[LLVMdev] Code generation problem

John McCall rjmccall at apple.com
Mon Feb 8 14:07:32 PST 2010

On Feb 8, 2010, at 1:53 PM, Russell Wallace wrote:
> I was testing my implementation of the usual ternary function just
> now, and it's giving the wrong answer; I would attribute this to an
> error in my parser/abstract syntax tree code, but looking at the
> output, as far as I can see, the intermediate code is correct at the
> point where I hand it to LLVM's JIT. Could someone look and see if I'm
> doing something wrong, misunderstanding the semantics of the
> intermediate code format or something?

The condition on a branch instruction is not evaluated with C condition
semantics.  It must be of i1 type, i.e. either true or false.  Your IR should
(presumably) be more like:

  %c1 = icmp ne i32 1, i32 0
   br i1 %c1, label %yes, label %no
  %c2 = icmp ne i32 2, i32 0
  br i1 %c2, ....

Also, if you are producing IR yourself, you should always be running the IR
verification pass (at least in debug mode), which should catch structural
problems like this (and many others).


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