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Comment by flohofwoe | Hacker News Reader

It should indeed, and in hindsight it would have been better to move int to 64 bits (especially for C's integer promotion, which only really makes sense when the promotion happens to the register width.

But porting 32-bit code to 64-bit was a big deal back then, and C99 with its new fixed-width integer types overlapped with the first AMD64 CPUs (and Microsoft's MSVC didn't start to support C99 until around 2015 anyway), I guess keeping int on 32-bits in the popular compilers was deemed 'safer' for porting existing code. I guess we can already be lucky that all the big compilers agreed on the same int width.

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I've always believed that they kept 32 bits ints on 64 bits CPU as a default because going full 64 bits would make the code and data structures bigger for "no reason" (it's not often that one hits the 4.10^9 limit in system code (if you don't count timestamps, that is)). For instance, a load-register-with-immediate instruction would normally take 5 bytes (opcode+value) on 32 bits, but 9 bytes on 64.

> would make... data structures bigger for "no reason"

This was the main reason that in stayed 32 bits. Not because larger footprint of structs would be catastrophic — it wouldn't: the 64-bit migration was motivated mainly by the fact that the 32-bit architectures couldn't easily address all that actually existing physical memory past 4 GiB (and virtual memory as well) — but because lots and I mean lots of on-disk data structures were defined in terms of char/short/int, people routinely dumped/gulped their data structures as-is, with no marshalling, so changing the size of int would break literally everything that worked with on-disk data, starting with the filesystem implementations themselves.

I think backward compatibility was the main aim. Intel tried redesigning the architecture as 64-bit native (Itanium), but AMD done a better job at backward compatibility - and intel eventually adopted it as x86-64.

The amd64 design could run most 32-bit code with minimal changes. All the 32-bit instructions had the same encoding, besides push/pop which instead acted on 64-bits. The 64-bit instructions were basically opt-in, though a few opcodes (0x40..0x4F) had to be deprecated for the REX prefix.

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