Hello,
I have question to the counters used for the condition coverage implementation
in tree-profile.cc
/* Stores the incoming edge and previous counters (in SSA form) on that edge
for the node e->deston that edge for the node e->dest. The counters record
the seen-true (0), seen-false (1), and current-mask (2). They are stored in
an array rather than proper members for access-by-index as the code paths
tend to be identical for the different counters. */
struct counters
{
edge e;
tree counter[3];
tree& operator [] (size_t i) { return counter[i]; }
};
While working on the -fprofile-update=atomic support for 32-bit targets which
lack support for 64-bit atomic operations, I noticed that some atomic
no-operations are generated for the instrumented code
(https://gcc.gnu.org/pipermail/gcc-patches/2025-August/692555.html). For
example:
int a(int i);
int b(int i);
int f(int i)
{
if (i) {
return a(i);
} else {
return b(i);
}
}
gcc -O2 -fprofile-update=atomic -fcondition-coverage -S -o - test.c
-fdump-tree-all
;; Function f (f, funcdef_no=0, decl_uid=4621, cgraph_uid=1, symbol_order=0)
int f (int i)
{
int _1;
int _6;
int _8;
<bb 2> [local count: 1073741824]:
if (i_3(D) != 0)
goto <bb 3>; [50.00%]
else
goto <bb 4>; [50.00%]
<bb 3> [local count: 536870912]:
__atomic_fetch_or_8 (&__gcov8.f[0], 1, 0);
__atomic_fetch_or_8 (&__gcov8.f[1], 0, 0);
_8 = a (i_3(D)); [tail call]
goto <bb 5>; [100.00%]
<bb 4> [local count: 536870912]:
__atomic_fetch_or_8 (&__gcov8.f[0], 0, 0);
__atomic_fetch_or_8 (&__gcov8.f[1], 1, 0);
_6 = b (0); [tail call]
<bb 5> [local count: 1073741824]:
# _1 = PHI <_8(3), _6(4)>
return _1;
}
The __atomic_fetch_or_8 (&__gcov8.f[1], 0, 0) and __atomic_fetch_or_8
(&__gcov8.f[0], 0, 0) could be optimized away. Since GCC is able to figure out
that the masks are compile-time constants wouldn't it be possible to use a
simple uint64_t for the current-mask (2) in struct counters?
I am not sure how the phi node stuff works in resolve_counter() since I am not
a compiler expert.
Kind regards, Sebastian
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