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Alignment not respected in containers for over-aligned enumeration types #41741

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mclow opened this issue Jun 25, 2019 · 2 comments
Closed

Alignment not respected in containers for over-aligned enumeration types #41741

mclow opened this issue Jun 25, 2019 · 2 comments
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bugzilla invalid libc++

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@mclow
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@mclow mclow commented Jun 25, 2019

Bugzilla Link 42396
Resolution INVALID
Resolved on Jun 25, 2019 15:41
Version unspecified
OS All
Attachments Reproducer
CC @dwblaikie,@mclow,@zygoloid

Extended Description

Inspired by: https://stackoverflow.com/questions/56760874/discrepancy-in-c-between-over-aligned-struct-and-enum-within-container

Given an enumeration type with a specified alignment, the elements in a vector of such a type should be aligned correctly. This appears not to be the case - and can lead to UBSAN going off.

In the reproducer, I also check the size of a 16-element array of these two types.
Clang says they're different sizes - that suggests to me that there's a clang problem here.

The alignment in the containers bit could be related to #23008 . (might even be the same bug)

@mclow
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@mclow mclow commented Jun 25, 2019

Now I'm very confused.

====
struct alignas(16) byte_struct {std::uint8_t value;};
enum alignas(16) byte_enum : std::uint8_t { one=1, two, three };

int main() {
std::cout << "struct: " << sizeof(byte_struct) << std::endl;
std::cout << "enum: " << sizeof(byte_enum ) << std::endl;
std::cout << "01 structs: " << sizeof(byte_struct[1]) << std::endl;
std::cout << "01 enums: " << sizeof(byte_enum [1]) << std::endl;
std::cout << "02 structs: " << sizeof(byte_struct[2]) << std::endl;
std::cout << "02 enums: " << sizeof(byte_enum [2]) << std::endl;
std::cout << "16 structs: " << sizeof(byte_struct[16]) << std::endl;
std::cout << "16 enums: " << sizeof(byte_enum [16]) << std::endl;
}

prints (with a reasonably recent clang):

struct: 16
enum: 1
01 structs: 16
01 enums: 16
02 structs: 32
02 enums: 16
16 structs: 256
16 enums: 16

@mclow
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@mclow mclow commented Jun 25, 2019

Richard pointed out (on SO) that this is core issue 2354. Extended alignment and object representation, which was resolved in Kona by removing the ability to put 'alignas' on enumeration types.

Closing as invalid.

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 10, 2021
This issue was closed.
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