https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80274
Bug ID: 80274
Summary: There is an unoptimized direct memory write of 16bit
value on x86_64 with O2 and O3.
Product: gcc
Version: 7.0.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: zloten at mail dot ru
Target Milestone: ---
The next code:
void foo() {
*(unsigned short*)0xFFFF=1;
}
Compiles to:
foo():
mov eax, 1
mov WORD PTR ds:65535, ax
ret
But must be:
foo():
mov WORD PTR ds:65535, 1
ret
The bug occurs with any options at optimization levels O2, O3. But this is ok
at optimization level O.
The bug occurs since 4.8.1 version of GCC.
PS: For any other sizes (8bit, 32bit and 64bit) the output is correct:
mov BYTE PTR ds:65535, 1 ;unsigned byte
mov DWORD PTR ds:65535, 1 ;unsigned int
mov QWORD PTR ds:65535, 1 ;unsigned long long int
