[llvm] d177988 - [SCEV] Retain AddExpr flags when subtracting a foldable constant.

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2021-06-22T11:27:51+01:00
New Revision: d17798823ca2213af353e125f6f5233522dda7ce

URL: https://github.com/llvm/llvm-project/commit/d17798823ca2213af353e125f6f5233522dda7ce
DIFF: https://github.com/llvm/llvm-project/commit/d17798823ca2213af353e125f6f5233522dda7ce.diff

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

Currently we drop wrapping flags for expressions like (A + C1)<flags> - C2.

But we can retain flags under certain conditions:

* Adding a smaller constant is NUW if the original AddExpr was NUW.

* Adding a constant with the same sign and small magnitude is NSW, if the
  original AddExpr was NSW.

This can improve results after using `SimplifyICmpOperands`, which may
subtract one in order to use stricter predicates, as is the case for
`isKnownPredicate`.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D104319

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index f011020266c74..4af19e49fb6f7 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -2541,6 +2541,48 @@ const SCEV *ScalarEvolution::getAddExpr(SmallVectorImpl<const SCEV *> &Ops,
     }
   }
 
+  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;

diff  --git a/llvm/test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll b/llvm/test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll
index be51167dab8a9..84ee043b5c264 100644
--- a/llvm/test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll
+++ b/llvm/test/Analysis/LoopAccessAnalysis/reverse-memcheck-bounds.ll
@@ -16,7 +16,7 @@ target datalayout = "e-m:e-i64:64-i128:128-n32:64-S128"
 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

diff  --git a/llvm/test/Transforms/IndVarSimplify/eliminate-exit-no-dl.ll b/llvm/test/Transforms/IndVarSimplify/eliminate-exit-no-dl.ll
index 11c960e103f57..0abfb06bfe2b0 100644
--- a/llvm/test/Transforms/IndVarSimplify/eliminate-exit-no-dl.ll
+++ b/llvm/test/Transforms/IndVarSimplify/eliminate-exit-no-dl.ll
@@ -12,15 +12,12 @@
 define void @foo() {
 ; CHECK-LABEL: @foo(
 ; CHECK-NEXT:  bb:
-; CHECK-NEXT:    [[UMIN:%.*]] = call i64 @llvm.umin.i64(i64 add (i64 ptrtoint ([0 x i8]* @global to i64), i64 500), i64 add (i64 ptrtoint ([0 x i8]* @global to i64), i64 1))
-; CHECK-NEXT:    [[TMP0:%.*]] = sub i64 add (i64 ptrtoint ([0 x i8]* @global to i64), i64 1), [[UMIN]]
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp ne i64 [[TMP0]], 0
 ; CHECK-NEXT:    br label [[BB3:%.*]]
 ; CHECK:       bb3:
 ; 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:    br i1 [[TMP1]], 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]]