[llvm-commits] [llvm] r136731 - in /llvm/trunk/lib/CodeGen: SpillPlacement.cpp SpillPlacement.h

[Jakob Stoklund Olesen]

Author: stoklund
Date: Tue Aug  2 16:53:03 2011
New Revision: 136731

URL: http://llvm.org/viewvc/llvm-project?rev=136731&view=rev
Log:
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.

Modified:
    llvm/trunk/lib/CodeGen/SpillPlacement.cpp
    llvm/trunk/lib/CodeGen/SpillPlacement.h

Modified: llvm/trunk/lib/CodeGen/SpillPlacement.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SpillPlacement.cpp?rev=136731&r1=136730&r2=136731&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SpillPlacement.cpp (original)
+++ llvm/trunk/lib/CodeGen/SpillPlacement.cpp Tue Aug  2 16:53:03 2011
@@ -220,6 +220,7 @@
       0,           // DontCare,
       1,           // PrefReg,
       -1,          // PrefSpill
+      0,           // PrefBoth
       -HUGE_VALF   // MustSpill
     };
 

Modified: llvm/trunk/lib/CodeGen/SpillPlacement.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SpillPlacement.h?rev=136731&r1=136730&r2=136731&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SpillPlacement.h (original)
+++ llvm/trunk/lib/CodeGen/SpillPlacement.h Tue Aug  2 16:53:03 2011
@@ -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);