[LLE] Don't hoist conditionally executed loads

If the load is conditional we can't hoist its 0-iteration instance to
the preheader because that would make it unconditional.  Thus we would
access a memory location that the original loop did not access.

llvm-svn: 273991

llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp

@@ -119,6 +119,11 @@ bool doesStoreDominatesAllLatches(BasicBlock *StoreBlock, Loop *L,
                      });
 }
 
+/// \brief Return true if the load is not executed on all paths in the loop.
+static bool isLoadConditional(LoadInst *Load, Loop *L) {
+  return Load->getParent() != L->getHeader();
+}
+
 /// \brief The per-loop class that does most of the work.
 class LoadEliminationForLoop {
 public:
@@ -450,6 +455,12 @@ public:
       if (!doesStoreDominatesAllLatches(Cand.Store->getParent(), L, DT))
         continue;
 
+      // If the load is conditional we can't hoist its 0-iteration instance to
+      // the preheader because that would make it unconditional.  Thus we would
+      // access a memory location that the original loop did not access.
+      if (isLoadConditional(Cand.Load, L))
+        continue;
+
       // Check whether the SCEV difference is the same as the induction step,
       // thus we load the value in the next iteration.
       if (!Cand.isDependenceDistanceOfOne(PSE, L))

llvm/test/Transforms/LoopLoadElim/cond-load.ll

@@ -0,0 +1,42 @@
+; RUN: opt -S -loop-load-elim < %s | FileCheck %s
+
+; We can't hoist conditional loads to the preheader for the initial value.
+; E.g. in the loop below we'd access array[-1] if we did:
+;
+;   for(int i = 0 ; i < n ; i++ )
+;     array[i] = ( i > 0 ? array[i - 1] : 0 ) + 4;
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.11.0"
+
+define void @f(i32* %array, i32 %n) {
+entry:
+  %cmp10 = icmp sgt i32 %n, 0
+  br i1 %cmp10, label %for.body, label %for.cond.cleanup
+
+for.cond.cleanup:                                 ; preds = %cond.end, %entry
+  ret void
+
+for.body:                                         ; preds = %entry, %cond.end
+  %indvars.iv = phi i64 [ %indvars.iv.next, %cond.end ], [ 0, %entry ]
+; CHECK-NOT: %store_forwarded = phi
+  %cmp1 = icmp sgt i64 %indvars.iv, 0
+  br i1 %cmp1, label %cond.true, label %cond.end
+
+cond.true:                                        ; preds = %for.body
+  %0 = add nsw i64 %indvars.iv, -1
+  %arrayidx = getelementptr inbounds i32, i32* %array, i64 %0
+  %1 = load i32, i32* %arrayidx, align 4
+  br label %cond.end
+
+cond.end:                                         ; preds = %for.body, %cond.true
+  %cond = phi i32 [ %1, %cond.true ], [ 0, %for.body ]
+; CHECK: %cond = phi i32 [ %1, %cond.true ], [ 0, %for.body ]
+  %add = add nsw i32 %cond, 4
+  %arrayidx3 = getelementptr inbounds i32, i32* %array, i64 %indvars.iv
+  store i32 %add, i32* %arrayidx3, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %n
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+}