LLVM optimization passes break recursive code

Question

I have a problem regarding some LLVM optimization passes, which modify the compilation output, so that it is not functional anymore.

This is the input source code of a Fibonacci algorithm:

f<int> fib(int n) {
    if n <= 1 { return 1; }
    return fib(n - 1) + fib(n - 2);
}

f<int> main() {
    printf("Result: %d", fib(46));
    return 0;
}

Without optimization, my compiler spits out following IR code, which is working perfectly fine:

; ModuleID = 'Module'
source_filename = "Module"

@0 = private unnamed_addr constant [11 x i8] c"Result: %d\00", align 1

declare i32 @printf(i8*, ...)

define i32 @"fib(int)"(i32 %0) {
entry:
  %n = alloca i32, align 4
  store i32 %0, i32* %n, align 4
  %result = alloca i32, align 4
  %1 = load i32, i32* %n, align 4
  %le = icmp sle i32 %1, 1
  br i1 %le, label %then, label %end

then:                                             ; preds = %entry
  ret i32 1
  br label %end

end:                                              ; preds = %then, %entry
  %2 = load i32, i32* %n, align 4
  %sub = sub i32 %2, 1
  %3 = call i32 @"fib(int)"(i32 %sub)
  %4 = load i32, i32* %n, align 4
  %sub1 = sub i32 %4, 2
  %5 = call i32 @"fib(int)"(i32 %sub1)
  %add = add i32 %3, %5
  ret i32 %add
}

define i32 @main() {
main_entry:
  %result = alloca i32, align 4
  %0 = call i32 @"fib(int)"(i32 46)
  %1 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([11 x i8], [11 x i8]* @0, i32 0, i32 0), i32 %0)
  ret i32 0
}

Then I applied a few optimization passes to it. Here is my chain of passes:

fpm->add(llvm::createDeadCodeEliminationPass());
fpm->add(llvm::createLoopDeletionPass());
fpm->add(llvm::createDeadStoreEliminationPass());
fpm->add(llvm::createGVNPass());
fpm->add(llvm::createPromoteMemoryToRegisterPass());
fpm->add(llvm::createInstructionCombiningPass());
fpm->add(llvm::createReassociatePass());
fpm->add(llvm::createCFGSimplificationPass()); // Breaks recursion
fpm->add(llvm::createCorrelatedValuePropagationPass());
fpm->add(llvm::createLoopSimplifyPass());

With this optimization passes enabled, I get following IR code:

; ModuleID = 'Module'
source_filename = "Module"

@0 = private unnamed_addr constant [11 x i8] c"Result: %d\00", align 1

declare i32 @printf(i8*, ...)

define i32 @"fib(int)"(i32 %0) {
entry:
  %le = icmp slt i32 %0, 2
  %sub = add i32 %0, -1
  %1 = call i32 @"fib(int)"(i32 %sub)
  %sub1 = add i32 %0, -2
  %2 = call i32 @"fib(int)"(i32 %sub1)
  %add = add i32 %2, %1
  ret i32 %add
}

define i32 @main() {
main_entry:
  %0 = call i32 @"fib(int)"(i32 46)
  %1 = call i32 (i8*, ...) @printf(i8* noundef nonnull dereferenceable(1) getelementptr inbounds ([11 x i8], [11 x i8]* @0, i64 0, i64 0), i32 %0)
  ret i32 0
}

Obviously, this code does produce a stack overflow, because the recursion anchor is gone. It seems like the CFGSimplificationPass merges blocks in a wrong way / eliminates the if body although it is relevant. When I remove the 'createCFGSimplificationPass' line, the optimizations works and the executable outcome runs fine.

Now my question: What am I doing wrong? Or is this maybe a bug in LLVM?

Thanks for your help!

score 1 · Accepted Answer · answered Aug 06 '21 at 19:01

then:                                             ; preds = %entry
  ret i32 1
  br label %end

A block can't have two terminators, so this IR is invalid. This causes the optimizations to misbehave as they can't tell which one is the intended terminator.

To more easily catch errors like this in the future, you should use llvm::verifyModule to verify the IR you generate before you run additional passes on it.

LLVM optimization passes break recursive code

1 Answers1