[PATCH] document BLOCK_ABSTRACT_ORIGIN et al.

[Richard Biener]

On Wed, Feb 3, 2021 at 6:12 PM Martin Sebor <msebor@gmail.com> wrote:
>
> On 2/3/21 5:01 AM, Richard Biener wrote:
> > On Mon, Feb 1, 2021 at 5:20 PM Martin Sebor <msebor@gmail.com> wrote:
> >>
> >> I have pushed the tree.h comments in g:6a2053773b8.  I will wait
> >> for an approval of the changes to the manual.
> >
> > Sorry for not looking earlier.
>
> Sorry, I thought you were fine with the text after your first review.
> I'll adjust the tree.h comments when we're done, though I'd like to
> think the example in the manual will do a lot more to help make it
> clear than the comments in tree.h can.
>
> >
> > +/* The scope enclosing the scope NODE, or FUNCTION_DECL for the "outermost"
> > +   function scope.  Inlined functions are chained by this so that given
> > +   expression E and its TREE_BLOCK(E) B, BLOCK_SUPERCONTEXT(B) is the scope
> > +   in which E has been made or into which E has been inlined.   */
> >
> > I can't really understand what you are trying to say with the second
> > sentence.  There's
> > nothing really special about BLOCK_SUPERCONTEXT and inlines so I believe this
> > sentence only adds confusion.
>
> The sentence explains how SUPERCONTEXT chains inlined blocks.  In
> the manual diff I show an example:
>
>          void f0 (char *p, int n) { memset (p, 1, n); }
>
>        void f1 (char *p, int n) { f0 (p + 1, n + 1); }
>
>      void f2 (char *p, int n) { f1 (p + 1, n + 1); }
>
>    int a[6];
>    void f3 (char *p, int n) { f2 (a, 3); }
>
> The blocks for all calls inlined into f3 are chained like so:
>
>        CALL_EXPR: memset              E
>
>        BLOCK #13             <--+     TREE_BLOCK (E)
>    +-- SUPERCONTEXT: BLOCK #12  |
>    |     ABSTRACT_ORIGIN: BLOCK #0 --+
>    |                            |    |
>    +-> BLOCK #12 (f1)        <--|-+  |
>    +-- SUPERCONTEXT: BLOCK #10  | |  |
>    |     SUBBLOCKS: BLOCK #13 --|-|  |
>    |     ABSTRACT_ORIGIN: f0 ---+ |  |
>    |                              |  |
>    +-> BLOCK #10 (f2)         <-+ |  |
>    +--- SUPERCONTEXT: BLOCK #8  | |  |
>    |    SUBBLOCKS: BLOCK #12 ---|-|  |
>    |    ABSTRACT_ORIGIN: f1 ------+  |
>    |                            |    |
>    +-> BLOCK #8 (f3)            |    |
>    +---- SUPERCONTEXT: BLOCK #0 |    |
>    |     SUBBLOCKS: BLOCK #10 --|    |
>    |     ABSTRACT_ORIGIN: f2 ---+    |
>    |                                 |
>    +-> BLOCK #0 (f3) <---------------+
>          SUPERCONTEXT: f3
>          SUBBLOCKS: BLOCK #8
>
> Does the following sound better? (Dropping the "in which E has been
> made.")
>
>    Inlined functions are chained by this so that given expression E
>    and its TREE_BLOCK(E) B, BLOCK_SUPERCONTEXT(B) is the scope into
>    which E has been inlined.

Oh, I see what you mean.  But this is misleading for the case where f0
has any blocks:

 f0 (..) { { int tem; { memset (..); } } if () { }... }

because BLOCK_SUPERCONTEXT is simply the parent scope ( { int tem; ... } ) and
not yet the artifical scope we generate to wrap f0.  To figure the
scope a block was
inlined to you'd have to do sth like

  b = TREE_BLOCK (E);
  gcc_assert (BLOCK_ABSTRACT_ORIGIN (b)); // it was inlined
  while (!inlined_function_outer_scope_p (b))
    b = BLOCK_SUPERCONTEXT (b);
  now BLOCK_SUPERCONTEXT (b) is the block the function containing E was
  inlined to.

