Cast performance from size_t to double

Question

TL;DR: Why is multiplying/casting data in size_t slow and why does this vary per platform?

I\'m having some performance issues that I don\'

Answer 1

On x86, the conversion of uint64_t to floating point is slower because there are only instructions to convert int64_t, int32_t and int16_t. int16_t and in 32-bit mode int64_t can only be converted using x87 instructions, not SSE.

When converting uint64_t to floating point, GCC 4.2.1 first converts the value as if it were an int64_t and then adds 2⁶⁴ if it was negative to compensate. (When using the x87, on Windows and *BSD or if you changed the precision control, beware that the conversion ignores precision control but the addition respects it.)

An uint32_t is first extended to int64_t.

When converting 64-bit integers in 32-bit mode on processors with certain 64-bit capabilities, a store-to-load forwarding issue may cause stalls. The 64-bit integer is written as two 32-bit values and read back as one 64-bit value. This can be very bad if the conversion is part of a long dependency chain (not in this case).