llvm_ir — consolidated language guide — DedSec Project

LLVM IR

Working model

LLVM IR is a typed intermediate representation used by compiler toolchains, not a portable assembly language for direct execution on hardware.

Focused source example

define i32 @add(i32 %a, i32 %b) {
entry:
  %sum = add i32 %a, %b
  ret i32 %sum
}

Identify the function signature, named basic block and SSA value. Compare verifier acceptance of a correct return type with an intentionally mismatched one.

Evidence to collect

Record the exact LLVM_IR compiler or runtime version, required dependencies, and output from harmless test data. Predict an observation before checking it, and keep both the initial and revised source to show which change caused the result. Do not claim that a displayed fragment was executed if the platform is unavailable.

Failure mode worth investigating

An IR file's syntax and supported attributes vary by LLVM version; verify with tools from the same release.

LLVM IR: applied exercises

Run the provided module

The companion 00_starter.ll defines i32 @main() and returns i32 0. Work on a copied exercise.ll, leaving the starter untouched. With compatible LLVM command-line tools installed:

llvm-as exercise.ll -o exercise.bc
lli exercise.bc
printf 'exit status: %s\n' "$?"

An ordinary successful run should exit with status zero. lli executes the module; it does not print the returned integer as text. Record llvm-as --version and lli --version; version mismatch can explain parser failures. This recipe has not been executed here.

Predict an observable change

Change only ret i32 0 to ret i32 7, assemble again, and predict a shell status of 7 on an environment that runs main as the entry point. Do not assert that the shell can preserve arbitrary 32-bit exit values: operating systems commonly expose only a limited exit-status range. For a return value visible without a shell, a separate call harness is better than relying on $?.

Deliberate type mismatch

In a disposable copy, replace the terminator with ret i64 7 while keeping define i32 @main(). LLVM IR uses explicitly typed instructions; the function's return type and ret operand type must agree. Run llvm-as, keep the actual diagnostic, then restore ret i32 7. Do not solve the mismatch by changing random integers until the assembler accepts the file.

Control-flow experiment

Construct a second version with blocks entry, positive and nonpositive. Compare an icmp sgt i32 result with zero, branch using br i1, and ensure each reachable block ends in a terminator. Make every return i32; if you instead merge branches using phi, supply one incoming value per predecessor. Inspect with llvm-dis to see the instruction structure.

Independent review

Explain why i32 is part of the semantic contract, why a basic block cannot silently fall through, and why assembling is not the same as executing. Record one valid result, one invalid type result, the LLVM version and any target limitations. Reference: https://llvm.org/docs/LangRef.html