https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78726
Bug ID: 78726
Summary: [5/6/7 Regression] Incorrect unsigned arithmetic
optimization
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: babokin at gmail dot com
Target Milestone: ---
Created attachment 40279
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40279&action=edit
reproducer
-O2 produces different result than -O0 for unsigned arithmetic.
Reproduces with gcc 5, 6, and 7, while 4.9 works fine.
> cat driver.cpp
#include <stdio.h>
unsigned char A = 36;
unsigned char B = 173;
unsigned long int C;
extern void foo ();
int main () {
foo ();
printf("%lu\n", C);
return 0;
}
> cat func.cpp
extern unsigned char A;
extern unsigned char B;
extern unsigned long int C;
void foo() {
unsigned a = ~A;
C = a * B * B + 1023094746 * a;
}
> g++ -O0 -o no_opt driver.cpp func.cpp
> ./no_opt
799092689
> g++ -O2 -o opt driver.cpp func.cpp
> ./opt
800200062
