Overly optimized simultaneous copy of struct fields corrupts destination struct

[samuelsadok]

Bugzilla Link	50623
Resolution	FIXED
Resolved on	Jun 10, 2021 12:51
Version	12.0
OS	Linux
Blocks	#48661
CC	@dwblaikie,@efriedma-quic,@RKSimon,@zygoloid,@rotateright,@tstellar
Fixed by commit(s)	2ef81cb dd763ac 4232693 b54ccef

Extended Description

Clang 12.0.0 modifies a field of a struct that should not be modified. This is a regression from clang 11.0.1.

Input File:

struct S { char val1; char padding; short val2; };

void func(struct S src, struct S* dest) {
    dest->val1 = src.val1;
    dest->val2 = src.val2;
}

Output from clang 12.0.0 (clang -c test.c -o test.o -O1 && objdump -S test.o -M intel):

0000000000000000 <func>:
   0:	89 3e                	mov    DWORD PTR [rsi],edi
   2:	c3                   	ret    

Output from clang 11.0.1:

0000000000000000 <func>:
   0:	40 88 3e             	mov    BYTE PTR [rsi],dil
   3:	c1 ef 10             	shr    edi,0x10
   6:	66 89 7e 02          	mov    WORD PTR [rsi+0x2],di
   a:	c3                   	ret    

The output from clang 12.0.0 is incorrect because it modifies the field dest->padding.

See also here: https://godbolt.org/z/dsr9soqbf
"armv8-a clang (trunk)" exhibits the same problem.

[samuelsadok]

assigned to @rotateright

[efriedma-quic]

https://reviews.llvm.org/D86420 ?

Slightly more clear testcase:

struct S { char val1; char padding; short val2; };
void func(struct S *dest, int x) {
dest->val1 = x;
dest->val2 = x >> 16;
}

[rotateright]

https://reviews.llvm.org/D86420 ?

Yes...ouch.

That patch allowed matching multiple types, but I failed to make sure that the types match each other and/or map all of the stored bytes.

Working on a fix now.

[rotateright]

Should be fixed after:
https://reviews.llvm.org/rGdd763ac79196

Leaving status as open as a candidate for the 12.0.1 release.

[jyknight]

*** Bug llvm/llvm-bugzilla-archive#50635 has been marked as a duplicate of this bug. ***

[tstellar]

Merged: b54ccef

[samuelsadok]

Thanks for the quick response time!

[jyknight]

mentioned in issue llvm/llvm-bugzilla-archive#50635