[Bug c++/101240] New: [missed optimization] Transitivity of less-than and less-or-equal
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101240
Bug ID: 101240
Summary: [missed optimization] Transitivity of less-than and
less-or-equal
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: kyrylo.bohdanenko at gmail dot com
Target Milestone: ---
Consider the following C++ code
#define ALWAYS_TRUE(x) do { if (x) __builtin_unreachable(); } while (false)
int divide(int a, int b) {
ALWAYS_TRUE(a == b);
return a / b;
}
void test_array(unsigned (&arr)[3]) {
ALWAYS_TRUE(a[0] < a[1] && a[1] < a[2]);
return a[0] < a[2];
}
The first function is optimiozed away:
divide(int, int):
mov eax, 1
ret
While the second still does the comparison:
test_array(unsigned int (&) [3]):
mov eax, DWORD PTR [rdi+8]
cmp DWORD PTR [rdi], eax
setbal
ret
It would be nice if GCC could deduce that
a < b && b < c --> a < c
And optimize that second function (provided no other UB is involved)
[Bug tree-optimization/106076] New: Sub-optimal code is generated for checking bitfields via proxy functions
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106076
Bug ID: 106076
Summary: Sub-optimal code is generated for checking bitfields
via proxy functions
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: kyrylo.bohdanenko at gmail dot com
Target Milestone: ---
Consider the following struct:
#include
struct SomeClass {
uint16_t dummy1 : 1;
uint16_t cfg2 : 1;
uint16_t cfg3 : 1;
uint16_t dummy2 : 1;
uint16_t dummy3 : 1;
uint16_t dummy4 : 1;
uint16_t cfg1 : 1;
uint16_t dummy5 : 1;
uint16_t cfg4 : 1;
constexpr bool checkA() const { return cfg1 || cfg2 || cfg3; }
constexpr bool checkB() const { return cfg4; }
constexpr bool checkA_B() const { return (cfg1 || cfg2 || cfg3) || cfg4; }
constexpr bool checkA_B_SLOW() const { return checkA() || checkB(); }
};
For the following functions (which do the same thing) GCC generates different
assembly.
bool check(const SomeClass& rt) {
return rt.checkA_B();
}
bool check_SLOW(const SomeClass& rt) {
return rt.checkA_B_SLOW();
}
Compiled as:
g++ -std=c++17 -S
The assembly:
; demangled: check(SomeClass const&)
_Z5checkRK9SomeClass:
endbr64
testw $326, (%rdi)
setne %al
ret
; demangled: check_SLOW(SomeClass const&)
_Z10check_SLOWRK9SomeClass:
endbr64
movzwl (%rdi), %edx
movl$1, %eax
testb $70, %dl
jne .L3
movzbl %dh, %eax
andl$1, %eax
.L3:
ret
As we can see, during check_SLOW GCC decided to check the result on
byte-by-byte basis introducing a conditional jump in between. It looks like GCC
did not fully analyse the code after inlining checkA() and checkB().
FYI, the same code on Clang produces the 1st option of ASM for both functions.
[Bug tree-optimization/106076] Sub-optimal code is generated for checking bitfields via proxy functions
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106076 --- Comment #1 from Kyrylo Bohdanenko --- The provided assembly is for -O2/-O3
[Bug c++/95454] type-level nodiscard not applied to constructors
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95454
Kyrylo Bohdanenko changed:
What|Removed |Added
CC||kyrylo.bohdanenko at gmail dot
com
--- Comment #2 from Kyrylo Bohdanenko ---
Still relevant as of the current trunk.
A workaround would be to declare the constructor(s) [[nodiscard]]:
struct Data {
[[nodiscard]] Data() {}
};
int main() {
Data{};
}
This actually issues a warning:
: In function 'int main()':
:6:11: warning: ignoring return value of 'Data::Data()', declared with
attribute 'nodiscard' [-Wunused-result]
Sandbox: https://godbolt.org/z/3Koj8rra7
[Bug c++/113108] New: Internal compiler error when choosing overload for operator=
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113108
Bug ID: 113108
Summary: Internal compiler error when choosing overload for
operator=
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: kyrylo.bohdanenko at gmail dot com
Target Milestone: ---
The following code causes GCC internal error:
template
struct Foo {
Foo& operator=(Foo&&) = default;
T data;
};
template
void consume(Foo& (Foo::*)(Foo&&) ) {}
template
void consume(Foo& (Foo::*)(Foo&&) noexcept) {}
int main() {
consume(&Foo::operator=);
}
Output:
: In substitution of 'template void consume(Foo&
(Foo::*)(Foo&&) noexcept) [with T = ]':
:26:12: required from here
26 | consume(&Foo::operator=);
| ~~~^~
:26:12: internal compiler error: in nothrow_spec_p, at
cp/except.cc:1201
0x262b4bc internal_error(char const*, ...)
???:0
0xa4d583 fancy_abort(char const*, int, char const*)
???:0
0xca6a7f fn_type_unification(tree_node*, tree_node*, tree_node*, tree_node*
const*, unsigned int, tree_node*, unification_kind_t, int, conversion**, bool,
bool)
???:0
0xa7cfd9 build_new_function_call(tree_node*, vec**, int)
???:0
0xcc6c76 finish_call_expr(tree_node*, vec**, bool,
bool, int)
???:0
0xc4b8ba c_parse_file()
???:0
0xd9cda9 c_common_parse_file()
???:0
Please submit a full bug report, with preprocessed source (by using
-freport-bug).
Please include the complete backtrace with any bug report.
See
[Bug c++/113108] Internal compiler error when choosing overload for operator=
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113108 --- Comment #1 from Kyrylo Bohdanenko --- Compiled with -std=c++17 -O2 -Wall -Wextra -Wpedantic
[Bug c++/113108] Internal compiler error when choosing overload for operator=
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113108 --- Comment #2 from Kyrylo Bohdanenko --- Godbolt link (not the original example): https://godbolt.org/z/E1veMxcdx
