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Notes on how to kill the eeevil brtwoway, and make ppc branch selector
more target independant, generate better code, and be less conservative. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26809 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -18,6 +18,34 @@ unconditional branch, so we don't end up with things like:
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This occurs in SPASS.
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This occurs in SPASS.
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The power of diet coke came up with a solution to this today:
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We know the only two cases that can happen here are either:
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a) we have a conditional branch followed by a fallthrough to the next BB
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b) we have a conditional branch followed by an unconditional branch
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We also invented the BRTWOWAY node to model (b).
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Currently, these are modeled by the PPC_BRCOND node which is a 12-byte pseudo
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that codegens to
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bccinv false
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true:
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b truebb
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false:
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b falsebb
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However, realizing that for (a), we can bccinv directly to the fallthrough
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block, and for (b) we will already have another unconditional branch after
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the conditional branch (see SPASS case above), then we know that we don't need
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BRTWOWAY at all, and can just codegen PPC_BRCOND as
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bccinv +8
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b truebb
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This will also allow us to selectively not run the ppc branch selector, by just
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selecting PPC_BRCOND pseudo directly to the correct conditional branch
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instruction for small functions.
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===-------------------------------------------------------------------------===
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===-------------------------------------------------------------------------===
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* Codegen this:
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* Codegen this:
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