Compiler misoptimizes the logical expression

Original Reporter info from Mantis: frol

• Reporter name:

Description:

The following code outputs "YES YES" when compiled with -O0, -O1, -O3, -O4, but outputs "NO YES" with -O2.

Notes:

* the if expressions are completely the same, so there is no reason for the output to be different!
* the first WriteLn; is essential to reproduce the bug, but it can be replaced with some other computations.
* the issue is reproducible on latest Debian, Ubuntu, CentOS, AlpineLinux x86_64 with FPC installed from the official repositories and the prebuilt-binaries from FreePascal.org
* the issue is NOT reproducible on Arch Linux
* the issue affects FPC 3.0.0+ and is not reproducible on FPC 2.6.4

Steps to reproduce:

1. Compile the following code with fpc -O2 qq.pas and run the program:

var d: LongInt;
begin
    WriteLn;

    d := 828;
    if ((d mod 2) = 0) xor ((d < 0) and ((d mod 3) = 0)) then
        WriteLn('YES')
    else
        WriteLn('NO');

    if ((d mod 2) = 0) xor ((d < 0) and ((d mod 3) = 0)) then
        WriteLn('YES')
    else
        WriteLn('NO');
end.

2. The expected output is "YES YES", but the output is "NO YES"

Mantis conversion info:

• Mantis ID: 34653
• OS: Debian
• OS Build: 9.6
• Platform: Linux x86_64
• Version: 3.0.4
• Fixed in version: 3.3.1
• Fixed in revision: 40934 (#e24449bc)
• Monitored by: » frol (frol), » Vincent (Vincent Snijders)
• Target version: 3.3.1