[llvm] 4bf3b7d - Recommit "[LICM] Only create load in pre-header when promoting load."

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2022-04-13T17:20:39+02:00
New Revision: 4bf3b7dc929c8288e9e5631978ef060d9140b251

URL: https://github.com/llvm/llvm-project/commit/4bf3b7dc929c8288e9e5631978ef060d9140b251
DIFF: https://github.com/llvm/llvm-project/commit/4bf3b7dc929c8288e9e5631978ef060d9140b251.diff

LOG: Recommit "[LICM] Only create load in pre-header when promoting load."

This reverts the revert commit 1ddc719680c21f3.

This version of the patch sets the initial available value to poison,
which resolves an issue with the SSAUpdater breaking LCSSA form.

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/LICM.cpp
    llvm/test/Transforms/LICM/scalar-promote.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index 74ae5977a3663..a7e47fe797133 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -2156,21 +2156,26 @@ bool llvm::promoteLoopAccessesToScalars(
 
   // Set up the preheader to have a definition of the value.  It is the live-out
   // value from the preheader that uses in the loop will use.
-  LoadInst *PreheaderLoad = new LoadInst(
-      AccessTy, SomePtr, SomePtr->getName() + ".promoted",
-      Preheader->getTerminator());
-  if (SawUnorderedAtomic)
-    PreheaderLoad->setOrdering(AtomicOrdering::Unordered);
-  PreheaderLoad->setAlignment(Alignment);
-  PreheaderLoad->setDebugLoc(DebugLoc());
-  if (AATags)
-    PreheaderLoad->setAAMetadata(AATags);
-  SSA.AddAvailableValue(Preheader, PreheaderLoad);
-
-  MemoryAccess *PreheaderLoadMemoryAccess = MSSAU.createMemoryAccessInBB(
-      PreheaderLoad, nullptr, PreheaderLoad->getParent(), MemorySSA::End);
-  MemoryUse *NewMemUse = cast<MemoryUse>(PreheaderLoadMemoryAccess);
-  MSSAU.insertUse(NewMemUse, /*RenameUses=*/true);
+  LoadInst *PreheaderLoad = nullptr;
+  if (FoundLoadToPromote) {
+    PreheaderLoad =
+        new LoadInst(AccessTy, SomePtr, SomePtr->getName() + ".promoted",
+                     Preheader->getTerminator());
+    if (SawUnorderedAtomic)
+      PreheaderLoad->setOrdering(AtomicOrdering::Unordered);
+    PreheaderLoad->setAlignment(Alignment);
+    PreheaderLoad->setDebugLoc(DebugLoc());
+    if (AATags)
+      PreheaderLoad->setAAMetadata(AATags);
+
+    MemoryAccess *PreheaderLoadMemoryAccess = MSSAU.createMemoryAccessInBB(
+        PreheaderLoad, nullptr, PreheaderLoad->getParent(), MemorySSA::End);
+    MemoryUse *NewMemUse = cast<MemoryUse>(PreheaderLoadMemoryAccess);
+    MSSAU.insertUse(NewMemUse, /*RenameUses=*/true);
+    SSA.AddAvailableValue(Preheader, PreheaderLoad);
+  } else {
+    SSA.AddAvailableValue(Preheader, PoisonValue::get(AccessTy));
+  }
 
   if (VerifyMemorySSA)
     MSSAU.getMemorySSA()->verifyMemorySSA();
@@ -2181,7 +2186,7 @@ bool llvm::promoteLoopAccessesToScalars(
   if (VerifyMemorySSA)
     MSSAU.getMemorySSA()->verifyMemorySSA();
   // If the SSAUpdater didn't use the load in the preheader, just zap it now.
-  if (PreheaderLoad->use_empty())
+  if (PreheaderLoad && PreheaderLoad->use_empty())
     eraseInstruction(*PreheaderLoad, *SafetyInfo, MSSAU);
 
   return true;