So again, I think tree.h is not the place to document this.  There
BLOCK_SUPERCONTEXT
should simply say it's pointing  to the parent BLOCK.

In the texi documentation I'd separate out the representation of
inlines and clones,
eventually put it on the BLOCK_ABSTRACT_ORIGIN documentation.

[I did not review the texi part yet - I mainly want to avoid people
being even more
confused about the tree.h comments]

> >   #define BLOCK_SUPERCONTEXT(NODE) (BLOCK_CHECK (NODE)->block.supercontext)
> > +/* Points to the next scope at the same level of nesting as scope NODE.  */
> >   #define BLOCK_CHAIN(NODE) (BLOCK_CHECK (NODE)->block.chain)
> > +/* A BLOCK, or FUNCTION_DECL of the function from which a block has been
> > +   inlined.
> >
> > ... from which a block has been ultimatively copied for example by inlining.
> >
> > [clones also will have abstract origins]
> >
> >    In a scope immediately enclosing an inlined leaf expression,
> > +   points to the outermost scope into which it has been inlined (thus
> > +   bypassing all intermediate BLOCK_SUPERCONTEXTs). */
> >
> > ?
>
> This describes the long arrow on the right, pointing Block #13's
> ABSTRACT_ORIGIN down to Block #0.  All the other AO's point down
> to the next/enclosing block (arrows on the left).  I didn't expect
> this when I first worked with the blocks so it seemed like
> an important detail to mention.
>
> >
> > Maybe:  An inlined function is represented by a scope with
> > BLOCK_ABSTRACT_ORIGIN being the FUNCTION_DECL of the inlined function
> > containing the inlined functions scope tree as children.  All abstract origins
> > are ultimate, that is BLOCK_ABSTRACT_ORIGIN(NODE)
> > == BLOCK_ABSTRACT_ORIGIN(BLOCK_ABSTRACT_ORIGIN (NODE)).
>
> The first sentence sounds good to me as far as it goes but it
> doesn't capture the long arrow above.  (By children I assume you
> mean SUBBLOCKS, correct?)
>
> I don't follow what you're trying to say in the second sentence.
> The equality isn't true for Block #0 whose AO is null.  It also
> isn't true for Block #12 and the others whose AO is a DECL, not
> a block.
>
> What do you mean by "ultimate" in plain English?

Ultimate in the sense dwarf2out uses it.  DWARF wants to refer to
the abstract copy.  Originally (I think till GCC 9) BLOCK_ABSTRACT_ORIGIN
of a function inlined that had functions inlined into it pointed to the
inlined block of the inner inline but now it points to the original
outline copy BLOCK of the inner inline (uh, can you parse that?).
That simplifies things and is all we need.

> FWIW, if I were to try to explain it using the example I'd say
> only Block #13's AO is "ultimate:" it points down in the diagram
> to the block of the function into which the expression has
> ultimately been inlined.  The AO's of all the other intervening
> inlined blocks are the DECLs of the inlined callees (up-pointing
> arrows); they don't look ultimate to me in this sense.
>
> But however this is phrased I suspect it won't be perfectly clear
> without an example or a picture.

Which means giving partial info in tree.h isn't useful but confusing.

Richard.

