UB with union using

[safocl]

Operating System Info

Other

Other OS

all OS

OBS Studio Version

Git

OBS Studio Version (Other)

No response

OBS Studio Log URL

https://eel.is/c++draft/defns.undefined

OBS Studio Crash Log URL

No response

Expected Behavior

defined behavior

Current Behavior

[defns.undefined]

3.65 [defns.undefined] undefined behavior
behavior for which this document imposes no requirements
[Note 1: Undefined behavior may be expected when this document omits any explicit definition of behavior or when a program uses an incorrect construct or invalid data. Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message ([defns.diagnostic])), to terminating a translation or execution (with the issuance of a diagnostic message). Many incorrect program constructs do not engender undefined behavior; they are required to be diagnosed. Evaluation of a constant expression ([expr.const]) never exhibits behavior explicitly specified as undefined in [intro] through [cpp]. — end note]

these lines cause undefined behavior:

obs-studio/libobs/util/util_uint128.h

Line 83 in 8e129b1

out = util_add128(out, util_lshift64_internal_32(m));

obs-studio/libobs/util/util_uint128.h

Line 86 in 8e129b1

out = util_add128(out, util_lshift64_internal_64(m));

here output is union with uint32_t i32[4]; active member,

obs-studio/libobs/util/util_uint128.h

Line 49 in 8e129b1

out.i32[3] = (uint32_t)val;

but read as

struct {
			uint64_t low;
			uint64_t high;
};

here:

obs-studio/libobs/util/util_uint128.h

Line 36 in 8e129b1

val = (a.low & 0xFFFFFFFFULL) + (b.low & 0xFFFFFFFFULL);

uint32_t i32[4]; and struct { uint64_t low; uint64_t high; }; are not same standard-layout!

https://eel.is/c++draft/basic.life#2.sentence-3
https://eel.is/c++draft/class.union#general-2
https://eel.is/c++draft/basic.life#8
https://en.cppreference.com/w/cpp/language/union

other links:
https://gitlab.com/libeigen/eigen/-/issues/2898
arduino/ArduinoCore-API#225

Steps to Reproduce

compile and using

Anything else we should know?

No response

[safocl]

simple example:
https://godbolt.org/z/MTdTWrW44