[LDV][RAGreedy] Inform LiveDebugVariables about new VRegs added by InlineSpiller

Summary:
Make sure RAGreedy informs LiveDebugVariables about new VRegs
that is introduced at spill by InlineSpiller.

Consider this example

 LDV: !"var"	 [48r;128r):0 Loc0=%2

 48B   %2 = ...
 ...
 128B  %7 = ADD %2, ...

If %2 is spilled the InlineSpiller will insert spill/reload
instructions and introduces some new vregs. So we get

 48B   %4 = ...
 56B   spill %4
 ...
 120B  reload %5
 128B  %3 = ADD %5, ...

In the past we did not inform LDV about this, and when reintroducing
DBG_VALUE instruction LDV still got information that "var" had the
location of the spilled register %2 for the interval [48r;128r).
The result was bad, since we mapped "var" to the spill slot even
before the spill happened:

 %4 = ...
 DBG_VALUE %spill.0, !"var"
 spill %4 to %spill.0
 ...
 reload %5
 %3 = ADD %5, ...

This patch will inform LDV about the interval split introduced
due to spilling. So the location map in LDV will become

 !"var"	[48r;56r):1 [56r;120r):0 [120r;128r):2 Loc0=%2 Loc1=%4 Loc2=%5

And when inserting DBG_VALUE instructions we get

 %4 = ...
 DBG_VALUE %4, !"var"
 spill %4 to %spill.0
 DBG_VALUE %spill.0, !"var"
 ...
 reload %5
 DBG_VALUE %5, !"var"
 %3 = ADD %5, ...

Fixes: https://bugs.llvm.org/show_bug.cgi?id=38899

Reviewers: jmorse, vsk, aprantl

Reviewed By: jmorse

Subscribers: dstenb, wuzish, MatzeB, qcolombet, nemanjai, hiraditya, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69584

2 files changed, 32 insertions, 17 deletions

diff --git a/llvm/lib/CodeGen/LiveDebugVariables.cpp b/llvm/lib/CodeGen/LiveDebugVariables.cpp
index 253bdbb11d5..cace04430ad 100644
--- a/llvm/lib/CodeGen/LiveDebugVariables.cpp
+++ b/llvm/lib/CodeGen/LiveDebugVariables.cpp
@@ -255,6 +255,25 @@ public:
     return locations.size() - 1;
   }
 
+  /// Remove (recycle) a location number. If \p LocNo still is used by the
+  /// locInts nothing is done.
+  void removeLocationIfUnused(unsigned LocNo) {
+    // Bail out if LocNo still is used.
+    for (LocMap::const_iterator I = locInts.begin(); I.valid(); ++I) {
+      DbgValueLocation Loc = I.value();
+      if (Loc.locNo() == LocNo)
+        return;
+    }
+    // Remove the entry in the locations vector, and adjust all references to
+    // location numbers above the removed entry.
+    locations.erase(locations.begin() + LocNo);
+    for (LocMap::iterator I = locInts.begin(); I.valid(); ++I) {
+      DbgValueLocation Loc = I.value();
+      if (!Loc.isUndef() && Loc.locNo() > LocNo)
+        I.setValueUnchecked(Loc.changeLocNo(Loc.locNo() - 1));
+    }
+  }
+
   /// Ensure that all virtual register locations are mapped.
   void mapVirtRegs(LDVImpl *LDV);
 
@@ -1073,23 +1092,14 @@ UserValue::splitLocation(unsigned OldLocNo, ArrayRef<unsigned> NewRegs,
     }
   }
 
-  // Finally, remove any remaining OldLocNo intervals and OldLocNo itself.
-  locations.erase(locations.begin() + OldLocNo);
-  LocMapI.goToBegin();
-  while (LocMapI.valid()) {
-    DbgValueLocation v = LocMapI.value();
-    if (v.locNo() == OldLocNo) {
-      LLVM_DEBUG(dbgs() << "Erasing [" << LocMapI.start() << ';'
-                        << LocMapI.stop() << ")\n");
-      LocMapI.erase();
-    } else {
-      // Undef values always have location number UndefLocNo, so don't change
-      // locNo in that case. See getLocationNo().
-      if (!v.isUndef() && v.locNo() > OldLocNo)
-        LocMapI.setValueUnchecked(v.changeLocNo(v.locNo() - 1));
-      ++LocMapI;
-    }
-  }
+  // Finally, remove OldLocNo unless it is still used by some interval in the
+  // locInts map. One case when OldLocNo still is in use is when the register
+  // has been spilled. In such situations the spilled register is kept as a
+  // location until rewriteLocations is called (VirtRegMap is mapping the old
+  // register to the spill slot). So for a while we can have locations that map
+  // to virtual registers that have been removed from both the MachineFunction
+  // and from LiveIntervals.
+  removeLocationIfUnused(OldLocNo);
 
   LLVM_DEBUG({
     dbgs() << "Split result: \t";
diff --git a/llvm/lib/CodeGen/RegAllocGreedy.cpp b/llvm/lib/CodeGen/RegAllocGreedy.cpp
index d27db678f02..fc12a499438 100644
--- a/llvm/lib/CodeGen/RegAllocGreedy.cpp
+++ b/llvm/lib/CodeGen/RegAllocGreedy.cpp
@@ -3126,6 +3126,11 @@ unsigned RAGreedy::selectOrSplitImpl(LiveInterval &VirtReg,
     spiller().spill(LRE);
     setStage(NewVRegs.begin(), NewVRegs.end(), RS_Done);
 
+    // Tell LiveDebugVariables about the new ranges. Ranges not being covered by
+    // the new regs are kept in LDV (still mapping to the old register), until
+    // we rewrite spilled locations in LDV at a later stage.
+    DebugVars->splitRegister(VirtReg.reg, LRE.regs(), *LIS);
+
     if (VerifyEnabled)
       MF->verify(this, "After spilling");
   }