[Bug middle-end/114289] Non-optimal assembly for accessing bit-fields in packed structs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114289
--- Comment #3 from Dan Stahlke ---
Variants that generate more reasonable results:
struct foo {
int x:32;
} __attribute__((packed));
struct foo {
int x:16;
} __attribute__((packed));
struct foo {
int x:31;
};
struct foo {
int x:31;
} __attribute__((packed, aligned(4)));
[Bug tree-optimization/114289] Non-optimal assembly for accessing bit-fields in packed structs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114289
Dan Stahlke changed:
What|Removed |Added
Target|x86_64-linux-gnu|
Keywords|missed-optimization |
Component|middle-end |tree-optimization
--- Comment #1 from Dan Stahlke ---
Variants that generate more reasonable results:
struct foo {
int x:32;
} __attribute__((packed));
struct foo {
int x:16;
} __attribute__((packed));
struct foo {
int x:31;
};
struct foo {
int x:31;
} __attribute__((packed, aligned(4)));
[Bug tree-optimization/114289] New: Non-optimal assembly for accessing bit-fields in packed structs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114289
Bug ID: 114289
Summary: Non-optimal assembly for accessing bit-fields in
packed structs
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: dan at stahlke dot org
Target Milestone: ---
// gcc -O3 -g -Wall -march=haswell -mavx2
// https://godbolt.org/z/Yfb9dnYx4
struct foo {
int x:31;
} __attribute__((packed));
int fx(struct foo const *o) { return o->x; }
Disassembly:
movzx eax, BYTE PTR [rdi+1]
movzx edx, BYTE PTR [rdi]
sal rax, 8
or rax, rdx
movzx edx, BYTE PTR [rdi+2]
sal rdx, 16
or rdx, rax
movzx eax, BYTE PTR [rdi+3]
and eax, 127
sal rax, 24
or rax, rdx
sal rax, 33
sar rax, 33
ret
Compare to clang:
mov eax, dword ptr [rdi]
add eax, eax
sar eax
ret
[Bug tree-optimization/105329] [12/13 Regression] Bogus restrict warning when assigning 1-char string literal to std::string since r12-3347-g8af8abfbbace49e6
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105329 --- Comment #25 from Dan Stahlke --- The test case I just posted appears in the bug 105651 discussion. So maybe or maybe not related to the present discussion.
[Bug tree-optimization/105329] [12/13 Regression] Bogus restrict warning when assigning 1-char string literal to std::string since r12-3347-g8af8abfbbace49e6
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105329
Dan Stahlke changed:
What|Removed |Added
CC||dan at stahlke dot org
--- Comment #24 from Dan Stahlke ---
Here is another test case, broken on 12.2 with "-std=c++20 -Wrestrict -O3" but
working on trunk and working on 12.2 with -O2 rather than -O3.
#include
#include // std::ignore
int main()
{
std::string s = "b";
std::ignore = "a" + std::move(s);
}
[Bug c++/106893] New: auto deduces wrong type for function pointer
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106893
Bug ID: 106893
Summary: auto deduces wrong type for function pointer
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: dan at stahlke dot org
Target Milestone: ---
This works in 11.3.0 but not in 12.2.0.
struct IntegerCoordinate {
int x() const;
int y() const;
};
template
struct CoordTraits
{
static auto GetX(T const ) { return p.x(); }
static auto GetY(T const ) { return p.y(); }
using Ordinate = decltype(GetX(T{}));
};
template
struct Node {
static constexpr auto GetX = Traits::GetX; // works because of the `using
Ordinate = ...`
static constexpr auto GetY = Traits::GetY; // compilation error
// This compiles
//static constexpr auto GetX = ::GetX;
//static constexpr auto GetY = ::GetY;
// This compiles
//using fptr = int(*)(IntegerCoordinate const &);
//static constexpr fptr GetX = Traits::GetX;
//static constexpr fptr GetY = Traits::GetY;
// Not needed for minimal test case
auto x(auto const ) const { return GetX(p); }
auto y(auto const ) const { return GetY(p); }
};
int main() {
Node> node;
// Not needed for minimal test case
node.x(IntegerCoordinate{});
node.y(IntegerCoordinate{});
}
$ g++ -std=c++20 gcc-function-ptr-bug.cpp -c -o gcc-function-ptr-bug.o
gcc-function-ptr-bug.cpp: In instantiation of ‘constexpr auto (* const
Node >::GetY)(const IntegerCoordinate&)’:
gcc-function-ptr-bug.cpp:17:27: required from ‘struct
Node >’
gcc-function-ptr-bug.cpp:32:42: required from here
gcc-function-ptr-bug.cpp:17:27: error: invalid conversion from ‘int (*)(const
IntegerCoordinate&)’ to ‘auto (*)(const IntegerCoordinate&)’ [-fpermissive]
17 | static constexpr auto GetY = Traits::GetY; // compilation error
| ^~~~
| |
| int (*)(const IntegerCoordinate&)
gcc-function-ptr-bug.cpp: In instantiation of ‘auto Node::y(const
auto:2&) const [with auto:2 = IntegerCoordinate; Traits =
CoordTraits]’:
gcc-function-ptr-bug.cpp:35:11: required from here
gcc-function-ptr-bug.cpp:28:46: error: use of
‘Node >::GetY’ before deduction of ‘auto’
28 | auto y(auto const ) const { return GetY(p); }
| ^~~
[Bug c++/102970] New: stable_sort uninitialized value with -funroll-loops -fno-tree-vectorize
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102970 Bug ID: 102970 Summary: stable_sort uninitialized value with -funroll-loops -fno-tree-vectorize Product: gcc Version: 11.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: dan at stahlke dot org Target Milestone: --- Created attachment 51682 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51682=edit demonstration source code The attached file, compiled with 11.2.0, using the command line in the comment, causes std::stable_sort to access uninitialized memory. In fact, it already goes wrong by the time it passes the inputs to the first comparison operation. The problem goes away if the copy constructor of the box struct is defaulted (see the "#if"). It only happens with "-funroll-loops -fno-tree-vectorize". Here is a Godbolt: https://godbolt.org/z/6PsdPj6q3