>
> Martin
>
> >
> >   #define BLOCK_ABSTRACT_ORIGIN(NODE) (BLOCK_CHECK (NODE)->block.abstract_origin)
> >
> >
> >> On 1/27/21 5:54 PM, Martin Sebor wrote:
> >>> Attached is an updated patch for both tree.h and the internals manual
> >>> documenting the most important BLOCK_ macros and what they represent.
> >>>
> >>> On 1/21/21 2:52 PM, Martin Sebor wrote:
> >>>> On 1/18/21 6:25 AM, Richard Biener wrote:
> >>>>>> PS Here are my notes on the macros and the two related functions:
> >>>>>>
> >>>>>> BLOCK: Denotes a lexical scope.  Contains BLOCK_VARS of variables
> >>>>>> declared in it, BLOCK_SUBBLOCKS of scopes nested in it, and
> >>>>>> BLOCK_CHAIN pointing to the next BLOCK.  Its BLOCK_SUPERCONTEXT
> >>>>>> point to the BLOCK of the enclosing scope.  May have
> >>>>>> a BLOCK_ABSTRACT_ORIGIN and a BLOCK_SOURCE_LOCATION.
> >>>>>>
> >>>>>> BLOCK_SUPERCONTEXT: The scope of the enclosing block, or FUNCTION_DECL
> >>>>>> for the "outermost" function scope.  Inlined functions are chained by
> >>>>>> this so that given expression E and its TREE_BLOCK(E) B,
> >>>>>> BLOCK_SUPERCONTEXT(B) is the scope (BLOCK) in which E has been made
> >>>>>> or into which E has been inlined.  In the latter case,
> >>>>>>
> >>>>>> BLOCK_ORIGIN(B) evaluates either to the enclosing BLOCK or to
> >>>>>> the enclosing function DECL.  It's never null.
> >>>>>>
> >>>>>> BLOCK_ABSTRACT_ORIGIN(B) is the FUNCTION_DECL of the function into
> >>>>>> which it has been inlined, or null if B is not inlined.
> >>>>>
> >>>>> It's the BLOCK or FUNCTION it was inlined _from_, not were it was
> >>>>> inlined to.
> >>>>> It's the "ultimate" source, thus the abstract copy of the block or
> >>>>> function decl
> >>>>> (for the outermost scope, aka inlined_function_outer_scope_p).  It
> >>>>> corresponds
> >>>>> to what you'd expect for the DWARF abstract origin.
> >>>>
> >>>> Thanks for the correction!  It's just the "innermost" block that
> >>>> points to the "ultimate" destination into which it's been inlined.
> >>>>
> >>>>>
> >>>>> BLOCK_ABSTRACT_ORIGIN can be NULL (in case it isn't an inline instance).
> >>>>>
> >>>>>> BLOCK_ABSTRACT_ORIGIN: A BLOCK, or FUNCTION_DECL of the function
> >>>>>> into which a block has been inlined.  In a BLOCK immediately enclosing
> >>>>>> an inlined leaf expression points to the outermost BLOCK into which it
> >>>>>> has been inlined (thus bypassing all intermediate BLOCK_SUPERCONTEXTs).
> >>>>>>
> >>>>>> BLOCK_FRAGMENT_ORIGIN: ???
> >>>>>> BLOCK_FRAGMENT_CHAIN: ???
> >>>>>
> >>>>> that's for scope blocks split by hot/cold partitioning and only
> >>>>> temporarily
> >>>>> populated.
> >>>>
> >>>> Thanks, I now see these documented in detail in tree.h.
> >>>>
> >>>>>
> >>>>>> bool inlined_function_outer_scope_p(BLOCK)   [tree.h]
> >>>>>>      Returns true if a BLOCK has a source location.
> >>>>>>      True for all but the innermost (no SUBBLOCKs?) and outermost blocks
> >>>>>>      into which an expression has been inlined. (Is this always true?)
> >>>>>>
> >>>>>> tree block_ultimate_origin(BLOCK)   [tree.c]
> >>>>>>      Returns BLOCK_ABSTRACT_ORIGIN(BLOCK), AO, after asserting that
> >>>>>>      (DECL_P(AO) && DECL_ORIGIN(AO) == AO) || BLOCK_ORIGIN(AO) == AO).
> >>>>
> >>>> The attached diff adds the comments above to tree.h.
> >>>>
> >>>> I looked for a good place in the manual to add the same text but I'm
> >>>> not sure.  Would the Blocks @subsection in generic.texi be appropriate?
> >>>>
> >>>> Martin
> >>>
> >>>
> >>
>