the condition included
> at the end of the email (mostly to provoke a chuckle)
> necessary for the "p->f" format. If it's not true then
> the MEM form is emitted.

Yes, there's some loss of information so we don't
"pretty" print the MEM.

>
> What is most interesting from this whole exercise
> the question of why am I seeing offsets in
> the GIMPLE form? I'm seeing offsets where
> the symbolic form using field seems to make
> more sense. I'm also seeing accesses with
> offsets that are multiples of the structure size.
> That kind of idiom seems more appropriate at the
> RTL level.

That seems to be an unrelated question?  Note that GIMPLE
is much closer to RTL than you think - GIMPLE is _not_ an AST.
You see offsets whenever symbolic (COMPONENT_REF I suppose)
is eventually not semantically correct.

Richard.

>
> Thanks,
>
> Gary
>
> TREE_CODE (node) == MEM_REF
>   && integer_zerop (TREE_OPERAND (node, 1))
>   /* Dump the types of INTEGER_CSTs explicitly, for we can't
>      infer them and MEM_ATTR caching will share MEM_REFs
>      with differently-typed op0s.  */
>   && TREE_CODE (TREE_OPERAND (node, 0)) != INTEGER_CST
>   /* Released SSA_NAMES have no TREE_TYPE.  */
>   && TREE_TYPE (TREE_OPERAND (node, 0)) != NULL_TREE
>   /* Same pointer types, but ignoring POINTER_TYPE vs.
>      REFERENCE_TYPE.  */
>   && (TREE_TYPE (TREE_TYPE (TREE_OPERAND (node, 0)))
>       == TREE_TYPE (TREE_TYPE (TREE_OPERAND (node, 1))))
>   && (TYPE_MODE (TREE_TYPE (TREE_OPERAND (node, 0)))
>       == TYPE_MODE (TREE_TYPE (TREE_OPERAND (node, 1))))
>   && (TYPE_REF_CAN_ALIAS_ALL (TREE_TYPE (TREE_OPERAND (node, 0)))
>       == TYPE_REF_CAN_ALIAS_ALL (TREE_TYPE (TREE_OPERAND (node, 1))))
>   /* Same value types ignoring qualifiers.  */
>   && (TYPE_MAIN_VARIANT (TREE_TYPE (node))
>       == TYPE_MAIN_VARIANT
>       (TREE_TYPE (TREE_TYPE (TREE_OPERAND (node, 1)))))
>   && (!(flags & TDF_ALIAS)
>       || MR_DEPENDENCE_CLIQUE (node) == 0))
>
> ________________________________
> From: Richard Biener <richard.guenther@gmail.com>
> Sent: Monday, January 3, 2022 5:49 AM
> To: Gary Oblock <gary@amperecomputing.com>
> Cc: gcc@gcc.gnu.org <gcc@gcc.gnu.org>
> Subject: Re: Why do these two trees print differently
>
> [EXTERNAL EMAIL NOTICE: This email originated from an external sender. Please be mindful of safe email handling and proprietary information protection practices.]
>
>
> On Wed, Dec 15, 2021 at 7:10 AM Gary Oblock via Gcc <gcc@gcc.gnu.org> wrote:
> >
> > This is one of those things that has always puzzled
> > me so I thought I break down and finally ask.
> >
> > There are two ways a memory reference (tree) prints:
> >
> > MEM[(struct arc_t *)_684].flow
> >
> > and
> >
> > _684->flow
> >
> > Poking under the hood of them, the tree codes and
> > operands are identical so what am I missing?
>
> Try dumping with -gimple, that should show you the difference.
>
> >
> > Thanks,
> >
> > Gary
> >
> >
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