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

[Sebastian Huber]

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. 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.

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