Reapply [LSR] Create fewer redundant instructions.

[gberry]

Summary

Fix LSRInstance::HoistInsertPosition() to check the original insert
position block first for a canonical insertion point that is dominated
by all inputs. This leads to SCEV being able to reuse more instructions
since it currently tracks the instructions it creates for reuse by
keeping a table of <Value, insert point> pairs.

Originally reviewed in http://reviews.llvm.org/D18001

[qcolombet]

LGTM.

[gberry]

Thanks Quentin, but this patch exposes what I believe is a bug in the interaction between LSR and SCEV. I'm still in the process of resolving that issue.
If you're interested, the relevant email thread on llvm-dev has the Subject line "LSR/SCEV problem/question"

[qcolombet]

Hi Geoff,

llvm/trunk/lib/Transforms/Scalar/LoopStrengthReduce.cpp

 
 /// Helper for AdjustInsertPositionForExpand. Climb up the dominator tree far as
 /// we can go while still being dominated by the input positions. This helps
 /// canonicalize the insert position, which encourages sharing.
 BasicBlock::iterator
 LSRInstance::HoistInsertPosition(BasicBlock::iterator IP,
                                  const SmallVectorImpl<Instruction *> &Inputs)
                                                                          const {
+  Instruction *Tentative = &*IP;
   for (;;) {
-    const Loop *IPLoop = LI.getLoopFor(IP->getParent());
-    unsigned IPLoopDepth = IPLoop ? IPLoop->getLoopDepth() : 0;
-
-    BasicBlock *IDom;
-    for (DomTreeNode *Rung = DT.getNode(IP->getParent()); ; ) {
-      if (!Rung) return IP;
-      Rung = Rung->getIDom();
-      if (!Rung) return IP;
-      IDom = Rung->getBlock();
-
-      // Don't climb into a loop though.
-      const Loop *IDomLoop = LI.getLoopFor(IDom);
-      unsigned IDomDepth = IDomLoop ? IDomLoop->getLoopDepth() : 0;
-      if (IDomDepth <= IPLoopDepth &&
-          (IDomDepth != IPLoopDepth || IDomLoop == IPLoop))
-        break;
-    }
-
     bool AllDominate = true;
     Instruction *BetterPos = nullptr;
-    Instruction *Tentative = IDom->getTerminator();
     // Don't bother attempting to insert before a catchswitch, their basic block
     // cannot have other non-PHI instructions.
     if (isa<CatchSwitchInst>(Tentative))
       return IP;
 
     for (Instruction *Inst : Inputs) {
       if (Inst == Tentative || !DT.dominates(Inst, Tentative)) {
         AllDominate = false;
         break;
       }
       // Attempt to find an insert position in the middle of the block,
       // instead of at the end, so that it can be used for other expansions.
-      if (IDom == Inst->getParent() &&
+      if (Tentative->getParent() == Inst->getParent() &&
           (!BetterPos || !DT.dominates(Inst, BetterPos)))
         BetterPos = &*std::next(BasicBlock::iterator(Inst));
     }
     if (!AllDominate)
       break;
     if (BetterPos)
       IP = BetterPos->getIterator();
     else
       IP = Tentative->getIterator();
+
+    const Loop *IPLoop = LI.getLoopFor(IP->getParent());
+    unsigned IPLoopDepth = IPLoop ? IPLoop->getLoopDepth() : 0;
+
+    BasicBlock *IDom;
+    for (DomTreeNode *Rung = DT.getNode(IP->getParent()); ; ) {
+      if (!Rung) return IP;
+      Rung = Rung->getIDom();
+      if (!Rung) return IP;
+      IDom = Rung->getBlock();
+
+      // Don't climb into a loop though.
+      const Loop *IDomLoop = LI.getLoopFor(IDom);
+      unsigned IDomDepth = IDomLoop ? IDomLoop->getLoopDepth() : 0;
+      if (IDomDepth <= IPLoopDepth &&
+          (IDomDepth != IPLoopDepth || IDomLoop == IPLoop))
+        break;
+    }
+
+    Tentative = IDom->getTerminator();
   }
 
