[WebAssembly] Fix dominance check for PHIs in the StoreResult pass

When a block has no terminator instructions, getFirstTerminator() returns
end(), which can't be used in dominance checks. Check dominance for phi
operands separately.

Also, remove some bits from WebAssemblyRegStackify.cpp that were causing
trouble on the same testcase; they were left behind from an earlier
experiment.

Differential Revision: http://reviews.llvm.org/D15210

llvm-svn: 254662

1 files changed, 22 insertions, 11 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyStoreResults.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyStoreResults.cpp
index 3a7f50e3b14..4a8fc09878c 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyStoreResults.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyStoreResults.cpp
@@ -10,9 +10,12 @@
 /// \file
 /// \brief This file implements an optimization pass using store result values.
 ///
-/// WebAssembly's store instructions return the stored value, specifically to
-/// enable the optimization of reducing get_local/set_local traffic, which is
-/// what we're doing here.
+/// WebAssembly's store instructions return the stored value. This is to enable
+/// an optimization wherein uses of the stored value can be replaced by uses of
+/// the store's result value, making the stored value register more likely to
+/// be single-use, thus more likely to be useful to register stackifying, and
+/// potentially also exposing the store to register stackifying. These both can
+/// reduce get_local/set_local traffic.
 ///
 //===----------------------------------------------------------------------===//
 
@@ -89,14 +92,22 @@ bool WebAssemblyStoreResults::runOnMachineFunction(MachineFunction &MF) {
         for (auto I = MRI.use_begin(FromReg), E = MRI.use_end(); I != E;) {
           MachineOperand &O = *I++;
           MachineInstr *Where = O.getParent();
-          if (Where->getOpcode() == TargetOpcode::PHI)
-            Where = Where->getOperand(&O - &Where->getOperand(0) + 1)
-                        .getMBB()
-                        ->getFirstTerminator();
-          if (&MI == Where || !MDT.dominates(&MI, Where))
-            continue;
-          DEBUG(dbgs() << "Setting operand " << O << " in " << *Where <<
-                " from " << MI <<"\n");
+          if (Where->getOpcode() == TargetOpcode::PHI) {
+            // PHIs use their operands on their incoming CFG edges rather than
+            // in their parent blocks. Get the basic block paired with this use
+            // of FromReg and check that MI's block dominates it.
+            MachineBasicBlock *Pred =
+                Where->getOperand(&O - &Where->getOperand(0) + 1).getMBB();
+            if (!MDT.dominates(&MBB, Pred))
+              continue;
+          } else {
+            // For a non-PHI, check that MI dominates the instruction in the
+            // normal way.
+            if (&MI == Where || !MDT.dominates(&MI, Where))
+              continue;
+          }
+          DEBUG(dbgs() << "Setting operand " << O << " in " << *Where
+                       << " from " << MI << "\n");
           O.setReg(ToReg);
         }
       }