Wrong code generated on x86-64

Original Reporter info from Mantis: runewalsh

• Reporter name:

Description:

Code below should write "#######... 70%", but depending on compiler settings it writes garbage or even crashes.
I have this happening on x86-64-win64 with optimization levels greater than -O1.

Bug vanishes after replacing
pChar(result)[i] :=
with conventional
result[1 + i] :=

or after removing the second condition part, i. e.
(i = int32(divs) - 1) and (progress >= 1)

or after replacing any of 32-bit variables with 64-bit equivalent

so I can't even come close to guessing what's going on here.

Steps to reproduce:

function Bar(const progress: single; divs: uint32): string;
const
	BarSym: array[boolean] of char = ('.', '#');
var
	i: int32;
begin
	SetLength(result, divs);
	for i := 0 to int32(divs) - 1 do
		pChar(result)[i] := BarSym[(progress >= (0.75 + i) / divs) or (i = int32(divs) - 1) and (progress >= 1)];
end;

var
	s: string;

begin
	writeln(Bar(0.7, 10) + ' 70%');
end.

Mantis conversion info:

• Mantis ID: 38129
• Build: 63950
• Platform: x86-64-win64
• Fixed in version: 3.3.1
• Fixed in revision: 49095 (#cc64d9eb)
• Monitored by: » Vincent (Vincent Snijders), » @MageSlayer (Denis Golovan), » @CuriousKit (J. Gareth Moreton)