[IfConversion] Disallow TrueBB == FalseBB for valid diamonds

[uabelho]

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.

[uabelho]

I have no idea if this is the best fix for
https://bugs.llvm.org/show_bug.cgi?id=45302
but I saw that we've made a similar fix for triangles in 957e090ac9 and thought maybe this could be an easy way out in this case.

Found this when compiling for my out of tree backend with a non-standard pipeline, so in normal cases I suppose the code will be cleanup up before we reach ifconversion.

[efriedma]

LGTM

[uabelho]

Thanks!
Submitted in 893df2032d4.

llvm/lib/CodeGen/IfConversion.cpp

bool IfConverter::ValidDiamond(
     BBInfo &TrueBBI, BBInfo &FalseBBI,
     unsigned &Dups1, unsigned &Dups2,
     BBInfo &TrueBBICalc, BBInfo &FalseBBICalc) const {
   Dups1 = Dups2 = 0;
   if (TrueBBI.IsBeingAnalyzed || TrueBBI.IsDone ||
       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;
 
   if (!TT && blockAlwaysFallThrough(TrueBBI))
     TT = getNextBlock(*TrueBBI.BB);
   if (!FT && blockAlwaysFallThrough(FalseBBI))
     FT = getNextBlock(*FalseBBI.BB);
   if (TT != FT)

llvm/test/CodeGen/ARM/ifcvt_diamondSameTrueFalse.mir

+# 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