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What’s the use case for running a 32bit binary in a 64bit cpu/os? Is there any advantage? Or is it simply to avoid having to recompile twice to support two arch

What’s the use case for running a 32bit binary in a 64bit cpu/os? Is there any advantage? Or is it simply to avoid having to recompile twice to support two architectures?

rwmj 2y ago
Many server-class Aarch64 chips have dropped support for 32 bit so in those cases it's not even an option.
- adrian_b 2y ago
  Also some of the latest smartphone chips have a part of the cores that no longer can execute 32-bit code. The next generations will not include any core supporting the 32-bit ISA.
- ithkuil 2y ago
  You can still use 32-bit wide addresses in 64-bit mode and sign-extend them into 64-bit registers. If you have a statically compiled binary all you need is compiler support for 32-bit address mode in the otherwise 64-bit ready instruction set and on x86 you get access to more registers etc but you don't pay the price for the wider addresses stored in data structures. OTOH if you use shared libraries you also have to have your shared libraries compiled in that mode too. The difficulty of dealing with that depends also if you gave a hybrid system or a full 32 bit userspace
  - leni536 2y ago
    Yes, that's an alternate ABI with 32-bit addressing targeting the same 64-bit instruction set. But it's not trivial, as you described. You might also want some kernel support too, and there might not be enough demand to maintain that support.
  - namibj 2y ago
    AKA x32 on AMD64.
0x0 2y ago
Closed-source applications or games, I'd say.
Dalewyn 2y ago
Backwards compatibility with older binaries (some of which might not have devs anymore), and infrastructure and workflows already in place. Granted this might not actually be worth considering here, given the much shorter history unlike x86(-64).
nickcw 2y ago
Apart from backwards compatibility, 32 bit apps can use less memory with half size pointers and ints than 64 bit apps.
- the_mitsuhiko 2y ago
  That and there are some applications that have algorithms that scale really badly to the much larger size of the address space.
- Dwedit 2y ago
  I once read an article about a project that placed everything into 64KB size blocks. Any pointer within the block is 16-bit, and you can also have references to other blocks. First advantage is that the pointers become tiny, so the objects took up a lot less space. The other advantage was that when it was time to serialize and deserialize the data, you didn't have to do any processing at all.
  - dmitrygr 2y ago
    > placed everything into 64KB size blocks. Any pointer within the block is 16-bit, and you can also have references to other blocks. Just reinvented segmentation
- NavinF 2y ago
  That only applies to apps that allocate a ton of objects and refer to them using pointers instead of using a dynamic array with 16bit/32bit indexes
notepad0x90 2y ago
Under Linux, I have no idea. but aarch has a wide variety of use cases. 64bit means more transistors on cpus, and wider instructions. The people that implement arm's designs would like to save money on all that. if it is an embedded device or a limited use device, it may not need 64bit. Especially when you're mass manufacturing, every $0.01 counts. Just a side-note, I am always intrigued by 16bit thumb and aarch32 "interworking", basically flipping on/off the LSB of the PC(EIP/RIP equivalent register) to tell the cpu that the next instruction is thumb mode or a32, letting them mix different instruction sets within the same program.
t-3 2y ago
The advantage of 64-bit is only a wider integer range and address space which isn't needed for most applications, so trading those for portability is probably good. I've also heard that many people doing ARM assembly prefer A32 NEON over A64 NEON.
- pm215 2y ago
  Depends what you want to be portable to -- now that 64 bit only Arm CPUs are becoming more common, building for 32 bit means you cannot run on those systems.
caerwy 2y ago
some legacy applications were designed around having a 32 bit wordsize for pointers. so calculations of offsets into memory where a data structure includes pointers depends on a pointer size.
chasil 2y ago
For ARM, 64-bit has removed features of the instruction set. Notibly, conditional execution is gone. If you have a compiled binary but lack source, then this subsystem is helpful.

What’s the use case for running a 32bit binary in a 64bit cpu/os? Is there any advantage? Or is it simply to avoid having to recompile twice to support two architectures?

rwmj 2y ago

Many server-class Aarch64 chips have dropped support for 32 bit so in those cases it's not even an option.

adrian_b 2y ago
Also some of the latest smartphone chips have a part of the cores that no longer can execute 32-bit code. The next generations will not include any core supporting the 32-bit ISA.
ithkuil 2y ago
You can still use 32-bit wide addresses in 64-bit mode and sign-extend them into 64-bit registers. If you have a statically compiled binary all you need is compiler support for 32-bit address mode in the otherwise 64-bit ready instruction set and on x86 you get access to more registers etc but you don't pay the price for the wider addresses stored in data structures. OTOH if you use shared libraries you also have to have your shared libraries compiled in that mode too. The difficulty of dealing with that depends also if you gave a hybrid system or a full 32 bit userspace
- leni536 2y ago
  Yes, that's an alternate ABI with 32-bit addressing targeting the same 64-bit instruction set. But it's not trivial, as you described. You might also want some kernel support too, and there might not be enough demand to maintain that support.
- namibj 2y ago
  AKA x32 on AMD64.

0x0 2y ago

Closed-source applications or games, I'd say.

Dalewyn 2y ago

Backwards compatibility with older binaries (some of which might not have devs anymore), and infrastructure and workflows already in place. Granted this might not actually be worth considering here, given the much shorter history unlike x86(-64).

nickcw 2y ago

Apart from backwards compatibility, 32 bit apps can use less memory with half size pointers and ints than 64 bit apps.

the_mitsuhiko 2y ago
That and there are some applications that have algorithms that scale really badly to the much larger size of the address space.
Dwedit 2y ago
I once read an article about a project that placed everything into 64KB size blocks. Any pointer within the block is 16-bit, and you can also have references to other blocks. First advantage is that the pointers become tiny, so the objects took up a lot less space. The other advantage was that when it was time to serialize and deserialize the data, you didn't have to do any processing at all.
- dmitrygr 2y ago
  > placed everything into 64KB size blocks. Any pointer within the block is 16-bit, and you can also have references to other blocks. Just reinvented segmentation
NavinF 2y ago
That only applies to apps that allocate a ton of objects and refer to them using pointers instead of using a dynamic array with 16bit/32bit indexes

notepad0x90 2y ago

Under Linux, I have no idea. but aarch has a wide variety of use cases. 64bit means more transistors on cpus, and wider instructions. The people that implement arm's designs would like to save money on all that. if it is an embedded device or a limited use device, it may not need 64bit. Especially when you're mass manufacturing, every $0.01 counts. Just a side-note, I am always intrigued by 16bit thumb and aarch32 "interworking", basically flipping on/off the LSB of the PC(EIP/RIP equivalent register) to tell the cpu that the next instruction is thumb mode or a32, letting them mix different instruction sets within the same program.

t-3 2y ago

The advantage of 64-bit is only a wider integer range and address space which isn't needed for most applications, so trading those for portability is probably good. I've also heard that many people doing ARM assembly prefer A32 NEON over A64 NEON.

pm215 2y ago
Depends what you want to be portable to -- now that 64 bit only Arm CPUs are becoming more common, building for 32 bit means you cannot run on those systems.

caerwy 2y ago

some legacy applications were designed around having a 32 bit wordsize for pointers. so calculations of offsets into memory where a data structure includes pointers depends on a pointer size.

chasil 2y ago

For ARM, 64-bit has removed features of the instruction set. Notibly, conditional execution is gone. If you have a compiled binary but lack source, then this subsystem is helpful.