Re: -fprofile-update=atomic vs. 32-bit architectures

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

On Tue, Dec 6, 2022 at 2:11 PM Sebastian Huber
<sebastian.huber@embedded-brains.de> wrote:
>
> On 05/12/2022 08:44, Richard Biener wrote:
> > On Mon, Dec 5, 2022 at 8:26 AM Sebastian Huber
> > <sebastian.huber@embedded-brains.de>  wrote:
> >> On 08/11/2022 11:25, Richard Biener wrote:
> >>>> It would be great to have a code example for the construction of the "if
> >>>> (f()) f();".
> >>> I think for the function above we need to emit __atomic_fetch_add_8,
> >>> not the emulated form because we cannot insert the required control
> >>> flow (if (f()) f()) on an edge.  The __atomic_fetch_add_8 should then be
> >>> lowered after the instrumentation took place.
> >> Would it help to change the
> >>
> >>       if (__atomic_add_fetch_4 ((unsigned int *) &val, 1, __ATOMIC_RELAXED)
> >> == 0)
> >>         __atomic_fetch_add_4 (((unsigned int *) &val) + 1, 1,
> >> __ATOMIC_RELAXED);
> >>
> >> into
> >>
> >>       unsigned int v = __atomic_add_fetch_4 ((unsigned int *) &val, 1,
> >> __ATOMIC_RELAXED)
> >> == 0)
> >>       v = (unsigned int)(v == 0);
> >>       __atomic_fetch_add_4 (((unsigned int *) &val) + 1, 1,
> >> __ATOMIC_RELAXED);
> > that's supposed to add 'v' instead of 1?  Possibly use uint32_t here
> > (aka uint32_type_node).
> >
> >> to get rid of an inserted control flow?
> > That for sure wouldn't require any changes to how the profile
> > instrumentation works,
> > so yes it would be simpler.
>
> Yes, this seems to work. After a bit of trial and error I ended up with
> something in gimple_gen_edge_profiler() like this (endian support is
> missing):
>
>    else if (flag_profile_update == PROFILE_UPDATE_SPLIT_ATOMIC)
>      {
>        tree addr = tree_coverage_counter_addr (GCOV_COUNTER_ARCS, edgeno);
>        tree f = builtin_decl_explicit (BUILT_IN_ATOMIC_ADD_FETCH_4);
>        gcall *stmt1 = gimple_build_call (f, 3, addr, one,
>                                         build_int_cst (integer_type_node,
>                                                       MEMMODEL_RELAXED));
>        tree low = create_tmp_var (uint32_type_node);
>        gimple_call_set_lhs (stmt1, low);
>        tree is_zero = create_tmp_var (boolean_type_node);
>        gassign *stmt2 = gimple_build_assign (is_zero, EQ_EXPR, low,
>                                             build_zero_cst (uint32_type_node));
>        tree high_inc = create_tmp_var (uint32_type_node);
>        gassign *stmt3 = gimple_build_assign (high_inc, COND_EXPR, is_zero,
>                                             build_one_cst (uint32_type_node),
>                                             build_zero_cst (uint32_type_node));
>        tree addr_high = create_tmp_var (TREE_TYPE (addr));
>        gassign *stmt4 = gimple_build_assign (addr_high, addr);
>        gassign *stmt5 = gimple_build_assign (addr_high, POINTER_PLUS_EXPR,
>                                             addr_high,
>                                             build_int_cst (size_type_node, 4));
>        gcall *stmt6 = gimple_build_call (f, 3, addr_high, high_inc,
>                                         build_int_cst (integer_type_node,
>                                                        MEMMODEL_RELAXED));
>        gsi_insert_on_edge (e, stmt1);
>        gsi_insert_on_edge (e, stmt2);
>        gsi_insert_on_edge (e, stmt3);
>        gsi_insert_on_edge (e, stmt4);
>        gsi_insert_on_edge (e, stmt5);
>        gsi_insert_on_edge (e, stmt6);
>      }
>
> It can be probably simplified.

Likely.  I'd use the gimple_build () API from gimple-fold.h which
builds the expression(s) to a gimple_seq creating necessary temporaries
on-the-fly and then insert that sequence on the edge.

But even the above should work.

 The generated code:
>
>          .type   f, @function
> f:
>          lui     a4,%hi(__gcov0.f)
>          li      a3,1
>          addi    a4,a4,%lo(__gcov0.f)
>          amoadd.w a5,a3,0(a4)
>          lui     a4,%hi(__gcov0.f+4)
>          addi    a5,a5,1
>          seqz    a5,a5
>          addi    a4,a4,%lo(__gcov0.f+4)
>          amoadd.w zero,a5,0(a4)
>          li      a0,3
>          ret
>
> looks good for this code:
>
> int f(void)
> {
>    return 3;
> }
>
> The loading of the high address could be probably optimized from
>
>          lui     a4,%hi(__gcov0.f+4)
>          addi    a4,a4,%lo(__gcov0.f+4)
>
> to
>
>          addi    a4,a4,4
>
> I wasn't able to figure out how to do this.

I think that's something for the backend - we're not good
CSEing parts of an "invariant" address and the above might
be the form required when relocations are needed.

Richard.

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