https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92425
Bug ID: 92425
Summary: Incorrect logical AND on 64bit variable using 32bit
register
Product: gcc
Version: 9.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: akobets at mail dot ru
Target Milestone: ---
Created attachment 47200
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=47200&action=edit
Assembly output
=== prog.cpp ===
unsigned long long a, b;
int start()
{
a = b & 0xFFF;
return 1;
}
================
Compiling
gcc -c -fno-PIC -Wall -Wextra -save-temps -fverbose-asm -masm=intel
-march=core2 -mtune=generic -O2 -fno-rtti -fno-default-inline --no-exceptions
-O2 prog.cpp
No errors
No warnings
Produces wrong assembly code
=== cut ===
_Z5startv:
.LFB0:
.cfi_startproc
endbr64
# prog.cpp:5: a = b & 0xFFF;
mov rax, QWORD PTR b[rip] # tmp87, b
and eax, 4095 # tmp87,
# prog.cpp:5: a = b & 0xFFF;
mov QWORD PTR a[rip], rax # a, tmp87
# prog.cpp:7: }
mov eax, 1 #,
ret
.cfi_endproc
=== cut ===
There is 'and' on 32bit register eax, while 'b' is 64bit variable. Bits 32...63
of 'b' stays unchanged, but must be zeroed.
