Semantically revert 151015. Add a comment on why we should be able to assert

the dominance once the dominates method is fixed and why we can use the builder's
insertion point.
Fixes pr12048.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151125 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/ScalarEvolutionExpander.cpp

@@ -31,11 +31,20 @@ using namespace llvm;
 Value *SCEVExpander::ReuseOrCreateCast(Value *V, Type *Ty,
                                        Instruction::CastOps Op,
                                        BasicBlock::iterator IP) {
-  // All new or reused instructions must strictly dominate their uses.
+  // This function must be called with the builder having a valid insertion
-  // It would be nice to assert this here, but we don't always know where
+  // point. It doesn't need to be the actual IP where the uses of the returned
-  // the next instructions will be added as the caller can move the
+  // cast will be added, but it must dominate such IP.
-  // Builder's InsertPt before creating them and we might be called with
+  // We use this precondition to assert that we can produce a cast that will
-  // an invalid InsertPt.
+  // dominate all its uses. In particular, this is crussial for the case
+  // where the builder's insertion point *is* the point where we were asked
+  // to put the cast.
+  // Since we don't know the the builder's insertion point is actually
+  // where the uses will be added (only that it dominates it), we are
+  // not allowed to move it.
+  BasicBlock::iterator BIP = Builder.GetInsertPoint();
+
+  // FIXME: enable once our implementation of dominates is fixed.
+  //  assert(BIP == IP || SE.DT->dominates(IP, BIP));
 
   // Check to see if there is already a cast!
   for (Value::use_iterator UI = V->use_begin(), E = V->use_end();
@@ -44,8 +53,9 @@ Value *SCEVExpander::ReuseOrCreateCast(Value *V, Type *Ty,
     if (U->getType() == Ty)
       if (CastInst *CI = dyn_cast<CastInst>(U))
         if (CI->getOpcode() == Op) {
-          // If the cast isn't where we want it, fix it.
+          // If the cast isn't where we want it or if it doesn't dominate
-          if (BasicBlock::iterator(CI) != IP) {
+          // a use in BIP, fix it.
+          if (BasicBlock::iterator(CI) != IP || BIP == IP) {
             // Create a new cast, and leave the old cast in place in case
             // it is being used as an insert point. Clear its operand
             // so that it doesn't hold anything live.

test/Transforms/LoopStrengthReduce/pr12048.ll

@@ -0,0 +1,38 @@
+; RUN: opt < %s -loop-reduce
+
+define void @resolve_name() nounwind uwtable ssp {
+  br label %while.cond40.preheader
+while.cond132.while.cond.loopexit_crit_edge:
+  br label %while.cond40.preheader
+while.cond40.preheader:
+  br label %while.cond40
+while.cond40:
+  %indvars.iv194 = phi i8* [ null, %while.cond40.preheader ], [ %scevgep, %while.body51 ]
+  %tmp.1 = phi i8* [ undef, %while.cond40.preheader ], [ %incdec.ptr, %while.body51 ]
+  switch i8 undef, label %while.body51 [
+    i8 0, label %if.then59
+  ]
+while.body51:                                     ; preds = %land.end50
+  %incdec.ptr = getelementptr inbounds i8* %tmp.1, i64 1
+  %scevgep = getelementptr i8* %indvars.iv194, i64 1
+  br label %while.cond40
+if.then59:                                        ; preds = %while.end
+  br i1 undef, label %if.then64, label %if.end113
+if.then64:                                        ; preds = %if.then59
+  %incdec.ptr88.tmp.2 = select i1 undef, i8* undef, i8* undef
+  br label %if.end113
+if.end113:                                        ; preds = %if.then64, %if.then59
+  %tmp.4 = phi i8* [ %incdec.ptr88.tmp.2, %if.then64 ], [ undef, %if.then59 ]
+  %tmp.4195 = ptrtoint i8* %tmp.4 to i64
+  br  label %while.cond132.preheader
+while.cond132.preheader:                          ; preds = %if.end113
+  %cmp133173 = icmp eq i8* %tmp.1, %tmp.4
+  br i1 %cmp133173, label %while.cond40.preheader, label %while.body139.lr.ph
+while.body139.lr.ph:                              ; preds = %while.cond132.preheader
+  %scevgep198 = getelementptr i8* %indvars.iv194, i64 0
+  %scevgep198199 = ptrtoint i8* %scevgep198 to i64
+  br label %while.body139
+while.body139:                                    ; preds = %while.body139, %while.body139.lr.ph
+  %start_of_var.0177 = phi i8* [ %tmp.1, %while.body139.lr.ph ], [ null, %while.body139 ]
+  br i1 undef, label %while.cond132.while.cond.loopexit_crit_edge, label %while.body139
+}