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Bitcode does not allow cross-architectural builds. This is a common misconception. IR (& bitcode) includes architecture and platform-specific ABI. What it does

Bitcode does not allow cross-architectural builds. This is a common misconception. IR (& bitcode) includes architecture and platform-specific ABI. What it does allow is for better optimizations as the LLVM backend optimizer improves.

jevinskie 10y ago
I would imagine that with enough engineering effort, a cross-architecture "porting" of IR would be feasible. I doubt Apple will bother to do that when they can just force developers to rebuild and republish lest they get left out of the App Store.
- LeoNatan25 10y ago
  Outside of a Java-style high-level VM, cross-platform cross-architecture in the C world requires compilation. You cannot use what is not there. When compiling a C language, macros are used to determine architecture and platform, and extra code is simply not compiled. The most simplest of examples is endianness handling, which would be totally broken if Intel-compiled code is automagically made to run on arm.
  - astrange 10y ago
    iOS and macOS are both little-endian, but there are many other differences between platforms (like pointer alignment, SIMD size, ObjC ABI) and bitcode makes no effort at all to accommodate them. Bitcode can used to recompile for minor ARM updates, compiler bugs, new optimizations etc without having to get developers to submit new binaries.
  - pjmlp 10y ago
    > Outside of a Java-style high-level VM, cross-platform cross-architecture in the C world requires compilation. Kind of. On IBM i, C compiles to TIMI bytecode just like everything else. For producing actual native code directly from the compiler you need the Metal C compiler, or the POSIX compatibility environment (PASE). The TenDRA C and C++ compilers also used bytecode (TenDRA Distribution Format). http://www.tendra.org/tdf-guide http://www.tendra.org/tdf-guide
caf 10y ago
This is potentially a problem, as better optimisers can bring to life previously dormant bugs in the code.
sneak 10y ago
Ahh, good to know.
pjmlp 10y ago
Standard bitcode does not, but there are people at LLVM and Apple working on a portable version of it, if you check LLVM talks.

jevinskie 10y ago

I would imagine that with enough engineering effort, a cross-architecture "porting" of IR would be feasible. I doubt Apple will bother to do that when they can just force developers to rebuild and republish lest they get left out of the App Store.

LeoNatan25 10y ago
Outside of a Java-style high-level VM, cross-platform cross-architecture in the C world requires compilation. You cannot use what is not there. When compiling a C language, macros are used to determine architecture and platform, and extra code is simply not compiled. The most simplest of examples is endianness handling, which would be totally broken if Intel-compiled code is automagically made to run on arm.
- astrange 10y ago
  iOS and macOS are both little-endian, but there are many other differences between platforms (like pointer alignment, SIMD size, ObjC ABI) and bitcode makes no effort at all to accommodate them. Bitcode can used to recompile for minor ARM updates, compiler bugs, new optimizations etc without having to get developers to submit new binaries.
- pjmlp 10y ago
  > Outside of a Java-style high-level VM, cross-platform cross-architecture in the C world requires compilation. Kind of. On IBM i, C compiles to TIMI bytecode just like everything else. For producing actual native code directly from the compiler you need the Metal C compiler, or the POSIX compatibility environment (PASE). The TenDRA C and C++ compilers also used bytecode (TenDRA Distribution Format). http://www.tendra.org/tdf-guide http://www.tendra.org/tdf-guide

caf 10y ago

This is potentially a problem, as better optimisers can bring to life previously dormant bugs in the code.

sneak 10y ago

Ahh, good to know.

pjmlp 10y ago

Standard bitcode does not, but there are people at LLVM and Apple working on a portable version of it, if you check LLVM talks.