https://bugs.llvm.org/show_bug.cgi?id=37774
Bug ID: 37774
Summary: Incorrect result at self-comparison of uninitialized
value
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
I have tested a program using self-comparison of uninitialized value as below.
--- test.c ---
#include <stdio.h>
int main() {
int a;
if (a == a) puts("equal");
else puts("not equal");
return 0;
}
--------------
A correct result is output at -O0, but an incorrect result is output at -O1.
$ clang --version
clang version 7.0.0 (trunk 334448)
$ clang -O0 test.c
test.c:4:9: warning: self-comparison always evaluates to true
[-Wtautological-compare]
if (a == a) puts("equal");
^
1 warning generated.
$ ./a.out
equal
$ clang -O1 test.c
test.c:4:9: warning: self-comparison always evaluates to true
[-Wtautological-compare]
if (a == a) puts("equal");
^
1 warning generated.
$ ./a.out
not equal
This is a rare case and I think no one writes a program like this, but I think
self-comparison program should always pass true route even if an uninitialized
value is used.
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