diff  --git a/llvm/test/Transforms/LICM/scalar-promote.ll b/llvm/test/Transforms/LICM/scalar-promote.ll
index ad60bc87b910a..12f1164cd7d27 100644
--- a/llvm/test/Transforms/LICM/scalar-promote.ll
+++ b/llvm/test/Transforms/LICM/scalar-promote.ll
@@ -605,10 +605,9 @@ Out:
 define void @test_sink_store_only() writeonly {
 ; CHECK-LABEL: @test_sink_store_only(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[GLB_PROMOTED:%.*]] = load i8, i8* @glb, align 1
 ; CHECK-NEXT:    br label [[LOOP_HEADER:%.*]]
 ; CHECK:       loop.header:
-; CHECK-NEXT:    [[DIV1:%.*]] = phi i8 [ [[GLB_PROMOTED]], [[ENTRY:%.*]] ], [ [[DIV:%.*]], [[LOOP_LATCH:%.*]] ]
+; CHECK-NEXT:    [[DIV1:%.*]] = phi i8 [ poison, [[ENTRY:%.*]] ], [ [[DIV:%.*]], [[LOOP_LATCH:%.*]] ]
 ; CHECK-NEXT:    [[I:%.*]] = phi i8 [ 0, [[ENTRY]] ], [ [[ADD:%.*]], [[LOOP_LATCH]] ]
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i8 [[I]], 4
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP_LATCH]], label [[EXIT:%.*]]
@@ -669,6 +668,58 @@ exit:
   ret void
 }
 
+define void @sink_store_lcssa_phis(i32* %ptr, i1 %c) {
+; CHECK-LABEL: @sink_store_lcssa_phis(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[LOOP_1_HEADER:%.*]]
+; CHECK:       loop.1.header:
+; CHECK-NEXT:    br label [[LOOP_2_HEADER:%.*]]
+; CHECK:       loop.2.header:
+; CHECK-NEXT:    br i1 false, label [[LOOP_3_HEADER_PREHEADER:%.*]], label [[LOOP_1_LATCH:%.*]]
+; CHECK:       loop.3.header.preheader:
+; CHECK-NEXT:    br label [[LOOP_3_HEADER:%.*]]
+; CHECK:       loop.3.header:
+; CHECK-NEXT:    [[I_11:%.*]] = phi i32 [ [[I_1:%.*]], [[LOOP_3_LATCH:%.*]] ], [ poison, [[LOOP_3_HEADER_PREHEADER]] ]
+; CHECK-NEXT:    [[I_1]] = phi i32 [ 1, [[LOOP_3_LATCH]] ], [ 0, [[LOOP_3_HEADER_PREHEADER]] ]
+; CHECK-NEXT:    br i1 true, label [[LOOP_3_LATCH]], label [[LOOP_2_LATCH:%.*]]
+; CHECK:       loop.3.latch:
+; CHECK-NEXT:    br label [[LOOP_3_HEADER]]
+; CHECK:       loop.2.latch:
+; CHECK-NEXT:    [[I_11_LCSSA:%.*]] = phi i32 [ [[I_11]], [[LOOP_3_HEADER]] ]
+; CHECK-NEXT:    store i32 [[I_11_LCSSA]], i32* [[PTR:%.*]], align 4
+; CHECK-NEXT:    br label [[LOOP_2_HEADER]]
+; CHECK:       loop.1.latch:
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[LOOP_1_HEADER]], label [[EXIT:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %loop.1.header
+
+loop.1.header:
+  br label %loop.2.header
+
+loop.2.header:
+  br i1 false, label %loop.3.header, label %loop.1.latch
+
+loop.3.header:
+  %i.1 = phi i32 [ 1, %loop.3.latch ], [ 0, %loop.2.header ]
+  br i1 true, label %loop.3.latch, label %loop.2.latch
+
+loop.3.latch:
+  store i32 %i.1, i32* %ptr, align 4
+  br label %loop.3.header
+
+loop.2.latch:
+  br label %loop.2.header
+
+loop.1.latch:
+  br i1 %c, label %loop.1.header, label %exit
+
+exit:
+  ret void
+}
+
 !0 = !{!4, !4, i64 0}
 !1 = !{!"omnipotent char", !2}
 !2 = !{!"Simple C/C++ TBAA"}