[llvm] r285080 - [EarlyCSE] Make MemorySSA memory dependency check more aggressive.

[Geoff Berry via llvm-commits]

Author: gberry
Date: Tue Oct 25 11:18:47 2016
New Revision: 285080

URL: http://llvm.org/viewvc/llvm-project?rev=285080&view=rev
Log:
[EarlyCSE] Make MemorySSA memory dependency check more aggressive.

Now that MemorySSA keeps track of whether MemoryUses are optimized, use
getClobberingMemoryAccess() to check MemoryUse memory dependencies since
it should no longer be so expensive.

This is a follow-up change to https://reviews.llvm.org/D25881

Modified:
    llvm/trunk/lib/Transforms/Scalar/EarlyCSE.cpp
    llvm/trunk/test/Transforms/EarlyCSE/memoryssa.ll

Modified: llvm/trunk/lib/Transforms/Scalar/EarlyCSE.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/EarlyCSE.cpp?rev=285080&r1=285079&r2=285080&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/EarlyCSE.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/EarlyCSE.cpp Tue Oct 25 11:18:47 2016
@@ -496,12 +496,11 @@ private:
   void removeMSSA(Instruction *Inst) {
     if (!MSSA)
       return;
-    // FIXME: Removing a store here can leave MemorySSA in an unoptimized state
-    // by creating MemoryPhis that have identical arguments and by creating
+    // Removing a store here can leave MemorySSA in an unoptimized state by
+    // creating MemoryPhis that have identical arguments and by creating
     // MemoryUses whose defining access is not an actual clobber.  We handle the
-    // phi case here, but the non-optimized MemoryUse case is not handled.  Once
-    // MemorySSA tracks whether uses are optimized this will be taken care of on
-    // the MemorySSA side.
+    // phi case eagerly here.  The non-optimized MemoryUse case is lazily
+    // updated by MemorySSA getClobberingMemoryAccess.
     if (MemoryAccess *MA = MSSA->getMemoryAccess(Inst)) {
       // Optimize MemoryPhi nodes that may become redundant by having all the
       // same input values once MA is removed.
@@ -564,17 +563,8 @@ bool EarlyCSE::isSameMemGeneration(unsig
   // LaterInst, if LaterDef dominates EarlierInst then it can't occur between
   // EarlierInst and LaterInst and neither can any other write that potentially
   // clobbers LaterInst.
-  // FIXME: Use getClobberingMemoryAccess only for stores since it is currently
-  // fairly expensive to call on MemoryUses since it does an AA check even for
-  // MemoryUses that were already optimized by MemorySSA construction.  Once
-  // MemorySSA optimized use tracking change has been committed we can use
-  // getClobberingMemoryAccess for MemoryUses as well.
-  MemoryAccess *LaterMA = MSSA->getMemoryAccess(LaterInst);
-  MemoryAccess *LaterDef;
-  if (auto *LaterUse = dyn_cast<MemoryUse>(LaterMA))
-    LaterDef = LaterUse->getDefiningAccess();
-  else
-    LaterDef = MSSA->getWalker()->getClobberingMemoryAccess(LaterInst);
+  MemoryAccess *LaterDef =
+      MSSA->getWalker()->getClobberingMemoryAccess(LaterInst);
   return MSSA->dominates(LaterDef, MSSA->getMemoryAccess(EarlierInst));
 }
 

Modified: llvm/trunk/test/Transforms/EarlyCSE/memoryssa.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/EarlyCSE/memoryssa.ll?rev=285080&r1=285079&r2=285080&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/EarlyCSE/memoryssa.ll (original)
+++ llvm/trunk/test/Transforms/EarlyCSE/memoryssa.ll Tue Oct 25 11:18:47 2016
@@ -5,6 +5,7 @@
 
 @G1 = global i32 zeroinitializer
 @G2 = global i32 zeroinitializer
+ at G3 = global i32 zeroinitializer
 
 ;; Simple load value numbering across non-clobbering store.
 ; CHECK-LABEL: @test1(
@@ -67,3 +68,41 @@ end:
   store i32 %sum, i32* @G2
   ret void
 }
+
+
+;; Check that MemoryPhi optimization and MemoryUse re-optimization
+;; happens during EarlyCSE, enabling more load CSE opportunities.
+; CHECK-LABEL: @test_memphiopt2(
+; CHECK-NOMEMSSA-LABEL: @test_memphiopt2(
+define void @test_memphiopt2(i1 %c, i32* %p) {
+; CHECK-LABEL: entry:
+; CHECK-NOMEMSSA-LABEL: entry:
+entry:
+; CHECK: load
+; CHECK-NOMEMSSA: load
+  %v1 = load i32, i32* @G1
+; CHECK: store
+; CHECK-NOMEMSSA: store
+  store i32 %v1, i32* @G2
+  br i1 %c, label %then, label %end
+
+; CHECK-LABEL: then:
+; CHECK-NOMEMSSA-LABEL: then:
+then:
+; CHECK: load
+; CHECK-NOMEMSSA: load
+  %pv = load i32, i32* %p
+; CHECK-NOT: store
+; CHECK-NOMEMSSA-NOT: store
+  store i32 %pv, i32* %p
+  br label %end
+
+; CHECK-LABEL: end:
+; CHECK-NOMEMSSA-LABEL: end:
+end:
+; CHECK-NOT: load
+; CHECK-NOMEMSSA: load
+  %v2 = load i32, i32* @G1
+  store i32 %v2, i32* @G3
+  ret void
+}