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Teach LoopSimplify how to merge multiple loop exits into a single exit,

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when one of them can be converted to a trivial icmp and conditional
branch.

This addresses what is essentially a phase ordering problem.
SimplifyCFG knows how to do this transformation, but it doesn't do so
if the primary block has any instructions in it other than an icmp and
a branch. In the given testcase, the block contains other instructions,
however they are loop-invariant and can be hoisted. SimplifyCFG doesn't
have LoopInfo though, so it can't hoist them. And, it's important that
the blocks be merged before LoopRotation, as it doesn't support
multiple-exit loops.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74396 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman
2009-06-27 21:30:38 +00:00
parent cfb32203bc
commit 4b35f83b91
4 changed files with 128 additions and 1 deletions
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7
include/llvm/Transforms/Utils/Local.h
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@ -19,6 +19,7 @@ namespace llvm {
class User;
class BasicBlock;
class BranchInst;
class Instruction;
class Value;
class Pass;
@ -94,6 +95,12 @@ void MergeBasicBlockIntoOnlyPred(BasicBlock *BB);
///
bool SimplifyCFG(BasicBlock *BB);
/// FoldBranchToCommonDest - If this basic block is ONLY a setcc and a branch,
/// and if a predecessor branches to us and one of our successors, fold the
/// setcc into the predecessor and use logical operations to pick the right
/// destination.
bool FoldBranchToCommonDest(BranchInst *BI);
/// DemoteRegToStack - This function takes a virtual register computed by an
/// Instruction and replaces it with a slot in the stack frame, allocated via
/// alloca. This allows the CFG to be changed around without fear of