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
This commit is contained in:
Jakob Stoklund Olesen 2011-08-02 21:53:03 +00:00
parent f78faf84a8
commit 0e0a8806d4
2 changed files with 11 additions and 1 deletions

View File

@ -220,6 +220,7 @@ void SpillPlacement::addConstraints(ArrayRef<BlockConstraint> LiveBlocks) {
0, // DontCare,
1, // PrefReg,
-1, // PrefSpill
0, // PrefBoth
-HUGE_VALF // MustSpill
};

View File

@ -71,6 +71,7 @@ public:
DontCare, ///< Block doesn't care / variable not live.
PrefReg, ///< Block entry/exit prefers a register.
PrefSpill, ///< Block entry/exit prefers a stack slot.
PrefBoth, ///< Block entry prefers both register and stack.
MustSpill ///< A register is impossible, variable must be spilled.
};
@ -79,6 +80,11 @@ public:
unsigned Number; ///< Basic block number (from MBB::getNumber()).
BorderConstraint Entry : 8; ///< Constraint on block entry.
BorderConstraint Exit : 8; ///< Constraint on block exit.
/// True when this block changes the value of the live range. This means
/// the block has a non-PHI def. When this is false, a live-in value on
/// the stack can be live-out on the stack without inserting a spill.
bool ChangesValue;
};
/// prepare - Reset state and prepare for a new spill placement computation.
@ -96,7 +102,10 @@ public:
/// live out.
void addConstraints(ArrayRef<BlockConstraint> LiveBlocks);
/// addPrefSpill - Add PrefSpill constraints to all blocks listed.
/// addPrefSpill - Add PrefSpill constraints to all blocks listed. This is
/// equivalent to calling addConstraint with identical BlockConstraints with
/// Entry = Exit = PrefSpill, and ChangesValue = false.
///
/// @param Blocks Array of block numbers that prefer to spill in and out.
void addPrefSpill(ArrayRef<unsigned> Blocks);