[WebAssembly] Make register stackification more conservative

Register stackification currently checks VNInfo for changes. Make that
more accurate by testing each intervening instruction for any other defs
to the same virtual register.

Patch by Jacob Gravelle

Differential Revision: https://reviews.llvm.org/D24942

llvm-svn: 282886

1 files changed, 15 insertions, 19 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
index 047ce4104f7..6dae653af59 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
@@ -274,11 +274,11 @@ static bool HasOneUse(unsigned Reg, MachineInstr *Def,
 // TODO: Compute memory dependencies in a way that uses AliasAnalysis to be
 // more precise.
 static bool IsSafeToMove(const MachineInstr *Def, const MachineInstr *Insert,
-                         AliasAnalysis &AA, const LiveIntervals &LIS,
-                         const MachineRegisterInfo &MRI) {
+                         AliasAnalysis &AA, const MachineRegisterInfo &MRI) {
   assert(Def->getParent() == Insert->getParent());
 
   // Check for register dependencies.
+  SmallVector<unsigned, 4> MutableRegisters;
   for (const MachineOperand &MO : Def->operands()) {
     if (!MO.isReg() || MO.isUndef())
       continue;
@@ -301,21 +301,11 @@ static bool IsSafeToMove(const MachineInstr *Def, const MachineInstr *Insert,
       return false;
     }
 
-    // Ask LiveIntervals whether moving this virtual register use or def to
-    // Insert will change which value numbers are seen.
-    //
-    // If the operand is a use of a register that is also defined in the same
-    // instruction, test that the newly defined value reaches the insert point,
-    // since the operand will be moving along with the def.
-    const LiveInterval &LI = LIS.getInterval(Reg);
-    VNInfo *DefVNI =
-        (MO.isDef() || Def->definesRegister(Reg)) ?
-        LI.getVNInfoAt(LIS.getInstructionIndex(*Def).getRegSlot()) :
-        LI.getVNInfoBefore(LIS.getInstructionIndex(*Def));
-    assert(DefVNI && "Instruction input missing value number");
-    VNInfo *InsVNI = LI.getVNInfoBefore(LIS.getInstructionIndex(*Insert));
-    if (InsVNI && DefVNI != InsVNI)
-      return false;
+    // If one of the operands isn't in SSA form, it has different values at
+    // different times, and we need to make sure we don't move our use across
+    // a different def.
+    if (!MO.isDef() && !MRI.hasOneDef(Reg))
+      MutableRegisters.push_back(Reg);
   }
 
   bool Read = false, Write = false, Effects = false, StackPointer = false;
@@ -323,7 +313,8 @@ static bool IsSafeToMove(const MachineInstr *Def, const MachineInstr *Insert,
 
   // If the instruction does not access memory and has no side effects, it has
   // no additional dependencies.
-  if (!Read && !Write && !Effects && !StackPointer)
+  bool HasMutableRegisters = !MutableRegisters.empty();
+  if (!Read && !Write && !Effects && !StackPointer && !HasMutableRegisters)
     return true;
 
   // Scan through the intervening instructions between Def and Insert.
@@ -343,6 +334,11 @@ static bool IsSafeToMove(const MachineInstr *Def, const MachineInstr *Insert,
       return false;
     if (StackPointer && InterveningStackPointer)
       return false;
+
+    for (unsigned Reg : MutableRegisters)
+      for (const MachineOperand &MO : I->operands())
+        if (MO.isReg() && MO.isDef() && MO.getReg() == Reg)
+          return false;
   }
 
   return true;
@@ -781,7 +777,7 @@ bool WebAssemblyRegStackify::runOnMachineFunction(MachineFunction &MF) {
         // supports intra-block moves) and it's MachineSink's job to catch all
         // the sinking opportunities anyway.
         bool SameBlock = Def->getParent() == &MBB;
-        bool CanMove = SameBlock && IsSafeToMove(Def, Insert, AA, LIS, MRI) &&
+        bool CanMove = SameBlock && IsSafeToMove(Def, Insert, AA, MRI) &&
                        !TreeWalker.IsOnStack(Reg);
         if (CanMove && HasOneUse(Reg, Def, MRI, MDT, LIS)) {
           Insert = MoveForSingleUse(Reg, Op, Def, MBB, Insert, LIS, MFI, MRI);