   return IP;
 }
 
 /// Determine an input position which will be dominated by the operands and
 /// which will dominate the result.
 BasicBlock::iterator

llvm/trunk/test/Transforms/LoopStrengthReduce/AArch64/lsr-reuse.ll

+; RUN: llc -mtriple=arm64-unknown-unknown -print-lsr-output < %s 2>&1 | FileCheck %s
+
+declare void @foo(i64)
+
+; Verify that redundant adds aren't inserted by LSR.
+; CHECK-LABEL: @bar(
+define void @bar(double* %A) {
+entry:
+  br label %while.cond
+
+while.cond:
+; CHECK-LABEL: while.cond:
+; CHECK: add i64 %lsr.iv, 1
+; CHECK-NOT: add i64 %lsr.iv, 1
+; CHECK-LABEL: land.rhs:
+  %indvars.iv28 = phi i64 [ %indvars.iv.next29, %land.rhs ], [ 50, %entry ]
+  %cmp = icmp sgt i64 %indvars.iv28, 0
+  br i1 %cmp, label %land.rhs, label %while.end
+
+land.rhs:
+  %indvars.iv.next29 = add nsw i64 %indvars.iv28, -1
+  %arrayidx = getelementptr inbounds double, double* %A, i64 %indvars.iv.next29
+  %Aload = load double, double* %arrayidx, align 8
+  %cmp1 = fcmp oeq double %Aload, 0.000000e+00
+  br i1 %cmp1, label %while.cond, label %if.end
+
+while.end:
+  %indvars.iv28.lcssa = phi i64 [ %indvars.iv28, %while.cond ]
+  tail call void @foo(i64 %indvars.iv28.lcssa)
+  br label %if.end
+
+if.end:
+  ret void
+}

llvm/trunk/test/Transforms/LoopStrengthReduce/scev-insertpt-bug.ll

+; RUN: opt < %s -loop-reduce -S
+
+; Test that SCEV insertpoint's don't get corrupted and cause an
+; invalid instruction to be inserted in a block other than its parent.
+; See http://reviews.llvm.org/D20703 for context.
+define void @test() {
+entry:
+  %bf.load = load i32, i32* null, align 4
+  %bf.clear = lshr i32 %bf.load, 1
+  %div = and i32 %bf.clear, 134217727
+  %sub = add nsw i32 %div, -1
+  %0 = zext i32 %sub to i64
+  br label %while.cond
+
+while.cond:                                       ; preds = %cond.end, %entry
+  %indvars.iv = phi i64 [ %indvars.iv.next, %cond.end ], [ 0, %entry ]
+  %cmp = icmp eq i64 %indvars.iv, %0
+  br i1 %cmp, label %cleanup16, label %while.body
+
+while.body:                                       ; preds = %while.cond
+  %1 = trunc i64 %indvars.iv to i32
+  %mul = shl i32 %1, 1
+  %add = add nuw i32 %mul, 2
+  %cmp3 = icmp ult i32 %add, 0
+  br i1 %cmp3, label %if.end, label %if.then
+
+if.then:                                          ; preds = %while.body
+  unreachable
+
+if.end:                                           ; preds = %while.body
+  br i1 false, label %cond.end, label %cond.true
+
+cond.true:                                        ; preds = %if.end
+  br label %cond.end
+
+cond.end:                                         ; preds = %cond.true, %if.end
+  %add7 = add i32 %1, 1
+  %cmp12 = icmp ugt i32 %add7, %sub
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  br i1 %cmp12, label %if.then13, label %while.cond
+
+if.then13:                                        ; preds = %cond.end
+  unreachable
+
+cleanup16:                                        ; preds = %while.cond
+  ret void
+}