[llvm] r254662 - [WebAssembly] Fix dominance check for PHIs in the StoreResult pass

Dan Gohman via llvm-commits llvm-commits at lists.llvm.org
Thu Dec 3 15:07:03 PST 2015


Author: djg
Date: Thu Dec  3 17:07:03 2015
New Revision: 254662

URL: http://llvm.org/viewvc/llvm-project?rev=254662&view=rev
Log:
[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

Modified:
    llvm/trunk/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
    llvm/trunk/lib/Target/WebAssembly/WebAssemblyStoreResults.cpp
    llvm/trunk/test/CodeGen/WebAssembly/store-results.ll

Modified: llvm/trunk/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp?rev=254662&r1=254661&r2=254662&view=diff
==============================================================================
--- llvm/trunk/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp (original)
+++ llvm/trunk/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp Thu Dec  3 17:07:03 2015
@@ -81,6 +81,7 @@ static void ImposeStackOrdering(MachineI
 // more precise.
 static bool IsSafeToMove(const MachineInstr *Def, const MachineInstr *Insert,
                          AliasAnalysis &AA) {
+  assert(Def->getParent() == Insert->getParent());
   bool SawStore = false, SawSideEffects = false;
   MachineBasicBlock::const_iterator D(Def), I(Insert);
   for (--I; I != D; --I)
@@ -155,17 +156,15 @@ bool WebAssemblyRegStackify::runOnMachin
             Def->getOpcode() == WebAssembly::ARGUMENT_F64)
           continue;
 
-        // Single-use expression trees require defs that have one use, or that
-        // they be trivially clonable.
+        // Single-use expression trees require defs that have one use.
         // TODO: Eventually we'll relax this, to take advantage of set_local
         // returning its result.
         if (!MRI.hasOneUse(Reg))
           continue;
 
-        // For now, be conservative and don't look across block boundaries,
-        // unless we have something trivially clonable.
+        // For now, be conservative and don't look across block boundaries.
         // TODO: Be more aggressive.
-        if (Def->getParent() != &MBB && !Def->isMoveImmediate())
+        if (Def->getParent() != &MBB)
           continue;
 
         // Don't move instructions that have side effects or memory dependencies

Modified: llvm/trunk/lib/Target/WebAssembly/WebAssemblyStoreResults.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/WebAssembly/WebAssemblyStoreResults.cpp?rev=254662&r1=254661&r2=254662&view=diff
==============================================================================
--- llvm/trunk/lib/Target/WebAssembly/WebAssemblyStoreResults.cpp (original)
+++ llvm/trunk/lib/Target/WebAssembly/WebAssemblyStoreResults.cpp Thu Dec  3 17:07:03 2015
@@ -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::runOnMachi
         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);
         }
       }

Modified: llvm/trunk/test/CodeGen/WebAssembly/store-results.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/WebAssembly/store-results.ll?rev=254662&r1=254661&r2=254662&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/WebAssembly/store-results.ll (original)
+++ llvm/trunk/test/CodeGen/WebAssembly/store-results.ll Thu Dec  3 17:07:03 2015
@@ -16,3 +16,46 @@ entry:
   store i32 0, i32* %p
   ret i32 0
 }
+
+; Test interesting corner cases for wasm-store-results, in which the operand of
+; a store ends up getting used by a phi, which needs special handling in the
+; dominance test, since phis use their operands on their incoming edges.
+
+%class.Vec3 = type { float, float, float }
+
+ at pos = global %class.Vec3 zeroinitializer, align 4
+
+; CHECK-LABEL: foo:
+; CHECK: i32.store $discard=, $pop0, $0
+define void @foo() {
+for.body.i:
+  br label %for.body5.i
+
+for.body5.i:
+  %i.0168.i = phi i32 [ 0, %for.body.i ], [ %inc.i, %for.body5.i ]
+  %conv6.i = sitofp i32 %i.0168.i to float
+  store volatile float 0.0, float* getelementptr inbounds (%class.Vec3, %class.Vec3* @pos, i32 0, i32 0)
+  %inc.i = add nuw nsw i32 %i.0168.i, 1
+  %exitcond.i = icmp eq i32 %inc.i, 256
+  br i1 %exitcond.i, label %for.cond.cleanup4.i, label %for.body5.i
+
+for.cond.cleanup4.i:
+  ret void
+}
+
+; CHECK-LABEL: bar:
+; CHECK: i32.store $discard=, $0, $pop0
+define void @bar() {
+for.body.i:
+  br label %for.body5.i
+
+for.body5.i:
+  %i.0168.i = phi float [ 0.0, %for.body.i ], [ %inc.i, %for.body5.i ]
+  store volatile float 0.0, float* getelementptr inbounds (%class.Vec3, %class.Vec3* @pos, i32 0, i32 0)
+  %inc.i = fadd float %i.0168.i, 1.0
+  %exitcond.i = fcmp oeq float %inc.i, 256.0
+  br i1 %exitcond.i, label %for.cond.cleanup4.i, label %for.body5.i
+
+for.cond.cleanup4.i:
+  ret void
+}




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