Deadstore optimisation can break a for-loop

Original Reporter info from Mantis: bunnylin @bunnylin

• Reporter name: Kirinn

Description:

Run this tiny program to reproduce. When compiled without deadstore optimisation, it runs as expected; i = 1 in the for-loop.

When compiled with -gt -Oodfa,deadstore then i has a trash value within the for-loop, which shouldn't be possible.

Interestingly, this only happens if i is an unsigned type. Any signed type still works correctly.

Steps to reproduce:

procedure ForLoop;
var i : dword;
begin
	for i := 1 to 1 do begin
		writeln('i=',i);
		if i <> 1 then halt(1);
	end;
end;

begin
ForLoop;
end.
------------------
Compile with:
fpc -gt -Oodfa,deadstore

Mantis conversion info:

• Mantis ID: 38698
• OS: Windows/Linux
• Platform: x86_64
• Version: 3.2.0
• Fixed in version: 3.3.1
• Monitored by: » @MageSlayer (Denis Golovan)