Fix for PR27750. Correctly handle the case where the FallThrough block == TBB in getFallThroughMBB.

[DavidKreitzer]

Summary

An explanation of the problem in https://llvm.org/bugs/show_bug.cgi?id=27750.

[congh]

LGTM. Thanks for the fix, David!

lib/Target/X86/X86InstrInfo.cpp

bool X86InstrInfo::isUnpredicatedTerminator(const MachineInstr &MI) const {
   // Conditional branch is a special case.
   if (MI.isBranch() && !MI.isBarrier())
     return true;
   if (!MI.isPredicable())
     return true;
   return !isPredicated(MI);
 }
 
-// Given a MBB and its TBB, find the FBB which was a fallthrough MBB (it may not
-// be a fallthorough MBB now due to layout changes). Return nullptr if the
-// fallthough MBB cannot be identified.
+// Given a MBB and its TBB, find the FBB which was a fallthrough MBB (it may
+// not be a fallthrough MBB now due to layout changes). Return nullptr if the
+// fallthrough MBB cannot be identified.
 static MachineBasicBlock *getFallThroughMBB(MachineBasicBlock *MBB,
                                             MachineBasicBlock *TBB) {
+  // Look for non-EHPad successors other than TBB. If we find exactly one, it
+  // is the fallthrough MBB. If we find zero, then TBB is both the target MBB
+  // and fallthrough MBB. If we find more than one, we cannot identify the
+  // fallthrough MBB and should return nullptr.
   MachineBasicBlock *FallthroughBB = nullptr;
   for (auto SI = MBB->succ_begin(), SE = MBB->succ_end(); SI != SE; ++SI) {
-    if ((*SI)->isEHPad() || *SI == TBB)
+    if ((*SI)->isEHPad() || (*SI == TBB && FallthroughBB))
       continue;
     // Return a nullptr if we found more than one fallthrough successor.
-    if (FallthroughBB)
+    if (FallthroughBB && FallthroughBB != TBB)
       return nullptr;
     FallthroughBB = *SI;
   }
   return FallthroughBB;
 }
 
 bool X86InstrInfo::AnalyzeBranchImpl(
     MachineBasicBlock &MBB, MachineBasicBlock *&TBB, MachineBasicBlock *&FBB,

test/CodeGen/X86/fp-une-cmp.ll

 if.then:
   tail call void @a()
   br label %if.end
 
 if.end:
   ret void
 }
 
+; Test that an FP oeq/une conditional branch can be inverted successfully even
+; when the true and false targets are the same (PR27750).
+; 
+; CHECK-LABEL: pr27750
+; CHECK: ucomiss
+; CHECK-NEXT: jne [[TARGET:.*]]
+; CHECK-NEXT: jp [[TARGET]]
+define void @pr27750(i32* %b, float %x, i1 %y) {
+entry:
+  br label %for.cond
+
+for.cond:
+  br label %for.cond1
+
+for.cond1:
+  br i1 %y, label %for.body3.lr.ph, label %for.end
+
+for.body3.lr.ph:
+  store i32 0, i32* %b, align 4
+  br label %for.end
+
+for.end:
+; After block %for.cond gets eliminated, the two target blocks of this
+; conditional block are the same.
+  %tobool = fcmp une float %x, 0.000000e+00
+  br i1 %tobool, label %for.cond, label %for.cond1
+}
+
 declare void @a()
 
 !1 = !{!"branch_weights", i32 1, i32 1000}