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
diff --git a/lib/CodeGen/SpillPlacement.h b/lib/CodeGen/SpillPlacement.h
index 9a4fc6d..9d1d676 100644
--- a/lib/CodeGen/SpillPlacement.h
+++ b/lib/CodeGen/SpillPlacement.h
@@ -71,6 +71,7 @@
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 @@
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 @@
/// 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);