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

[Sebastian Huber <sebastian.huber@embedded-brains.de>]

On 05.11.22 12:18, Richard Biener wrote:
> On Fri, Nov 4, 2022 at 9:28 AM Sebastian Huber
> <sebastian.huber@embedded-brains.de>  wrote:
>> Hello,
>>
>> even recent 32-bit architectures such as RISC-V do not support 64-bit
>> atomic operations.  Using -fprofile-update=atomic for the 32-bit RISC-V
>> RV32GC ISA yields:
>>
>> warning: target does not support atomic profile update, single mode is
>> selected
>>
>> For multi-threaded applications it is quite important to use atomic
>> counter increments to get valid coverage data. I think this fall back is
>> not really good. Maybe we should consider using this approach from Jakub
>> Jelinek for 32-bit architectures lacking 64-bit atomic operations:
>>
>>     if (__atomic_add_fetch_4 ((unsigned int *) &val, 1, __ATOMIC_RELAXED)
>> == 0)
>>       __atomic_fetch_add_4 (((unsigned int *) &val) + 1, 1,
>> __ATOMIC_RELAXED);
>>
>> https://patchwork.ozlabs.org/project/gcc/patch/19c4a81d-6ecd-8c6e-b641-e257c1959baf@suse.cz/#1447334
>>
>> Last year I added the TARGET_GCOV_TYPE_SIZE target hook to optionally
>> reduce the gcov type size to 32 bits. I am not really sure if this was a
>> good idea. Longer running executables may observe counter overflows
>> leading to invalid coverage data. If someone wants atomic updates, then
>> the updates should be atomic even if this means to use a library
>> implementation (libatomic).
>>
>> What about the following approach if -fprofile-update=atomic is given:
>>
>> 1. Use 64-bit atomics if available.
>>
>> 2. Use
>>
>>     if (__atomic_add_fetch_4 ((unsigned int *) &val, 1, __ATOMIC_RELAXED)
>> == 0)
>>       __atomic_fetch_add_4 (((unsigned int *) &val) + 1, 1,
>> __ATOMIC_RELAXED);
>>
>> if 32-bit atomics are available.
>>
>> 3. Else use a library call (libatomic).
> sounds good, though a library call would really be prohibitly expensive?

I someone wants to profile a multi-threaded application and selects 
-fprofile-update=atomic, then probably a correct result is preferred. 
You still have the option to select -fprofile-update=prefer-atomic.

For 2. I have to modify:

void
gimple_gen_edge_profiler (int edgeno, edge e)
{
   tree one;

   one = build_int_cst (gcov_type_node, 1);

   if (flag_profile_update == PROFILE_UPDATE_ATOMIC)
     {
       /* __atomic_fetch_add (&counter, 1, MEMMODEL_RELAXED); */
       tree addr = tree_coverage_counter_addr (GCOV_COUNTER_ARCS, edgeno);
       tree f = builtin_decl_explicit (TYPE_PRECISION (gcov_type_node) > 32
				      ? BUILT_IN_ATOMIC_FETCH_ADD_8:
				      BUILT_IN_ATOMIC_FETCH_ADD_4);
       gcall *stmt = gimple_build_call (f, 3, addr, one,
				       build_int_cst (integer_type_node,
						      MEMMODEL_RELAXED));
       gsi_insert_on_edge (e, stmt);
     }

Is

if (WORDS_BIG_ENDIAN)

the right way to check for big/little endian?

How do I get ((unsigned int *) &val) + 1 from tree addr?

It would be great to have a code example for the construction of the "if 
(f()) f();".

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