https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95971
Bug ID: 95971
Summary: [10 regression] Optimizer converts a false boolean
value into a true boolean value
Product: gcc
Version: 10.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: 0xe2.0x9a.0x9b at gmail dot com
Target Milestone: ---
Hello. I have found an optimization issue that is triggered by the -O2
optimization option in GCC 10.1.0.
The source code (see below) contains an infinite while(cond){} loop. The loop
condition is expected to always evaluate to true. The optimizer incorrectly
derives that the loop condition evaluates to false and removes the loop. It is
possible that the issue is related to optimizations of the delete operator in
C++.
Reproducibility:
g++ 10.1.0 -O0: not reproducible
g++ 10.1.0 -O1: not reproducible
g++ 10.1.0 -O2: REPRODUCIBLE
g++ 10.1.0 -O3: not reproducible
g++ 9.3.0 -O2: not reproducible
clang++ 10 -O2: not reproducible
Full source code:
$ cat a.cc
void xbool(bool value);
struct A {
char *a = (char*)1;
~A() { delete a; }
bool isZero() { return a == (void*)0; }
};
int main() {
A a;
xbool(a.isZero());
while(!a.isZero());
xbool(a.isZero()); // This line isn't required to trigger the issue
return 0;
}
$ cat b.cc
void xbool(bool value) {}
$ cat Makefile
test:
g++ -c -O2 a.cc
g++ -c b.cc
g++ -o a a.o b.o
time ./a
Dump of assembler code for function main:
push %rbp
xor %edi,%edi // %rdi := false
sub $0x10,%rsp
movq $0x1,0x8(%rsp)
callq xbool(bool)
mov $0x1,%edi // %rdi := true
callq xbool(bool)
lea 0x8(%rsp),%rdi
callq A::~A()
add $0x10,%rsp
xor %eax,%eax
pop %rbp
retq
mov %rax,%rbp
jmpq main.cold
In the assembler code: The compiler correctly passes zero (false) in the 1st
call to function xbool(bool), then incorrectly passes one (true) in the 2nd
call to function xbool(bool).
The source code initializes A::a to (char*)1 in order to keep the code as small
as possible to trigger the issue. A::a could have been initialized to a valid
delete-able heap address, but this would unnecessarily enlarge the source code.
The GCC version string on my machine is "g++ (Gentoo 10.1.0-r1 p2) 10.1.0".
Please confirm the reproducibility of this issue.
