[ShrinkWrap] Fix FindIDom to only have one kind of failure

[mkuper]

Summary

FindIDom could fail in two different ways - it could return either nullptr or the block itself, depending on the circumstances.
Some users checked one error condition, while others checked the other.

Make it always return nullptr on failure.
This fixes PR26004.

[qcolombet]

Hi Michael,

Good catch!
LGTM.

Thanks,
-Quentin

llvm/trunk/lib/CodeGen/ShrinkWrap.cpp

 MachineBasicBlock *FindIDom(MachineBasicBlock &Block, ListOfBBs BBs,
                             DominanceAnalysis &Dom) {
   MachineBasicBlock *IDom = &Block;
   for (MachineBasicBlock *BB : BBs) {
     IDom = Dom.findNearestCommonDominator(IDom, BB);
     if (!IDom)
       break;
   }
+  if (IDom == &Block)
+    return nullptr;
   return IDom;
 }
 
 void ShrinkWrap::updateSaveRestorePoints(MachineBasicBlock &MBB,
                                          RegScavenger *RS) {
   // Get rid of the easy cases first.
   if (!Save)
     Save = &MBB;
if (!RestorePostDominatesSave)
       Restore = MPDT->findNearestCommonDominator(Restore, Save);
 
     // Fix (C).
     if (Save && Restore &&
         (MLI->getLoopFor(Save) || MLI->getLoopFor(Restore))) {
       if (MLI->getLoopDepth(Save) > MLI->getLoopDepth(Restore)) {
         // Push Save outside of this loop if immediate dominator is different
         // from save block. If immediate dominator is not different, bail out.
-        MachineBasicBlock *IDom = FindIDom<>(*Save, Save->predecessors(), *MDT);
-        if (IDom != Save)
-          Save = IDom;
-        else {
-          Save = nullptr;
+        Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
+        if (!Save)
           break;
-        }
       } else {
         // If the loop does not exit, there is no point in looking
         // for a post-dominator outside the loop.
         SmallVector<MachineBasicBlock*, 4> ExitBlocks;
         MLI->getLoopFor(Restore)->getExitingBlocks(ExitBlocks);
         // Push Restore outside of this loop.
         // Look for the immediate post-dominator of the loop exits.
         MachineBasicBlock *IPdom = Restore;

llvm/trunk/test/CodeGen/X86/shrinkwrap-hang.ll

+; RUN: llc %s -o - -enable-shrink-wrap=true | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-f64:32:64-f80:32-n8:16:32-S128"
+target triple = "i686-pc-linux"
+
+@b = global i32 1, align 4
+@a = common global i32 0, align 4
+
+declare void @fn1() #0
+
+; CHECK-LABEL: fn2:
+define void @fn2() #0 {
+entry:
+  %0 = load i32, i32* @b, align 4
+  %tobool = icmp eq i32 %0, 0
+  br i1 %tobool, label %if.end, label %lbl
+
+lbl:                                              ; preds = %if.end, %entry
+  store i32 0, i32* @b, align 4
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %lbl
+  tail call void @fn1()
+  %1 = load i32, i32* @b, align 4
+  %tobool1 = icmp eq i32 %1, 0
+  br i1 %tobool1, label %if.end3, label %lbl
+
+if.end3:                                          ; preds = %if.end
+  ret void
+}
+
+attributes #0 = { norecurse nounwind "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="pentium4" "target-features"="+fxsr,+mmx,+sse,+sse2" "unsafe-fp-math"="false" "use-soft-float"="false" }