TIL: Inspecting LLVM inlining decisions with LLVM logging

I was curious about how LLVM makes decisions on whether to inline a function or not, and wanted to get a high level overview of its strategy. Thankfully, it seems like there is some relevant logging in InlineCost.cpp that we can take advantage of!

All we have to do is compile Clang in debug mode, then invoke clang with the arguments -mllvm -debug. All this does is pass the -debug flag to the llvm backend, which enables logging statements for us.

For example, if we wanted to analyze why the function foo may or may not be inlined below:

#include <stdio.h>
#include <stdlib.h>

int foo(int a) {
    if (a < 10) return 5;

    for (int i = 0; i < a; i++) {
        printf("%u ", i);
    return 0;

int main(int argc, char** argv) {
    if (argc < 2) {
        puts("Please enter an integer.");
    // You'll want error handling in a real application here,
    // but for simplicity we're omitting it.
    int i = strtol(argv[1], 0, 10);
    return foo(i);  

We would invoke clang main.c -mllvm -debug -O2 and look at the outputted logs. Here are the interesting log lines after redirecting the logs to a file:

  Initial cost: -35
  NumConstantArgs: 0
  NumConstantOffsetPtrArgs: 0
  NumAllocaArgs: 0
  NumConstantPtrCmps: 0
  NumConstantPtrDiffs: 0
  NumInstructionsSimplified: 4
  NumInstructions: 14
  SROACostSavings: 0
  SROACostSavingsLost: 0
  LoadEliminationCost: 0
  ContainsNoDuplicateCall: 0
  Cost: 170
  Threshold: 225
Inlining (cost=170, threshold=225), Call:   %call2 = call i32 @foo(i32 noundef %conv), !range !9

Note that the -debug argument won't work if clang is not compiled in debug mode.