[IndVars] Fix usage of SCEVExpander to not mess with SCEVConstant. PR38674

This patch removes the function `expandSCEVIfNeeded` which behaves not as
it was intended. This function tries to make a lookup for exact existing expansion
and only goes to normal expansion via `expandCodeFor` if this lookup hasn't found
anything. As a result of this, if some instruction above the loop has a `SCEVConstant`
SCEV, this logic will return this instruction when asked for this `SCEVConstant` rather
than return a constant value. This is both non-profitable and in some cases leads to
breach of LCSSA form (as in PR38674).

Whether or not it is possible to break LCSSA with this algorithm and with some
non-constant SCEVs is still in question, this is still being investigated. I wasn't
able to construct such a test so far, so maybe this situation is impossible. If it is,
it will go as a separate fix.

Rather than do it, it is always correct to just invoke `expandCodeFor` unconditionally:
it behaves smarter about insertion points, and as side effect of this it will choose a
constant value for SCEVConstants. For other SCEVs it may end up finding a better insertion
point. So it should not be worse in any case.

NOTE: So far the only known case for which this transform may break LCSSA is mapping
of SCEVConstant to an instruction. However there is a suspicion that the entire algorithm
can compromise LCSSA form for other cases as well (yet not proved).

Differential Revision: https://reviews.llvm.org/D51286
Reviewed By: etherzhhb

llvm-svn: 341345

Author: Max Kazantsev

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

@@ -152,9 +152,6 @@ class IndVarSimplify {
 
   void sinkUnusedInvariants(Loop *L);
 
-  Value *expandSCEVIfNeeded(SCEVExpander &Rewriter, const SCEV *S, Loop *L,
-                            Instruction *InsertPt, Type *Ty);
-
 public:
   IndVarSimplify(LoopInfo *LI, ScalarEvolution *SE, DominatorTree *DT,
                  const DataLayout &DL, TargetLibraryInfo *TLI,
@@ -521,19 +518,6 @@ struct RewritePhi {
 
 } // end anonymous namespace
 
-Value *IndVarSimplify::expandSCEVIfNeeded(SCEVExpander &Rewriter, const SCEV *S,
-                                          Loop *L, Instruction *InsertPt,
-                                          Type *ResultTy) {
-  // Before expanding S into an expensive LLVM expression, see if we can use an
-  // already existing value as the expansion for S.
-  if (Value *ExistingValue = Rewriter.getExactExistingExpansion(S, InsertPt, L))
-    if (ExistingValue->getType() == ResultTy)
-      return ExistingValue;
-
-  // We didn't find anything, fall back to using SCEVExpander.
-  return Rewriter.expandCodeFor(S, ResultTy, InsertPt);
-}
-
 //===----------------------------------------------------------------------===//
 // rewriteLoopExitValues - Optimize IV users outside the loop.
 // As a side effect, reduces the amount of IV processing within the loop.
@@ -650,8 +634,7 @@ void IndVarSimplify::rewriteLoopExitValues(Loop *L, SCEVExpander &Rewriter) {
         }
 
         bool HighCost = Rewriter.isHighCostExpansion(ExitValue, L, Inst);
-        Value *ExitVal =
-            expandSCEVIfNeeded(Rewriter, ExitValue, L, Inst, PN->getType());
+        Value *ExitVal = Rewriter.expandCodeFor(ExitValue, PN->getType(), Inst);
 
         LLVM_DEBUG(dbgs() << "INDVARS: RLEV: AfterLoopVal = " << *ExitVal
                           << '\n'

