Delphi fastest FileSize for sizes > 10gb - delphi

Question

Wanted to check with you experts if there are any drawbacks in this funtion. Will it work properly on the various Windows OS ? I am using Delphi Seattle (32 and 64 bit exe's). I am using this instead of Findfirst for its speed.
function GetFileDetailsFromAttr(pFileName:WideString):int64;
var
wfad: TWin32FileAttributeData;
wSize:LARGE_INTEGER ;
begin
Result:=0 ;
if not GetFileAttributesEx(pwidechar(pFileName), GetFileExInfoStandard,#wfad) then
exit;
wSize.HighPart:=wfad.nFileSizeHigh ;
wSize.LowPart:=wfad.nFileSizeLow ;
result:=wsize.QuadPart ;
end;
The typical googled samples shown with this command does not work for filesize > 9GB
function GetFileAttributesEx():Int64 using
begin
...
result:=((&wfad.nFileSizeHigh) or (&wfad.nFileSizeLow))

Answer 1

Code with variant record is correct.
But this code
result:=((&wfad.nFileSizeHigh) or (&wfad.nFileSizeLow))
is just wrong, result cannot overcome 32-bit border
Code from link in comment
result := Int64(info.nFileSizeLow) or Int64(info.nFileSizeHigh shl 32);
is wrong because it does not account how compiler works with 32 and 64-bit values. Look at the next example showing how to treat this situation properly (for value d, e):
var
a, b: DWord;
c, d, e: Int64;
wSize:LARGE_INTEGER ;
begin
a := 1;
b := 1;
c := Int64(a) or Int64(b shl 32);
d := Int64(a) or Int64(b) shl 32;
wSize.LowPart := a;
wSize.HighPart := b;
e := wsize.QuadPart;
Caption := Format('$%x $%x $%x', [c, d, e]);
Note that in the expression for c 32-bit value is shifted by 32 bits left and looses set bit, then zero transforms to 64-bit.

Answer 2

Unbound to how you get the filesize: it would even be faster if you'd use a type (manual) that exists for ~25 years already to assign the filesize directly to the function's result instead of using an intermediate variable:
Int64Rec(result).Hi:= wfad.nFileSizeHigh;
Int64Rec(result).Lo:= wfad.nFileSizeLow;
end;
In case this isn't obvious to anyone here's what the compilation looks like:
Above: the intermediate variable w: LARGE_INTEGER first gets assigned the two 32bit parts and then is assigned itself to the function's result. Cost: 10 instructions.
Above: the record Int64Rec is used to cast the function's result and assign both 32bit parts directly, without the need of any other variable. Cost: 6 instructions.
Environment used: Delphi 7.0 (Build 8.1), compiler version 15.0, Win32 executable, code optimization: on.