https://bugs.llvm.org/show_bug.cgi?id=42672
Bug ID: 42672
Summary: DSE fails to remove stores to a structure overwritten
by asm.
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: gli...@google.com
CC: llvm-bugs@lists.llvm.org
In the following example:
void _raw_spin_lock(void)
{
#if STRUCT
struct {
unsigned short owner, next;
} lockval;
lockval.owner = 1;
lockval.next = 2;
#else
int lockval;
lockval = 3;
#endif
asm ("nop" : "=r" (lockval));
}
|lockval| is declared as assembly output that is overwritten by the inline
assembly (as specified by constraints).
If |lockval| is a scalar type, its initialization is correctly omitted.
However if |lockval| is a struct, DSE is unable to figure out that the stores
to its fields are effectively dead:
$ clang -O2 -USTRUCT -c spinlock.c && objdump -d spinlock.o
spinlock.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <_raw_spin_lock>:
0: c3 retq
$ clang -O2 -DSTRUCT -c spinlock.c && objdump -d spinlock.o
spinlock.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <_raw_spin_lock>:
0: c7 44 24 f8 01 00 02 movl $0x20001,-0x8(%rsp)
7: 00
8: 90 nop
9: c3 retq
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