What <4GB workloads would have worse performance in the Linux x32 ABI than x64?

Question

There is a relatively new Linux ABI referred to as x32, where the x86-64 processor is run in 32-bit mode, so pointers are still only 32-bits, but the 64-bit architecture specific registers are still used. So you're still limited to 4GB max memory use as in normal 32-bit, but your pointers use up less cache space than they do in 64-bit, you can do 64-bit arithmetic efficiently, and you get access to more registers (16) than you would in vanilla 32-bit (8).

Assuming you have a workload the fits nicely within 4GB, is there any way the performance of x32 could be worse than on x86-64?

It seems to me that if you don't need the extra memory space nothing is lost -- you should always get the same perf (when you already fit in cache) or better (when the pointer space savings lets you fit more in cache). But it wouldn't surprise me if there are paging/TLB/etc. details that I don't know about.

Answer 1

Certainly if you have a multithreaded program, the fact that data structures are smaller on x32 might cause cache line fighting between threads -- different objects might get allocated on the same cache line in x32 mode and different cache lines in x86_64 mode. If two threads modify those objects independently the cache ping-ponging could severely slow down the x32 code. Of course, this kind of cache effect could happen regardless of pointer size, but if the code has been tuned assuming 64-bit pointers, going to 32-bit pointers could de-tune things.

Answer 2

In X32 the processor is actually executing in "long mode", the same mode as for x86_64. That is, addresses as seen by the processor when doing addressing are still 64 bits, however the X32 ABI makes sure that all addresses are small enough to fit into 32 bits. As a result of this, in some case there is some slight overhead when pointers have to be zero extended from 32 bits to 64.

Also, needing x86/x86-64/x32 libraries in RAM, which I suppose is what one will end up with in practice (unless you're talking about some embedded or other tightly controlled system rather than a general purpose computer), may eat up some of the benefit of X32.