https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111405
Bug ID: 111405
Summary: Problem with incorrect optimizion for "constexpr"
function with possible overflow
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: 3180104919 at zju dot edu.cn
Target Milestone: ---
Created attachment 55891
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55891&action=edit
A demo file contains a funciton that will be wrongly optimized using -O2
I happened to find this problem when I did the CSAPP lab.
int isTmax(int x) {
// make it all of 1
// it's quite strange that the results of x + 1 + x and x + x + 1 are
different
int c = x + x + 1;
// check if it's all of 1
int flag_all_ones = !(~c);
// avoid -1
int flag_not_neg1 = !!(x + 1);
return flag_all_ones & flag_not_neg1;
}
This function will be incorrectly optimized to return zero only with "-O2"
compiler flag. But in fact isTmax(0x7fffffff) should return 1. Here's the
disassembly code using coredump:
000012ac <isTmax>:
// check if it's all of 1
int flag_all_ones = !(~c);
// avoid -1
int flag_not_neg1 = !!(x + 1);
return flag_all_ones & flag_not_neg1;
}
12ac: b8 00 00 00 00 mov $0x0,%eax
12b1: c3 ret
This function can be correctly compiled with no compiler optimization (-O0).
And this behaviour always occurs using the latest 2 version gcc compiler (from
11.0 to 12.0). But using clang or msvc, everything works well.
Thank you for your time.
