[PATCH] D28408: Fix LoopLoadElimination to keep original alignment on the inital hoisted store

[Mehdi AMINI via Phabricator via llvm-commits]

mehdi_amini created this revision.
mehdi_amini added a reviewer: anemet.
mehdi_amini added a subscriber: llvm-commits.
Herald added a subscriber: mzolotukhin.

This is fixing a bug where Loop Vectorization is widening a load but
with a lower alignment. Hoisting the load without propagating the alignment
will allow inst-combine to later deduce a higher alignment that what the pointer
actually is.


https://reviews.llvm.org/D28408

Files:
  llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp
  llvm/test/Transforms/LoopLoadElim/forward.ll


Index: llvm/test/Transforms/LoopLoadElim/forward.ll
===================================================================
--- llvm/test/Transforms/LoopLoadElim/forward.ll
+++ llvm/test/Transforms/LoopLoadElim/forward.ll
@@ -16,8 +16,8 @@
 ; CHECK-NOT: %found.conflict{{.*}} =
 
 entry:
-; for.body.ph:
-; CHECK: %load_initial = load i32, i32* %A
+; Make sure the hoisted load keeps the alignment
+; CHECK: %load_initial = load i32, i32* %A, align 1
   br label %for.body
 
 for.body:                                         ; preds = %for.body, %entry
@@ -34,7 +34,7 @@
   %a_p1 = add i32 %b, 2
   store i32 %a_p1, i32* %Aidx_next, align 4
 
-  %a = load i32, i32* %Aidx, align 4
+  %a = load i32, i32* %Aidx, align 1
 ; CHECK: %c = mul i32 %store_forwarded, 2
   %c = mul i32 %a, 2
   store i32 %c, i32* %Cidx, align 4
Index: llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp
+++ llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp
@@ -415,7 +415,9 @@
     Value *InitialPtr = SEE.expandCodeFor(PtrSCEV->getStart(), Ptr->getType(),
                                           PH->getTerminator());
     Value *Initial =
-        new LoadInst(InitialPtr, "load_initial", PH->getTerminator());
+        new LoadInst(InitialPtr, "load_initial", /* isVolatile */ false,
+                     Cand.Load->getAlignment(), PH->getTerminator());
+
     PHINode *PHI = PHINode::Create(Initial->getType(), 2, "store_forwarded",
                                    &L->getHeader()->front());
     PHI->addIncoming(Initial, PH);


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