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John Carbon
John Carbon

Reputation: 5

LLVM 'ret' instructions

I have followed the LLVM tutorial Kaleidoscope. Now I'm trying to extern this front-end compiler for language similar to Pascal. I have this code:

program main;

function rett(): integer;
begin
    rett := 1;
    exit;
    rett := 5;
end;

var a: integer;
begin
    a := rett();    
    writeln(a);
end.

An output of this program should be a single integer '1', because of the exit command (so the line rett := 5; should'n be execute). However the output is '5'. For this input I have generate this LLVM IR:

define i32 @rett() {
entry:
  %rett = alloca i32
  store i32 0, i32* %rett
  store i32 1, i32* %rett
  %rett1 = load i32, i32* %rett
  ret i32 %rett1
  store i32 5, i32* %rett
  %rett2 = load i32, i32* %rett
  ret i32 %rett2
}

define i32 @main() {
entry:
  %a = alloca i32
  %main = alloca i32
  store i32 0, i32* %main
  store i32 0, i32* %a
  %calltmp = call i32 @rett()
  store i32 %calltmp, i32* %a
  %a1 = load i32, i32* %a
  %calltmp2 = call i32 (i8*, ...) @printf([4 x i8]* @.str, i32 %a1)
  %main3 = load i32, i32* %main
  ret i32 %main3
}

Why is not execute the first 'ret' command? Is there something what am I missing?

Update: In documentations is written:

Each basic block may optionally start with a label (giving the basic block a symbol table entry), contains a list of instructions, and ends with a terminator instruction (such as a branch or function return).

Is that mean that a "terminator" instruction can by only at the end of basic block? And if yes how should I implement return and break command for my language?

Upvotes: 0

Views: 2043

Answers (1)

eush77
eush77

Reputation: 4088

I could not reproduce the problem (your example outputs 1), but I will answer the questions anyway.

Is that mean that a "terminator" instruction can by only at the end of basic block?

That's correct. In fact, opt shows that @rett function in your example consists of these two basic blocks:

CFG for @rett

And if yes how should I implement return and break command for my language?

With ret and br.

You don't ever need to generate a terminator amidst a basic block, because, as you have noted, any instructions past it simply won't execute, so you might as well not generate them.

Upvotes: 0

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