if's to switch transformation?

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1

hello everyone,

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?

cheers
Eric van Riet Paap

2

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.

Sorry!

-Chris

3

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.

This is what we used for the last couple of months. However, since the x86 backend is more actively being improved now I would prefer to not use the C backend anymore.

Thank for your swift reply.

cheers
Eric