File size calculation, Int64, and differences between 32bit and 64bit

Question

I had problems with the following code:

var
  FileSize : Int64;
...
FileSize := Info.nFileSizeLow or (Info.nFileSizeHigh shl 32);

I expecte

Answer 1

This is not really an answer, but it's too long for a comment.

I noticed Delphi gets confused when the result of an expression is to be written into a 64 bit variable, but the operands are 32 bit. I ran into this bug when I was implementing a hash function returning an 64 bit number. Your third variant works because you're first assigning the 64 bit variable, helping Delphi figure out it really needs to do 64 bit arithmetic.

I'm tempted to say both variants (1) and (2) are actually failing because Delphi generates 32 bit arithmetic and then assignes the result to the 64 bit variable. I'm tempted to say the variant that works well on your 32 bit machine benefits from some sort of "unlucky non-failure" (ie: the code is bad, but none the less it produces good results for the given test). The trouble is, COMPILED code doesn't change when moved from a 32bit machine to a 64 bit machine. If the code is the same, the input is the same, you'd have to pin the error on the CPU, but you know you didn't find an bug in your CPU, so you have to fall back and re-think your tests, or pin it on the "unluck non-failure".