[PATCH] Make ScalarEvolution less aggressive with respect to no-wrap flags.

[Sanjoy Das]

Hi atrick, majnemer,

ScalarEvolution currently lowers a subtraction recurrence to an add recurrence with the same no-wrap flags as the subtraction.  This is incorrect because `sub nsw X, Y` is not the same as `add nsw X, -Y` and `sub nuw X, Y` is not the same as `add nuw X, -Y`.  This patch fixes the issue, and adds a test case demonstrating the bug.

http://reviews.llvm.org/D7081

Files:
  lib/Analysis/ScalarEvolution.cpp
  test/Analysis/ScalarEvolution/nw-sub-is-not-nw-add.ll

Index: lib/Analysis/ScalarEvolution.cpp
===================================================================
--- lib/Analysis/ScalarEvolution.cpp
+++ lib/Analysis/ScalarEvolution.cpp
@@ -3154,8 +3154,9 @@
   if (LHS == RHS)
     return getConstant(LHS->getType(), 0);
 
-  // X - Y --> X + -Y
-  return getAddExpr(LHS, getNegativeSCEV(RHS), Flags);
+  // X - Y --> X + -Y.
+  // X -(nsw || nuw) Y --> X + -Y.
+  return getAddExpr(LHS, getNegativeSCEV(RHS));
 }
 
 /// getTruncateOrZeroExtend - Return a SCEV corresponding to a conversion of the
@@ -3461,12 +3462,6 @@
                   if (isKnownPositive(getMinusSCEV(getSCEV(GEP), Ptr)))
                     Flags = setFlags(Flags, SCEV::FlagNUW);
                 }
-              } else if (const SubOperator *OBO =
-                           dyn_cast<SubOperator>(BEValueV)) {
-                if (OBO->hasNoUnsignedWrap())
-                  Flags = setFlags(Flags, SCEV::FlagNUW);
-                if (OBO->hasNoSignedWrap())
-                  Flags = setFlags(Flags, SCEV::FlagNSW);
               }
 
               const SCEV *StartVal = getSCEV(StartValueV);
Index: test/Analysis/ScalarEvolution/nw-sub-is-not-nw-add.ll
===================================================================
--- /dev/null
+++ test/Analysis/ScalarEvolution/nw-sub-is-not-nw-add.ll
@@ -0,0 +1,41 @@
+; RUN: opt -S -indvars < %s | FileCheck %s
+
+; Check that SCEV does not assume sub nuw X Y == add nuw X, -Y
+define void @f(i32* %loc) {
+; CHECK-LABEL: @f
+ entry:
+  br label %loop
+
+ loop:
+  %idx = phi i32 [ 6, %entry ], [ %idx.dec, %loop ]
+  store i32 %idx, i32* %loc
+  %idx.dec = sub nuw i32 %idx, 1
+  %cond = icmp uge i32 %idx.dec, 5
+  br i1 %cond, label %loop, label %exit
+; CHECK-NOT: br i1 true, label %loop, label %exit
+
+ exit:
+  ret void
+}
+
+declare void @use_i1(i1)
+
+; Check that SCEV does not assume sub nsw X Y == add nsw X, -Y
+define void @g(i32 %lim) {
+; CHECK-LABEL: @g
+ entry:
+  br label %loop
+
+ loop:
+  %idx = phi i32 [ -1, %entry ], [ %idx.dec, %loop ]
+  %t = icmp sgt i32 %idx, 0
+; CHECK-NOT:   call void @use_i1(i1 false)
+; CHECK: call void @use_i1(i1 %t)
+  call void @use_i1(i1 %t)
+  %idx.dec = sub nsw i32 %idx, -2147483648
+  %cond = icmp eq i32 %idx.dec, %lim
+  br i1 %cond, label %loop, label %exit
+
+ exit:
+  ret void
+}

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