What is llvm bitcode

As mentioned above: every abbreviation has a corresponding abbreviation definition , the location of which in the stream determines its corresponding abbreviation ID. In other words: abbreviation IDs are never explicitly defined anywhere. So, how do we determine which abbreviations apply to the current block scope?

We apply three rules:. The non-default abbreviation IDs for each are:. Everything here is important background for interpreting the bitstream container, but getting actual IR semantics out of is an entirely different, separate beast. I intend to dedicate a subsequent post give it about a month to just that: taking the primitives here and hacking using them to build a Rust library that can interpret bitcode modules It already has support for VBR n decoding, too.

The bitcode is stored in a section called. The macOS platform uses Mach-O files. Using native executables with embedded bitcode offers two advantages over plain bitcode files. First, build systems for native projects, for example a Makefile , expect the result to be an executable. Embedding the bitcode instead of changing the output format improves compatibility with existing projects. Second, executables allow specifying library dependencies which is not possible with LLVM bitcode.

For GraalVM Enterprise users, the manual installation is required. A terminator instruction is at the end of each basic block, and determines where to transfer control flow once the basic block finishes executing. For instance ret terminators returns control flow back to the caller function, and br terminators branches control flow either conditionally or unconditionally.

This property simplifies data flow analysis. To handle variables that are assigned more than once in the original source code, a notion of phi instructions are used in LLVM IR. A phi instruction essentially returns one value from a set of incoming values, based on the control flow path taken during execution to reach the phi instruction. Each incoming value is therefore associated with a predecessor basic block.

The phi instruction sometimes referred to as phi nodes in the above example essentially models the set of possible incoming values as distinct assignment statements, exactly one of which is executed based on the control flow path taken to reach the basic block of the phi instruction during execution.

One way to illustrate the corresponding data flow is as follows:. In general, when developing compilers which translates source code into LLVM IR, all local variables of the source code may be transformed into SSA-form, with the exception of variables of which the address is taken.

To simplify the implementation of LLVM front-ends, one recommendation is to model local variables in the source language as memory allocated variables using alloca , model assignments to local variables as store to memory, and uses of local variables as load from memory. As long as the memory accesses follows certain patters, we may then rely on the mem2reg LLVM optimization pass to translate memory allocate local variables to registers in SSA-form using phi nodes where necessary.

The primary motivation for developing a pure Go library for interacting with LLVM IR was to make it more fun to code compilers and static analysis tools that rely on and interact with the LLVM compiler framework. In part because the compile time of projects relying on the official LLVM bindings for Go could be quite substantial Thanks to aykevl , the author of TinyGo , there are now ways to speed up the compile time by dynamically linking against a system-installed version of LLVM 4.

Another leading motivation was to try and design an idiomatic Go API from the ground up.