[PATCH] D104063: [SimplifyCFG] avoid crash on degenerate loop

[Sanjay Patel via Phabricator via llvm-commits]

spatel updated this revision to Diff 351413.
spatel added a comment.

Patch updated:
Make the bailout more general and try to explain the problem pattern in a code comment.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D104063/new/

https://reviews.llvm.org/D104063

Files:
  llvm/lib/Transforms/Utils/SimplifyCFG.cpp
  llvm/test/Transforms/SimplifyCFG/two-entry-phi-return.ll


Index: llvm/test/Transforms/SimplifyCFG/two-entry-phi-return.ll
===================================================================
--- llvm/test/Transforms/SimplifyCFG/two-entry-phi-return.ll
+++ llvm/test/Transforms/SimplifyCFG/two-entry-phi-return.ll
@@ -23,4 +23,33 @@
 
 }
 
+ at a = external dso_local global i32, align 4
+
+define i32 @PR50638() {
+; CHECK-LABEL: @PR50638(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    store i32 0, i32* @a, align 4
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  store i32 0, i32* @a, align 4
+  br label %pre.for
+
+pre.for:
+  %tobool.not = phi i1 [ false, %for ], [ true, %entry ]
+  br i1 %tobool.not, label %end, label %for
+
+for:
+  %cmp = phi i1 [ true, %pre.for ], [ false, %post.for ]
+  %storemerge = phi i32 [ 0, %pre.for ], [ 1, %post.for ]
+  store i32 %storemerge, i32* @a, align 4
+  br i1 %cmp, label %post.for, label %pre.for
+
+post.for:
+  br label %for
+
+end:
+  ret i32 0
+}
+
 !0 = !{!"branch_weights", i32 4, i32 64}
Index: llvm/lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2713,6 +2713,12 @@
       isa<ConstantInt>(IfCond))
     return false;
 
+  // Don't try to fold an unreachable block. For example, the phi node itself
+  // can't be the candidate if-condition for a select that we want to form.
+  if (auto *IfCondInst = dyn_cast<Instruction>(IfCond))
+    if (IfCondInst->getParent() == BB)
+      return false;
+
   // Okay, we found that we can merge this two-entry phi node into a select.
   // Doing so would require us to fold *all* two entry phi nodes in this block.
   // At some point this becomes non-profitable (particularly if the target


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