# WAMR XIP (Execution In Place) feature introduction
Some IoT devices may require to run the AOT file from flash or ROM which is read-only, so as to reduce the memory consumption, or resolve the issue that there is no executable memory available to run AOT code. In such case, the AOT code inside the AOT file shouldn't be duplicated into memory and shouldn't be modified (or patched) by the AOT relocations. To address this, WAMR implements the XIP (Execution In Place) feature, which generates the AOT relocations as few as possible:
- In the AOT code, an AOT function calls other functions with indirect mode: it doesn't call other functions directly, but looks up their pointers from the function pointer table passed by its first argument exec_env, and then calls the function pointer found. By this way the relocations to other functions are eliminated.
- Eliminate the calls to the LLVM intrinsic functions, or, replace calling them with calling runtime self implemented functions instead, e.g. the calling to `llvm.experimental.constrained.fadd.f32` is replaced by the calling to `aot_intrinsic_fadd_f32`.
The XIP file is an AOT file without (or with few) relocations to patch the AOT code (or text section). Developer can use the option `--enable-indirect-mode --disable-llvm-intrinsics` for wamrc to generate the AOT file, e.g.:
WAMR provides a default mapping table for some targets, but it may not be the best one for your target. And it doesn't cover all the supported targets.
So, wamrc provides the option `--enable-builtin-intrinsics=<intr1,intr2,...>` to make it possible to tune the intrinsic functions for your target.
Firstly, you should understand why we don't use the LLVM intrinsic functions directly. The reason is that the LLVM intrinsic functions can't map to the native instructions directly, e.g. the LLVM intrinsic function `i32.div_s` can't map to the native instruction if the target doesn't support the division instruction, it will be translated to a function call to the runtime function from libgcc/compiler-rt. This will cause the AOT code to have the relocations to the libgcc/compiler-rt, which is not acceptable for the XIP feature.
So, we need to replace the LLVM intrinsic functions with the runtime self implemented functions, which can be called through the function pointer table (--enable-indirect-mode) and don't have the relocations to the libgcc/compiler-rt (--disable-llvm-intrinsics).
For ARM Cortex-M55, since it has double precision floating point unit, so it can support f32/f64 operations. But as a 32-bit MCU, it can only support 32-bit integer operations. So we can use the following command to generate the XIP binary:
For ARM Cortex-M3, since it has no floating point unit, and it can only support 32-bit integer operations. So we can use the following command to generate the XIP binary:
