On 05.11.22 12:18, Richard Biener wrote:
On Fri, Nov 4, 2022 at 9:28 AM Sebastian Huber
<sebastian.hu...@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-e257c1959...@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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