Confusing misaligned atomic operation warning

[tschuett]

Bugzilla Link	46947
Version	trunk
OS	All
CC	@zygoloid

Extended Description

For the following code, I get an error: repro.cpp:13:3: warning: misaligned atomic operation may incur significant performance penalty [-Watomic-alignment] __atomic_load(&foo->bar, &dummy, __ATOMIC_RELAXED);

clang believes that the pointer is 8 byte aligned, but foo->bar is actually 16 byte aligned.

include

struct Foo { struct Bar { uint64_t a; uint64_t b; }; Bar bar; };

void braz(Foo *foo) { Foo::Bar dummy; __atomic_load(&foo->bar, &dummy, __ATOMIC_RELAXED); }

[ec04fc15-fa35-46f2-80e1-5d271f2ef708]

Listing the alignment and access size (== expected alignment) in the warning seems like a good idea.

[tschuett]

Maybe extend the warning: The pointer is 8 byte aligned, while the value requires an alignment of 16 bytes.

[tschuett]

I read through the source code and finally found the ASTRecordLayout. Then I realized that it is aligned on Foo::Bar::a, which is only 8 bytes. The 16 bytes is my misunderstanding.

But the error makes it hard to find the root cause. The root cause is not even visible in the expression: __atomic_load(&foo->bar, &dummy, __ATOMIC_RELAXED); It is foo->bar.a.

[ec04fc15-fa35-46f2-80e1-5d271f2ef708]

clang believes that the pointer is 8 byte aligned, but foo->bar is actually 16 byte aligned.

Why do you believe that to be the case?

[tschuett]

"misaligned atomic operation may incur significant performance penalty" This sentence is always true, but it is not actionable. It does not help me to solve the problem.

[tschuett]

I am starting to understand the problem. Foo is aligned on Foo::Bar::a. Thus the alignment of Foo is 8 bytes. Foo * is not aligned on Foo::Bar. But the error message is still confusing as it took me a while to find out what is misaligned.

The following solves my problem.

struct Foo { struct alignas (16) Bar { uint64_t a; uint64_t b; }; Bar bar; };

[tschuett]

Even void braz(Foo foo) { Foo::Bar dummy; __atomic_load((Foo::Bar )foo, &dummy, __ATOMIC_RELAXED); }

gives warnings.

[tschuett]

I added printf statements to CGAtomic.cpp and the inferred alignment is 8 bytes and sizeChars is 16 byte.