[PATCH] D104319: [SCEV] Retain AddExpr flags when subtracting a foldable constant.

[Florian Hahn via Phabricator via llvm-commits]

fhahn updated this revision to Diff 353322.
fhahn added a comment.

Thanks Eli! I split up the code to handle NSW & NUW separately. Is this along the lines of your suggestion?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D104319/new/

https://reviews.llvm.org/D104319

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll
  llvm/test/Transforms/IndVarSimplify/eliminate-exit-no-dl.ll


Index: llvm/test/Transforms/IndVarSimplify/eliminate-exit-no-dl.ll
===================================================================
--- llvm/test/Transforms/IndVarSimplify/eliminate-exit-no-dl.ll
+++ llvm/test/Transforms/IndVarSimplify/eliminate-exit-no-dl.ll
@@ -17,8 +17,7 @@
 ; CHECK-NEXT:    [[TMP:%.*]] = phi i8* [ [[TMP4:%.*]], [[BB7:%.*]] ], [ getelementptr inbounds ([0 x i8], [0 x i8]* @global, i64 0, i64 2), [[BB:%.*]] ]
 ; CHECK-NEXT:    [[TMP4]] = getelementptr inbounds i8, i8* [[TMP]], i64 -1
 ; CHECK-NEXT:    [[TMP6:%.*]] = load i8, i8* [[TMP4]], align 1
-; CHECK-NEXT:    [[TMP5:%.*]] = icmp ugt i8* [[TMP4]], getelementptr inbounds ([0 x i8], [0 x i8]* @global, i64 0, i64 500)
-; CHECK-NEXT:    br i1 [[TMP5]], label [[BB7]], label [[BB11:%.*]]
+; CHECK-NEXT:    br i1 false, label [[BB7]], label [[BB11:%.*]]
 ; CHECK:       bb7:
 ; CHECK-NEXT:    [[TMP8:%.*]] = zext i8 [[TMP6]] to i64
 ; CHECK-NEXT:    br i1 true, label [[BB11]], label [[BB3]]
Index: llvm/test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll
===================================================================
--- llvm/test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll
+++ llvm/test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll
@@ -16,7 +16,7 @@
 target triple = "aarch64--linux-gnueabi"
 
 ; CHECK: function 'f':
-; CHECK: (Low: (20000 + %a) High: (60004 + %a))
+; CHECK: (Low: (20000 + %a)<nuw> High: (60004 + %a))
 
 @B = common global i32* null, align 8
 @A = common global i32* null, align 8
Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -2534,6 +2534,48 @@
     }
   }
 
+  if (Ops.size() == 2) {
+    // Check if we have an expression of the form ((X + C1) - C2), where C1 and
+    // C2 can be folded in a way that allows retaining wrapping flags of (X +
+    // C1).
+    const SCEV *A = Ops[0];
+    const SCEV *B = Ops[1];
+    auto *AddExpr = dyn_cast<SCEVAddExpr>(B);
+    auto *C = dyn_cast<SCEVConstant>(A);
+    if (AddExpr && C && isa<SCEVConstant>(AddExpr->getOperand(0))) {
+      auto C1 = cast<SCEVConstant>(AddExpr->getOperand(0))->getAPInt();
+      auto C2 = C->getAPInt();
+      SCEV::NoWrapFlags PreservedFlags = SCEV::FlagAnyWrap;
+
+      APInt ConstAdd = C1 + C2;
+      auto AddFlags = AddExpr->getNoWrapFlags();
+      // Adding a smaller constant is NUW if the original AddExpr was NUW.
+      if (ScalarEvolution::maskFlags(AddFlags, SCEV::FlagNUW) ==
+              SCEV::FlagNUW &&
+          ConstAdd.ule(C1)) {
+        PreservedFlags =
+            ScalarEvolution::setFlags(PreservedFlags, SCEV::FlagNUW);
+      }
+
+      // Adding a constant with the same sign and small magnitude is NSW, if the
+      // original AddExpr was NSW.
+      if (ScalarEvolution::maskFlags(AddFlags, SCEV::FlagNSW) ==
+              SCEV::FlagNSW &&
+          C1.isSignBitSet() == ConstAdd.isSignBitSet() &&
+          ConstAdd.abs().ule(C1.abs())) {
+        PreservedFlags =
+            ScalarEvolution::setFlags(PreservedFlags, SCEV::FlagNSW);
+      }
+
+      if (PreservedFlags != SCEV::FlagAnyWrap) {
+        SmallVector<const SCEV *, 4> NewOps(AddExpr->op_begin(),
+                                            AddExpr->op_end());
+        NewOps[0] = getConstant(ConstAdd);
+        return getAddExpr(NewOps, PreservedFlags);
+      }
+    }
+  }
+
   // Skip past any other cast SCEVs.
   while (Idx < Ops.size() && Ops[Idx]->getSCEVType() < scAddExpr)
     ++Idx;


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