llvm/test/Transforms/IndVarSimplify/pr38674.ll

@@ -0,0 +1,141 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -indvars < %s | FileCheck %s
+
+; Check that we don't reuse %zext instead of %inc11 for LCSSA Phi node. Case
+; with constants SCEV.
+
+define i32 @test_01() {
+; CHECK-LABEL: @test_01(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_COND1_PREHEADER:%.*]]
+; CHECK:       for.cond1.preheader:
+; CHECK-NEXT:    br label [[FOR_COND4_PREHEADER:%.*]]
+; CHECK:       for.cond4.preheader:
+; CHECK-NEXT:    [[ZEXT:%.*]] = zext i16 1 to i32
+; CHECK-NEXT:    br label [[FOR_BODY6:%.*]]
+; CHECK:       for.cond4:
+; CHECK-NEXT:    [[CMP5:%.*]] = icmp ult i32 [[INC:%.*]], 2
+; CHECK-NEXT:    br i1 [[CMP5]], label [[FOR_BODY6]], label [[FOR_END:%.*]]
+; CHECK:       for.body6:
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 0, [[FOR_COND4_PREHEADER]] ], [ [[INC]], [[FOR_COND4:%.*]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq i32 [[IV]], [[ZEXT]]
+; CHECK-NEXT:    [[INC]] = add nuw nsw i32 [[IV]], 1
+; CHECK-NEXT:    br i1 [[TMP0]], label [[RETURN_LOOPEXIT:%.*]], label [[FOR_COND4]]
+; CHECK:       for.end:
+; CHECK-NEXT:    br i1 false, label [[FOR_COND4_PREHEADER]], label [[FOR_END9:%.*]]
+; CHECK:       for.end9:
+; CHECK-NEXT:    br i1 false, label [[FOR_COND1_PREHEADER]], label [[RETURN_LOOPEXIT3:%.*]]
+; CHECK:       return.loopexit:
+; CHECK-NEXT:    unreachable
+; CHECK:       return.loopexit3:
+; CHECK-NEXT:    br label [[RETURN:%.*]]
+; CHECK:       return:
+; CHECK-NEXT:    ret i32 1
+;
+entry:
+  br label %for.cond1.preheader
+
+for.cond1.preheader:                              ; preds = %for.end9, %entry
+  br label %for.cond4.preheader
+
+for.cond4.preheader:                              ; preds = %for.end, %for.cond1.preheader
+  %zext = zext i16 1 to i32
+  br label %for.body6
+
+for.cond4:                                        ; preds = %for.body6
+  %cmp5 = icmp ult i32 %inc, 2
+  br i1 %cmp5, label %for.body6, label %for.end
+
+for.body6:                                        ; preds = %for.cond4, %for.cond4.preheader
+  %iv = phi i32 [ 0, %for.cond4.preheader ], [ %inc, %for.cond4 ]
+  %0 = icmp eq i32 %iv, %zext
+  %inc = add nuw nsw i32 %iv, 1
+  br i1 %0, label %return.loopexit, label %for.cond4
+
+for.end:                                          ; preds = %for.cond4
+  br i1 false, label %for.cond4.preheader, label %for.end9
+
+for.end9:                                         ; preds = %for.end
+  %inc11 = add nuw nsw i32 0, 1
+  br i1 false, label %for.cond1.preheader, label %return.loopexit3
+
+return.loopexit:                                  ; preds = %for.body6
+  unreachable
+
+return.loopexit3:                                 ; preds = %for.end9
+  %inc11.lcssa = phi i32 [ %inc11, %for.end9 ]
+  br label %return
+
+return:                                           ; preds = %return.loopexit3
+  ret i32 %inc11.lcssa
+}
+
+; Same as test_01, but the instructions with the same SCEV have a non-constant
+; SCEV.
+define i32 @test_02(i32 %x) {
+; CHECK-LABEL: @test_02(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_COND1_PREHEADER:%.*]]
+; CHECK:       for.cond1.preheader:
+; CHECK-NEXT:    br label [[FOR_COND4_PREHEADER:%.*]]
+; CHECK:       for.cond4.preheader:
+; CHECK-NEXT:    [[ZEXT:%.*]] = mul i32 [[X:%.*]], 1
+; CHECK-NEXT:    br label [[FOR_BODY6:%.*]]
+; CHECK:       for.cond4:
+; CHECK-NEXT:    [[CMP5:%.*]] = icmp ult i32 [[INC:%.*]], 2
+; CHECK-NEXT:    br i1 [[CMP5]], label [[FOR_BODY6]], label [[FOR_END:%.*]]
+; CHECK:       for.body6:
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 0, [[FOR_COND4_PREHEADER]] ], [ [[INC]], [[FOR_COND4:%.*]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq i32 [[IV]], [[ZEXT]]
+; CHECK-NEXT:    [[INC]] = add nuw nsw i32 [[IV]], 1
+; CHECK-NEXT:    br i1 [[TMP0]], label [[RETURN_LOOPEXIT:%.*]], label [[FOR_COND4]]
+; CHECK:       for.end:
+; CHECK-NEXT:    br i1 false, label [[FOR_COND4_PREHEADER]], label [[FOR_END9:%.*]]
+; CHECK:       for.end9:
+; CHECK-NEXT:    [[INC11:%.*]] = add nuw nsw i32 0, [[X]]
+; CHECK-NEXT:    br i1 false, label [[FOR_COND1_PREHEADER]], label [[RETURN_LOOPEXIT3:%.*]]
+; CHECK:       return.loopexit:
+; CHECK-NEXT:    unreachable
+; CHECK:       return.loopexit3:
+; CHECK-NEXT:    [[INC11_LCSSA:%.*]] = phi i32 [ [[INC11]], [[FOR_END9]] ]
+; CHECK-NEXT:    br label [[RETURN:%.*]]
+; CHECK:       return:
+; CHECK-NEXT:    ret i32 [[INC11_LCSSA]]
+;
+entry:
+  br label %for.cond1.preheader
+
+for.cond1.preheader:                              ; preds = %for.end9, %entry
+  br label %for.cond4.preheader
+
+for.cond4.preheader:                              ; preds = %for.end, %for.cond1.preheader
+  %zext = mul i32 %x, 1
+  br label %for.body6
+
+for.cond4:                                        ; preds = %for.body6
+  %cmp5 = icmp ult i32 %inc, 2
+  br i1 %cmp5, label %for.body6, label %for.end
+
+for.body6:                                        ; preds = %for.cond4, %for.cond4.preheader
+  %iv = phi i32 [ 0, %for.cond4.preheader ], [ %inc, %for.cond4 ]
+  %0 = icmp eq i32 %iv, %zext
+  %inc = add nuw nsw i32 %iv, 1
+  br i1 %0, label %return.loopexit, label %for.cond4
+
+for.end:                                          ; preds = %for.cond4
+  br i1 false, label %for.cond4.preheader, label %for.end9
+
+for.end9:                                         ; preds = %for.end
+  %inc11 = add nuw nsw i32 0, %x
+  br i1 false, label %for.cond1.preheader, label %return.loopexit3
+
+return.loopexit:                                  ; preds = %for.body6
+  unreachable
+
+return.loopexit3:                                 ; preds = %for.end9
+  %inc11.lcssa = phi i32 [ %inc11, %for.end9 ]
+  br label %return
+
+return:                                           ; preds = %return.loopexit3
+  ret i32 %inc11.lcssa
+}