InstCombine: Remove infinite loop caused by FoldOpIntoPhi

FoldOpIntoPhi could create an infinite loop if the PHI could potentially
reach a BB it was considering inserting instructions into.  The
instructions it would insert would eventually lead to other combines
firing which would, again, lead to FoldOpIntoPhi firing.

The solution is to handicap FoldOpIntoPhi so that it doesn't attempt to
insert instructions that the PHI might reach.

This fixes PR21377.

llvm-svn: 221187

3 files changed, 28 insertions, 18 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index c32294f51fb..5a0b01eca5c 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -40,8 +40,10 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Analysis/AssumptionTracker.h"
+#include "llvm/Analysis/CFG.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/InstructionSimplify.h"
+#include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/MemoryBuiltins.h"
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/CFG.h"
@@ -794,13 +796,14 @@ Instruction *InstCombiner::FoldOpIntoPhi(Instruction &I) {
     // If the incoming non-constant value is in I's block, we will remove one
     // instruction, but insert another equivalent one, leading to infinite
     // instcombine.
-    if (NonConstBB == I.getParent())
+    if (isPotentiallyReachable(I.getParent(), NonConstBB, DT,
+                               getAnalysisIfAvailable<LoopInfo>()))
       return nullptr;
   }
 
   // If there is exactly one non-constant value, we can insert a copy of the
   // operation in that block.  However, if this is a critical edge, we would be
-  // inserting the computation one some other paths (e.g. inside a loop).  Only
+  // inserting the computation on some other paths (e.g. inside a loop).  Only
   // do this if the pred block is unconditionally branching into the phi block.
   if (NonConstBB != nullptr) {
     BranchInst *BI = dyn_cast<BranchInst>(NonConstBB->getTerminator());
diff --git a/llvm/test/Transforms/InstCombine/fold-phi.ll b/llvm/test/Transforms/InstCombine/fold-phi.ll
index bd01d58aa58..7954caf9754 100644
--- a/llvm/test/Transforms/InstCombine/fold-phi.ll
+++ b/llvm/test/Transforms/InstCombine/fold-phi.ll
@@ -17,23 +17,30 @@ end:
   ret float %add5
 }
 
-; CHECK: fold_phi
-define float @fold_phi(float %a) nounwind {
+; CHECK-LABEL: @pr21377(
+define void @pr21377(i32, i32) {
 entry:
-  br label %for.body
+  br label %while.cond.i
 
-for.body:
-; CHECK: phi float
-; CHECK-NEXT: br i1 undef
-  %sum.057 = phi float [ 0.000000e+00, %entry ], [ %add5, %bb0 ]
-  %add5 = fadd float %sum.057, 1.0 ;; Should be moved to the latch!
-  br i1 undef, label %bb0, label %end
+while.cond.i:                                     ; preds = %while.end.i, %entry
+  %g.0.i = phi i64 [ 0, %entry ], [ %phitmp5.i, %while.end.i ]
+  br i1 undef, label %fn2.exit, label %while.body.i
 
-; CHECK: bb0:
-bb0:
-; CHECK: fadd float
-  br label %for.body
+while.body.i:                                     ; preds = %while.cond.i
+  %conv.i = zext i32 %0 to i64
+  %phitmp3.i = or i64 %g.0.i, %conv.i
+  br label %while.cond3.i
 
-end:
-  ret float %add5
+while.cond3.i:                                    ; preds = %while.cond3.i, %while.body.i
+  %g.1.i = phi i64 [ %phitmp3.i, %while.body.i ], [ 0, %while.cond3.i ]
+  br i1 undef, label %while.end.i, label %while.cond3.i
+
+while.end.i:                                      ; preds = %while.cond3.i
+  %conv.i.i = zext i32 %1 to i64
+  %or7.i = or i64 %g.1.i, %conv.i.i
+  %phitmp5.i = and i64 %or7.i, 4294967295
+  br label %while.cond.i
+
+fn2.exit:                                         ; preds = %while.cond.i
+  ret void
 }
diff --git a/llvm/test/Transforms/InstCombine/pr12338.ll b/llvm/test/Transforms/InstCombine/pr12338.ll
index d34600f0fa5..614387a452c 100644
--- a/llvm/test/Transforms/InstCombine/pr12338.ll
+++ b/llvm/test/Transforms/InstCombine/pr12338.ll
@@ -6,7 +6,6 @@ entry:
 
 for.cond:
   %local = phi <1 x i32> [ <i32 0>, %entry ], [ %phi2, %cond.end47 ]
-; CHECK: sub <1 x i32> <i32 92>, %local
   %phi3 = sub <1 x i32> zeroinitializer, %local
   br label %cond.end
 
@@ -19,6 +18,7 @@ cond.end:
 
 cond.end47:
   %sum = add <1 x i32> %cond, <i32 92>
+; CHECK: sub <1 x i32> <i32 -92>, %cond
   %phi2 = sub <1 x i32> zeroinitializer, %sum
   br label %for.cond
 }