[SeparateConstOffsetFromGEP] Fix: b - a matched a - b during reun…

…iteExts

During the SeparateConstOffsetFromGEP pass, a - b and b - a will be
considered equivalent in some instances.

An example- the IR contains:

  BB1:
      %add = add %a, 511
      br label %BB2
  BB2:
      %sub2 = sub %b,  %a
      br label %BB3
  BB3:
      %sub1 = sub %add, %b
      %gep = getelementptr float, ptr %p, %sub1

Step 1 in the SeparateConstOffsetFromGEP pass, after split constant index:

  BB1:
      %add = add %a, 511
      br label %BB2
  BB2:
      %sub2 = sub %b,  %a
      br label %BB3
  BB3:
      %sub.t = sub %a, %b
      %gep.base = getelementptr float, ptr %p, %sub.t
      %gep = getelementptr float, ptr %gep.base, 511

Step 2, after reuniteExts:

  BB1:
      br label %BB2
  BB2:
      %sub2 = sub %b,  %a
      br label %BB3
  BB3:
      %gep.base = getelementptr float, ptr %p, %sub2
      %gep = getelementptr float, ptr %gep.base, 511

Obviously, reuniteExts treated a - b and b - a as equivalent.
This patch fixes that.

Reviewed By: nikic, spatel

Differential Revision: https://reviews.llvm.org/D143542

llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp

@@ -1232,8 +1232,8 @@ bool SeparateConstOffsetFromGEP::reuniteExts(Instruction *I) {
     }
   } else if (match(I, m_Sub(m_SExt(m_Value(LHS)), m_SExt(m_Value(RHS))))) {
     if (LHS->getType() == RHS->getType()) {
-      const SCEV *Key =
-          SE->getAddExpr(SE->getUnknown(LHS), SE->getUnknown(RHS));
+      const SCEV *Key = SE->getAddExpr(
+          SE->getUnknown(LHS), SE->getNegativeSCEV(SE->getUnknown(RHS)));
       if (auto *Dom = findClosestMatchingDominator(Key, I, DominatingSubs)) {
         Instruction *NewSExt = new SExtInst(Dom, I->getType(), "", I);
         NewSExt->takeName(I);
@@ -1253,8 +1253,8 @@ bool SeparateConstOffsetFromGEP::reuniteExts(Instruction *I) {
     }
   } else if (match(I, m_NSWSub(m_Value(LHS), m_Value(RHS)))) {
     if (programUndefinedIfPoison(I)) {
-      const SCEV *Key =
-          SE->getAddExpr(SE->getUnknown(LHS), SE->getUnknown(RHS));
+      const SCEV *Key = SE->getAddExpr(
+          SE->getUnknown(LHS), SE->getNegativeSCEV(SE->getUnknown(RHS)));
       DominatingSubs[Key].push_back(I);
     }
   }

llvm/test/Transforms/SeparateConstOffsetFromGEP/split-gep-sub.ll

@@ -28,13 +28,15 @@ define void @test_A_sub_B_add_ConstantInt(ptr %p) {
 ; CHECK-NEXT:    [[CMP26:%.*]] = icmp ult i32 [[SUB1]], 512
 ; CHECK-NEXT:    br i1 [[CMP26]], label [[COND_TRUE:%.*]], label [[COND_END]]
 ; CHECK:       cond.true:
-; CHECK-NEXT:    [[SUB22:%.*]] = sext i32 [[SUB1]] to i64
-; CHECK-NEXT:    [[TMP1:%.*]] = ptrtoint ptr [[P:%.*]] to i64
-; CHECK-NEXT:    [[TMP2:%.*]] = shl i64 [[SUB22]], 2
-; CHECK-NEXT:    [[TMP3:%.*]] = add i64 [[TMP1]], [[TMP2]]
-; CHECK-NEXT:    [[TMP4:%.*]] = add i64 [[TMP3]], 2044
-; CHECK-NEXT:    [[TMP5:%.*]] = inttoptr i64 [[TMP4]] to ptr
-; CHECK-NEXT:    store float 1.000000e+00, ptr [[TMP5]], align 4
+; CHECK-NEXT:    [[TMP1:%.*]] = sext i32 [[MUL]] to i64
+; CHECK-NEXT:    [[TMP2:%.*]] = sext i32 [[REM]] to i64
+; CHECK-NEXT:    [[SUB22:%.*]] = sub i64 [[TMP2]], [[TMP1]]
+; CHECK-NEXT:    [[TMP3:%.*]] = ptrtoint ptr [[P:%.*]] to i64
+; CHECK-NEXT:    [[TMP4:%.*]] = shl i64 [[SUB22]], 2
+; CHECK-NEXT:    [[TMP5:%.*]] = add i64 [[TMP3]], [[TMP4]]
+; CHECK-NEXT:    [[TMP6:%.*]] = add i64 [[TMP5]], 2044
+; CHECK-NEXT:    [[TMP7:%.*]] = inttoptr i64 [[TMP6]] to ptr
+; CHECK-NEXT:    store float 1.000000e+00, ptr [[TMP7]], align 4
 ; CHECK-NEXT:    br label [[COND_END]]
 ; CHECK:       cond.end:
 ; CHECK-NEXT:    [[INC]] = add nuw nsw i32 [[K]], 1