Fix a bug in IfConverter with nested predicates.

Prior to this patch, IfConverter may widen the cases where a sequence of
instructions were executed because of the way it uses nested predicates. This
result in incorrect execution.

For instance, Let A be a basic block that flows conditionally into B and B be a
predicated block.
B can be predicated with A.BrToBPredicate into A iff B.Predicate is less
"permissive" than A.BrToBPredicate, i.e., iff A.BrToBPredicate subsumes
B.Predicate.

The IfConverter was checking the opposite: B.Predicate subsumes
A.BrToBPredicate.

<rdar://problem/14379453>

llvm-svn: 187071

lib/CodeGen/IfConversion.cpp

@@ -720,9 +720,9 @@ bool IfConverter::FeasibilityAnalysis(BBInfo &BBI,
   if (BBI.IsDone || BBI.IsUnpredicable)
     return false;
 
-  // If it is already predicated, check if its predicate subsumes the new
+  // If it is already predicated, check if the new predicate subsumes
-  // predicate.
+  // its predicate.
-  if (BBI.Predicate.size() && !TII->SubsumesPredicate(BBI.Predicate, Pred))
+  if (BBI.Predicate.size() && !TII->SubsumesPredicate(Pred, BBI.Predicate))
     return false;
 
   if (BBI.BrCond.size()) {

test/CodeGen/ARM/2013-05-05-IfConvertBug.ll

@@ -69,3 +69,62 @@ bb6.i350:                                         ; preds = %bb2.i
 KBBlockZero.exit:                                 ; preds = %bb2.i
   indirectbr i8* undef, [label %KBBlockZero_return_1, label %KBBlockZero_return_0]
 }
+
+
+; If-converter was checking for the wrong predicate subsumes pattern when doing
+; nested predicates.
+; E.g., Let A be a basic block that flows conditionally into B and B be a
+; predicated block.
+; B can be predicated with A.BrToBPredicate into A iff B.Predicate is less
+; "permissive" than A.BrToBPredicate, i.e., iff A.BrToBPredicate subsumes
+; B.Predicate. 
+; <rdar://problem/14379453>
+
+; Hard-coded registers comes from the ABI.
+; CHECK: wrapDistance:
+; CHECK: cmp r1, #59
+; CHECK-NEXT: itt le
+; CHECK-NEXT: suble r0, r2, #1
+; CHECK-NEXT: bxle lr
+; CHECK-NEXT: subs [[REG:r[0-9]+]], #120
+; CHECK-NEXT: cmp [[REG]], r1
+; CHECK-NOT: it lt
+; CHECK-NEXT: bge [[LABEL:.+]]
+; Next BB
+; CHECK-NOT: cmplt
+; CHECK: cmp r0, #119
+; CHECK-NEXT: itt le
+; CHECK-NEXT: addle r0, r1, #1
+; CHECK-NEXT: bxle lr
+; Next BB
+; CHECK: [[LABEL]]:
+; CHECK-NEXT: subs r0, r1, r0
+; CHECK-NEXT: bx lr
+define i32 @wrapDistance(i32 %tx, i32 %sx, i32 %w) {
+entry:
+  %cmp = icmp slt i32 %sx, 60
+  br i1 %cmp, label %if.then, label %if.else
+
+if.then:                                          ; preds = %entry
+  %sub = add nsw i32 %w, -1
+  br label %return
+
+if.else:                                          ; preds = %entry
+  %sub1 = add nsw i32 %w, -120
+  %cmp2 = icmp slt i32 %sub1, %sx
+  %cmp3 = icmp slt i32 %tx, 120
+  %or.cond = and i1 %cmp2, %cmp3
+  br i1 %or.cond, label %if.then4, label %if.end5
+
+if.then4:                                         ; preds = %if.else
+  %add = add nsw i32 %sx, 1
+  br label %return
+
+if.end5:                                          ; preds = %if.else
+  %sub6 = sub nsw i32 %sx, %tx
+  br label %return
+
+return:                                           ; preds = %if.end5, %if.then4, %if.then
+  %retval.0 = phi i32 [ %sub, %if.then ], [ %add, %if.then4 ], [ %sub6, %if.end5 ]
+  ret i32 %retval.0
+}