[PATCH] D75400: [LICM] Allow freeze to hoist out of a loop

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune created this revision.
Herald added subscribers: llvm-commits, asbirlea, hiraditya.
Herald added a project: LLVM.

This patch allows LICM to hoist freeze instructions out of a loop.

Sinking freeze into a loop is not allowed because after sinking the instruction may yield different values for iterations.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D75400

Files:
  llvm/lib/Transforms/Scalar/LICM.cpp
  llvm/test/Transforms/LICM/freeze.ll


Index: llvm/test/Transforms/LICM/freeze.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LICM/freeze.ll
@@ -0,0 +1,21 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -licm -S < %s | FileCheck %s
+
+define void @licm(i1 %a) {
+; CHECK-LABEL: @licm(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[B:%.*]] = freeze i1 [[A:%.*]]
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    call void @use(i1 [[B]])
+; CHECK-NEXT:    br label [[LOOP]]
+;
+entry:
+  br label %loop
+loop:
+  %b = freeze i1 %a
+  call void @use(i1 %b)
+  br label %loop
+}
+
+declare void @use(i1)
Index: llvm/lib/Transforms/Scalar/LICM.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LICM.cpp
+++ llvm/lib/Transforms/Scalar/LICM.cpp
@@ -993,7 +993,7 @@
           isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
           isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
           isa<ShuffleVectorInst>(I) || isa<ExtractValueInst>(I) ||
-          isa<InsertValueInst>(I));
+          isa<InsertValueInst>(I) || isa<FreezeInst>(I));
 }
 /// Return true if all of the alias sets within this AST are known not to
 /// contain a Mod, or if MSSA knows thare are no MemoryDefs in the loop.
@@ -1231,6 +1231,10 @@
       return MSSA->isLiveOnEntryDef(Source) ||
              !CurLoop->contains(Source->getBlock());
     }
+  } else if (isa<FreezeInst>(&I)) {
+    // Freeze instruction cannot be sunk into a loop because it may yield a
+    // different value for each iteration after sinking.
+    return !Flags->IsSink;
   }
 
   assert(!I.mayReadOrWriteMemory() && "unhandled aliasing");
@@ -1945,7 +1949,7 @@
         // Note that proving a load safe to speculate requires proving
         // sufficient alignment at the target location.  Proving it guaranteed
         // to execute does as well.  Thus we can increase our guaranteed
-        // alignment as well. 
+        // alignment as well.
         if (!DereferenceableInPH || (InstAlignment > Alignment))
           if (isSafeToExecuteUnconditionally(*Load, DT, CurLoop, SafetyInfo,
                                              ORE, Preheader->getTerminator())) {


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