[LLVMdev] if's to switch transformation?

Chris Lattner sabre at nondot.org
Fri Dec 23 08:18:18 PST 2005

On Fri, 23 Dec 2005, Eric van Riet Paap wrote:
> I few days ago I had some code (for the PyPy project) that llc -march=c 
> converted to a switch statement and gcc compiled that nicely to a jumptable 
> in the .s file. Now I try to reproduce when directly going from a .ll to a .s 
> file. But I now see a long list of compare and jumps instead of the 
> jumptable. Is there a transformation that does this if->switch(ing) or is 
> that in the part of llc that generates the c file?

There is one of two things that are happening depending on what you mean.

1. Make sure you're running gccas/gccld to optimize the program.  These
    optimizations are what form the switch stmt.
2. If you're using the native backends, even if #1 is right, you won't get
    switch tables.  The current code generators lower the switches to
    branches.  This is on the near term (within a couple months) TODO list
    to improve, but remains suboptimal for now.

To get the optimization, the best approach is to use gccas/gccld combined 
with the C backend.




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