I’m putting this here so I have a place to look for this later. In macOS, iOS, tvOS, etc., there are a number of target conditionals that are set in “TargetConditionals.h” on Xcode which allow you to detect what you’re compiling for.

A number of these constants will probably never be seen in the wild. Certainly you’re not going to see a PowerPC running macOS Big Sur anytime soon.

I pulled this directly out of the comments.

Those are:

TARGET_CPU_*

These conditionals specify which microprocessor instruction set is being
generated. At most one of these is true, the rest are false.

• TARGET_CPU_PPC – Compiler is generating PowerPC instructions for 32-bit mode
• TARGET_CPU_PPC64 – Compiler is generating PowerPC instructions for 64-bit mode
• TARGET_CPU_68K – Compiler is generating 680×0 instructions
• TARGET_CPU_X86 – Compiler is generating x86 instructions for 32-bit mode
• TARGET_CPU_X86_64 – Compiler is generating x86 instructions for 64-bit mode
• TARGET_CPU_ARM – Compiler is generating ARM instructions for 32-bit mode
• TARGET_CPU_ARM64 – Compiler is generating ARM instructions for 64-bit mode
• TARGET_CPU_MIPS – Compiler is generating MIPS instructions
• TARGET_CPU_SPARC – Compiler is generating Sparc instructions
• TARGET_CPU_ALPHA – Compiler is generating Dec Alpha instructions

TARGET_OS_*

These conditionals specify in which Operating System the generated code will
run. Indention is used to show which conditionals are evolutionary subclasses.

The MAC/WIN32/UNIX conditionals are mutually exclusive.
The IOS/TV/WATCH conditionals are mutually exclusive.

• TARGET_OS_WIN32 – Generated code will run under 32-bit Windows
• TARGET_OS_UNIX – Generated code will run under some Unix (not OSX)
• TARGET_OS_MAC – Generated code will run under Mac OS X variant
• TARGET_OS_OSX – Generated code will run under OS X devices
• TARGET_OS_IPHONE – Generated code for firmware, devices, or simulator
• TARGET_OS_IOS – Generated code will run under iOS
• TARGET_OS_TV – Generated code will run under Apple TV OS
• TARGET_OS_WATCH – Generated code will run under Apple Watch OS
• TARGET_OS_BRIDGE – Generated code will run under Bridge devices
• TARGET_OS_MACCATALYST – Generated code will run under macOS
• TARGET_OS_SIMULATOR – Generated code will run under a simulator
• TARGET_OS_EMBEDDED – DEPRECATED: Use TARGET_OS_IPHONE and/or TARGET_OS_SIMULATOR instead
• TARGET_IPHONE_SIMULATOR – DEPRECATED: Same as TARGET_OS_SIMULATOR
• TARGET_OS_NANO – DEPRECATED: Same as TARGET_OS_WATCH

  +---------------------------------------------------------------------+
  |                            TARGET_OS_MAC                            |
  | +---+ +-----------------------------------------------+ +---------+ |
  | |   | |               TARGET_OS_IPHONE                | |         | |
  | |   | | +---------------+ +----+ +-------+ +--------+ | |         | |
  | |   | | |      IOS      | |    | |       | |        | | |         | |
  | |OSX| | |+-------------+| | TV | | WATCH | | BRIDGE | | |DRIVERKIT| |
  | |   | | || MACCATALYST || |    | |       | |        | | |         | |
  | |   | | |+-------------+| |    | |       | |        | | |         | |
  | |   | | +---------------+ +----+ +-------+ +--------+ | |         | |
  | +---+ +-----------------------------------------------+ +---------+ |
  +---------------------------------------------------------------------+

TARGET_RT_*

These conditionals specify in which runtime the generated code will
run. This is needed when the OS and CPU support more than one runtime
(e.g. Mac OS X supports CFM and mach-o).

• TARGET_RT_LITTLE_ENDIAN – Generated code uses little endian format for integers
• TARGET_RT_BIG_ENDIAN – Generated code uses big endian format for integers
• TARGET_RT_64_BIT – Generated code uses 64-bit pointers
• TARGET_RT_MAC_CFM – TARGET_OS_MAC is true and CFM68K or PowerPC CFM (TVectors) are used
• TARGET_RT_MAC_MACHO – TARGET_OS_MAC is true and Mach-O/dlyd runtime is used