[IfConversion] Disallow TrueBB == FalseBB for valid diamonds

Summary:
This fixes PR45302.

Previously the case

     BB1
     / \
    |   |
   TBB FBB
    |   |
     \ /
     BB2

was treated as a valid diamond also when TBB and FBB was the same basic
block. This then lead to a failed assertion in IfConvertDiamond.

Since TBB == FBB is quite a degenerated case of a diamond, we now
don't treat it as a valid diamond anymore, and thus we will avoid the
trouble of making IfConvertDiamond handle it correctly.

Reviewers: efriedma, kparzysz

Reviewed By: efriedma

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D77651

lib/CodeGen/IfConversion.cpp

@@ -972,6 +972,11 @@ bool IfConverter::ValidDiamond(
       FalseBBI.IsBeingAnalyzed || FalseBBI.IsDone)
     return false;
 
+  // If the True and False BBs are equal we're dealing with a degenerate case
+  // that we don't treat as a diamond.
+  if (TrueBBI.BB == FalseBBI.BB)
+    return false;
+
   MachineBasicBlock *TT = TrueBBI.TrueBB;
   MachineBasicBlock *FT = FalseBBI.TrueBB;
 

test/CodeGen/ARM/ifcvt_diamondSameTrueFalse.mir

@@ -0,0 +1,40 @@
+# RUN: llc -mtriple=thumbv7-apple-ios -o - %s -run-pass if-converter -verify-machineinstrs | FileCheck %s
+
+# Don't treat bb.1 as a valid diamond since IfConverter::IfConvertDiamond can't
+# handle it and used to hit an assertion instead.
+
+--- |
+  define void @func() minsize {
+    ret void
+  }
+...
+---
+name:            func
+body:             |
+  bb.0:
+
+    tBcc %bb.3, 1, $cpsr
+
+  bb.1:
+
+    tBcc %bb.2, 1, $cpsr
+    tB %bb.2, 14, $noreg
+
+  bb.2:
+
+  bb.3:
+  successors:
+    tBX_RET 14, _
+...
+
+# CHECK-LABEL: bb.0:
+# CHECK:         tBcc %bb.3, 1
+
+# CHECK-LABEL: bb.1:
+# CHECK:         tBcc %bb.2, 1
+# CHECK-NEXT:    tB %bb.2, 14
+
+# CHECK-LABEL: bb.2:
+
+# CHECK-LABEL: bb.3:
+# CHECK:         tBX_RET 14