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Extend the SpillPlacement interface with two new features.
The PrefBoth constraint is used for blocks that ideally want a live-in value both on the stack and in a register. This would be used by a block that has a use before interference forces a spill. Secondly, add the ChangesValue flag to BlockConstraint. This tells SpillPlacement if a live-in value on the stack can be reused as a live-out stack value for free. If the block redefines the virtual register, a spill would be required for that. This extra information will be used by SpillPlacement to more accurately calculate spill costs when a value can exist both on the stack and in a register. The simplest example is a basic block that reads the virtual register, but doesn't change its value. Spilling around such a block requires a reload, but no spill in the block. The spiller already knows this, but the spill placer doesn't. That can sometimes lead to suboptimal regions. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@136731 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -220,6 +220,7 @@ void SpillPlacement::addConstraints(ArrayRef<BlockConstraint> LiveBlocks) {
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0, // DontCare,
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1, // PrefReg,
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-1, // PrefSpill
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0, // PrefBoth
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-HUGE_VALF // MustSpill
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};
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@ -71,6 +71,7 @@ public:
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DontCare, ///< Block doesn't care / variable not live.
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PrefReg, ///< Block entry/exit prefers a register.
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PrefSpill, ///< Block entry/exit prefers a stack slot.
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PrefBoth, ///< Block entry prefers both register and stack.
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MustSpill ///< A register is impossible, variable must be spilled.
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};
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@ -79,6 +80,11 @@ public:
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unsigned Number; ///< Basic block number (from MBB::getNumber()).
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BorderConstraint Entry : 8; ///< Constraint on block entry.
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BorderConstraint Exit : 8; ///< Constraint on block exit.
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/// True when this block changes the value of the live range. This means
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/// the block has a non-PHI def. When this is false, a live-in value on
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/// the stack can be live-out on the stack without inserting a spill.
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bool ChangesValue;
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};
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/// prepare - Reset state and prepare for a new spill placement computation.
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@ -96,7 +102,10 @@ public:
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/// live out.
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void addConstraints(ArrayRef<BlockConstraint> LiveBlocks);
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/// addPrefSpill - Add PrefSpill constraints to all blocks listed.
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/// addPrefSpill - Add PrefSpill constraints to all blocks listed. This is
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/// equivalent to calling addConstraint with identical BlockConstraints with
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/// Entry = Exit = PrefSpill, and ChangesValue = false.
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///
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/// @param Blocks Array of block numbers that prefer to spill in and out.
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void addPrefSpill(ArrayRef<unsigned> Blocks);
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