Teach SCEV to report a max backedge count in one interesting case in

HowFarToZero; the case for a canonical loop.

llvm-svn: 139126

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -5184,8 +5184,12 @@ ScalarEvolution::HowFarToZero(const SCEV *V, const Loop *L) {
   // Handle unitary steps, which cannot wraparound.
   // 1*N = -Start; -1*N = Start (mod 2^BW), so:
   //   N = Distance (as unsigned)
-  if (StepC->getValue()->equalsInt(1) || StepC->getValue()->isAllOnesValue())
-    return Distance;
+  if (StepC->getValue()->equalsInt(1) || StepC->getValue()->isAllOnesValue()) {
+    ConstantRange CR = getUnsignedRange(Start);
+    const SCEV *MaxBECount = getConstant(CountDown ? CR.getUnsignedMax()
+                                                   : ~CR.getUnsignedMin());
+    return ExitLimit(Distance, MaxBECount);
+  }
 
   // If the recurrence is known not to wraparound, unsigned divide computes the
   // back edge count. We know that the value will either become zero (and thus

llvm/test/Analysis/ScalarEvolution/max-trip-count.ll

@@ -70,3 +70,34 @@ for.end:                                          ; preds = %for.body, %for.cond
 }
 
 declare i32 @printf(i8*, ...)
+
+; Before -indvars ran on this loop, SCEV solved a max loop trip count of
+; 2^31-2.  Afterwards, we can only solve 2^32-1.
+
+define void @test(i8* %a, i32 %n) nounwind {
+entry:
+  %cmp1 = icmp sgt i32 %n, 0
+  br i1 %cmp1, label %for.body.lr.ph, label %for.end
+
+for.body.lr.ph:                                   ; preds = %entry
+  %tmp = zext i32 %n to i64
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %for.body.lr.ph
+  %indvar = phi i64 [ %indvar.next, %for.body ], [ 0, %for.body.lr.ph ]
+  %arrayidx = getelementptr i8* %a, i64 %indvar
+  store i8 0, i8* %arrayidx, align 1
+  %indvar.next = add i64 %indvar, 1
+  %exitcond = icmp ne i64 %indvar.next, %tmp
+  br i1 %exitcond, label %for.body, label %for.cond.for.end_crit_edge
+
+for.cond.for.end_crit_edge:                       ; preds = %for.body
+  br label %for.end
+
+for.end:                                          ; preds = %for.cond.for.end_crit_edge, %entry
+  ret void
+}
+
+; CHECK: Determining loop execution counts for: @test
+; CHECK-NEXT: backedge-taken count is
+; CHECK-NEXT: max backedge-taken count is -1