[IfConversion] Disallow TBB == FBB for valid triangles

Summary:
Previously the case

     EBB
     | \_
     |  |
     | TBB
     |  /
     FBB

was treated as a valid triangle also when TBB and FBB was the same basic
block. This could then lead to an invalid CFG when we removed the edge
from EBB to TBB, since that meant we would also remove the edge from EBB
to FBB.

Since TBB == FBB is quite a degenerated case of a triangle, we now
don't treat it as a valid triangle anymore, and thus we will avoid the
trouble with updating the CFG.

Reviewers: efriedma, dmgreen, kparzysz

Reviewed By: efriedma

Subscribers: bjope, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67832

llvm-svn: 372943

lib/CodeGen/IfConversion.cpp

@@ -569,6 +569,9 @@ bool IfConverter::ValidTriangle(BBInfo &TrueBBI, BBInfo &FalseBBI,
                                 bool FalseBranch, unsigned &Dups,
                                 BranchProbability Prediction) const {
   Dups = 0;
+  if (TrueBBI.BB == FalseBBI.BB)
+    return false;
+
   if (TrueBBI.IsBeingAnalyzed || TrueBBI.IsDone)
     return false;
 

test/CodeGen/ARM/ifcvt_triangleSameCvtNext.mir

@@ -0,0 +1,29 @@
+# RUN: llc -mtriple=arm-apple-ios -run-pass=if-converter -verify-machineinstrs %s -o - | FileCheck %s
+...
+---
+name:            foo
+body:             |
+  bb.0:
+    Bcc %bb.2, 1, $cpsr
+
+  bb.1:
+    $sp = tADDspi $sp, 2, 14, _
+    B %bb.1
+
+  bb.2:
+    Bcc %bb.3, 0, $cpsr
+    B %bb.2
+
+  bb.3:
+    Bcc %bb.1, 1, $cpsr
+    B %bb.1
+...
+
+# Both branches in bb.3 jump to bb.1. IfConversion shouldn't treat this as a
+# tringle and insert the tADDspi in bb3, but leave it as it is.
+
+# CHECK:     bb.3:
+# CHECK:       successors: %bb.1
+# CHECK-NOT:   tADDspi
+# CHECK:       Bcc %bb.1, 1, $cpsr
+# CHECK:       B %bb.1