From c2ecf3efbf375fc82bb1cea6afd7448498f9ae75 Mon Sep 17 00:00:00 2001
From: Jakob Stoklund Olesen <stoklund@2pi.dk>
Date: Tue, 15 Nov 2011 01:15:30 +0000
Subject: Break false dependencies before partial register updates.

Two new TargetInstrInfo hooks lets the target tell ExecutionDepsFix
about instructions with partial register updates causing false unwanted
dependencies.

The ExecutionDepsFix pass will break the false dependencies if the
updated register was written in the previoius N instructions.

The small loop added to sse-domains.ll runs twice as fast with
dependency-breaking instructions inserted.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144602 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/CodeGen/ExecutionDepsFix.cpp | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

(limited to 'lib/CodeGen')

diff --git a/lib/CodeGen/ExecutionDepsFix.cpp b/lib/CodeGen/ExecutionDepsFix.cpp
index d094411116..050edce2ec 100644
--- a/lib/CodeGen/ExecutionDepsFix.cpp
+++ b/lib/CodeGen/ExecutionDepsFix.cpp
@@ -471,11 +471,34 @@ void ExeDepsFix::processDefs(MachineInstr *MI, bool Kill) {
     DEBUG(dbgs() << TRI->getName(RC->getRegister(rx)) << ":\t" << CurInstr
                  << '\t' << *MI);
 
+    // How many instructions since rx was last written?
+    unsigned Clearance = CurInstr - LiveRegs[rx].Def;
     LiveRegs[rx].Def = CurInstr;
 
     // Kill off domains redefined by generic instructions.
     if (Kill)
       kill(rx);
+
+    // Verify clearance before partial register updates.
+    unsigned Pref = TII->getPartialRegUpdateClearance(MI, i, TRI);
+    if (!Pref)
+      continue;
+    DEBUG(dbgs() << "Clearance: " << Clearance << ", want " << Pref);
+    if (Pref > Clearance) {
+      DEBUG(dbgs() << ": Break dependency.\n");
+      TII->breakPartialRegDependency(MI, i, TRI);
+      continue;
+    }
+
+    // The current clearance seems OK, but we may be ignoring a def from a
+    // back-edge.
+    if (!SeenUnknownBackEdge || Pref <= unsigned(CurInstr)) {
+      DEBUG(dbgs() << ": OK.\n");
+      continue;
+    }
+
+    // A def from an unprocessed back-edge may make us break this dependency.
+    DEBUG(dbgs() << ": Wait for back-edge to resolve.\n");
   }
 
   ++CurInstr;
@@ -663,6 +686,10 @@ bool ExeDepsFix::runOnMachineFunction(MachineFunction &mf) {
   for (unsigned i = 0, e = Loops.size(); i != e; ++i) {
     MachineBasicBlock *MBB = Loops[i];
     enterBasicBlock(MBB);
+    for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E;
+        ++I)
+      if (!I->isDebugValue())
+        processDefs(I, false);
     leaveBasicBlock(MBB);
   }
 
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