https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105860
Bug ID: 105860
Summary: Miscompilation causing clobbered union contents
Product: gcc
Version: 10.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: analyzer
Assignee: dmalcolm at gcc dot gnu.org
Reporter: tpg+gcc at mutabah dot net
Target Milestone: ---
Created attachment 53092
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53092&action=edit
Reproduction source file, compile with `gcc -O1`
Found while debugging this issue with auto-generated code:
https://github.com/thepowersgang/mrustc/issues/266#issuecomment-1147389581
gcc generates code that reads 32-bits from offset 8 of a union, and then writes
that value back to offset 4 before copying 64-bits from offset 4, causing data
corruption.
This issue is present on gcc 10.3.0 (`gcc (Ubuntu 10.3.0-1ubuntu1~20.04)
10.3.0`) and on gcc 11.1 (`gcc-11 (Ubuntu 11.1.0-1ubuntu1~20.04) 11.1.0`), but
not on gcc 9.4.0 (`gcc-9 (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0`)
Compiler flags required: `-O1`
Disassembly with comments pointing to the faulty instructions.
```
0000000000000000
<ZRQG3cM17rustc_middle0_0_02ty3sty20ExistentialPredicate0g3c_A2ty4fold12TypeFoldable0g15super_fold_with1gG3c_A2ty_E16Bou$c6411ae8e3203bad>:
0: f3 0f 1e fa endbr64
4: 41 56 push %r14
6: 41 55 push %r13
8: 41 54 push %r12
a: 49 89 fc mov %rdi,%r12
d: 55 push %rbp
e: 53 push %rbx
f: 48 83 ec 10 sub $0x10,%rsp
13: 8b 5c 24 40 mov 0x40(%rsp),%ebx
17: 8b 6c 24 48 mov 0x48(%rsp),%ebp ; Read word 3
1b: 81 fb 01 ff ff ff cmp $0xffffff01,%ebx
21: 74 4d je 70
<ZRQG3cM17rustc_middle0_0_02ty3sty20ExistentialPredicate0g3c_A2ty4fold12TypeFoldable0g15super_fold_with1gG3c_A2ty_E16Bou$c6411ae8e3203bad+0x70>
23: 81 fb 03 ff ff ff cmp $0xffffff03,%ebx
29: 74 5d je 88
<ZRQG3cM17rustc_middle0_0_02ty3sty20ExistentialPredicate0g3c_A2ty4fold12TypeFoldable0g15super_fold_with1gG3c_A2ty_E16Bou$c6411ae8e3203bad+0x88>
-- SNIP ---
88: 89 6c 24 44 mov %ebp,0x44(%rsp) ; Write word 3 over
word 2
8c: 48 8b 44 24 44 mov 0x44(%rsp),%rax ; Read words 2/3
(values from 3/3)
91: 48 89 47 04 mov %rax,0x4(%rdi) ; Write to words 2/3
of output
95: eb af jmp 46
<ZRQG3cM17rustc_middle0_0_02ty3sty20ExistentialPredicate0g3c_A2ty4fold12TypeFoldable0g15super_fold_with1gG3c_A2ty_E16Bou$c6411ae8e3203bad+0x46>
```
