Trunc() fails for high Int64 for 64-bit builds

Question

Why does the following code fail for 64-bit builds (but works for 32-bit builds)?

var
  TruncTmp: Extended;
begin
  TruncTmp := 9223372036854775296;
  TruncTmp := Trunc(TruncTmp); // this fails on 64-bit
  Assert(TruncTmp = 9223372036854775296);
end;

First chance exception at $0000000000405D70. Exception class $C0000090 with message 'c0000090 FLOAT_INVALID_OPERATION'.

Is this a bug or am I missing something?

Notes:

• I am using Delphi 10.2
• Numbers < 9223372036854775296 work fine

Answer 1

There is no "extended" (80-bit FP) type under Win64 - it is just an alias to "double" (64-bit FP). So you only have 53-bit of resolution in double.

So my guess it is that is as expected in your case.