Changes in directory llvm/lib/Target/PowerPC:
README.txt updated: 1.72 -> 1.73 --- Log message: Notes on how to kill the eeevil brtwoway, and make ppc branch selector more target independant, generate better code, and be less conservative. --- Diffs of the changes: (+28 -0) README.txt | 28 ++++++++++++++++++++++++++++ 1 files changed, 28 insertions(+) Index: llvm/lib/Target/PowerPC/README.txt diff -u llvm/lib/Target/PowerPC/README.txt:1.72 llvm/lib/Target/PowerPC/README.txt:1.73 --- llvm/lib/Target/PowerPC/README.txt:1.72 Thu Mar 16 16:25:55 2006 +++ llvm/lib/Target/PowerPC/README.txt Thu Mar 16 16:37:48 2006 @@ -18,6 +18,34 @@ This occurs in SPASS. +The power of diet coke came up with a solution to this today: + +We know the only two cases that can happen here are either: +a) we have a conditional branch followed by a fallthrough to the next BB +b) we have a conditional branch followed by an unconditional branch + +We also invented the BRTWOWAY node to model (b). + +Currently, these are modeled by the PPC_BRCOND node which is a 12-byte pseudo +that codegens to + bccinv false +true: + b truebb +false: + b falsebb + +However, realizing that for (a), we can bccinv directly to the fallthrough +block, and for (b) we will already have another unconditional branch after +the conditional branch (see SPASS case above), then we know that we don't need +BRTWOWAY at all, and can just codegen PPC_BRCOND as + +bccinv +8 +b truebb + +This will also allow us to selectively not run the ppc branch selector, by just +selecting PPC_BRCOND pseudo directly to the correct conditional branch +instruction for small functions. + ===-------------------------------------------------------------------------=== * Codegen this: _______________________________________________ llvm-commits mailing list llvm-commits@cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
