[PATCH] D87965: [InstCombine] replace phi values from unreachable blocks with 'undef'

[Sanjay Patel via Phabricator via llvm-commits]

This revision was automatically updated to reflect the committed changes.
spatel marked an inline comment as done.
Closed by commit rGcf75e83275d1: [InstCombine] replace zombie unreachable values with 'undef' before erasing (authored by spatel).

Changed prior to commit:
  https://reviews.llvm.org/D87965?vs=293018&id=293020#toc

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D87965/new/

https://reviews.llvm.org/D87965

Files:
  llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
  llvm/test/Transforms/InstCombine/phi.ll


Index: llvm/test/Transforms/InstCombine/phi.ll
===================================================================
--- llvm/test/Transforms/InstCombine/phi.ll
+++ llvm/test/Transforms/InstCombine/phi.ll
@@ -1181,3 +1181,39 @@
   %cmp1 = icmp ne i32 %a.0, 0
   ret i1  %cmp1
 }
+
+; This would crash trying to delete an instruction (conv)
+; that still had uses because the user (the phi) was not
+; updated to remove a use from an unreachable block (g.exit).
+
+define void @main(i1 %cond, i16 %x) {
+; CHECK-LABEL: @main(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_COND:%.*]]
+; CHECK:       for.cond:
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[FOR_END:%.*]], label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    unreachable
+; CHECK:       g.exit:
+; CHECK-NEXT:    br label [[FOR_COND]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %for.cond
+
+for.cond:
+  %p = phi double [ %conv, %g.exit ], [ undef, %entry ]
+  br i1 %cond, label %for.end, label %for.body
+
+for.body:
+  %conv = sitofp i16 %x to double
+  unreachable
+
+g.exit:
+  br label %for.cond
+
+for.end:
+  store double %p, double* undef
+  ret void
+}
Index: llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2813,6 +2813,9 @@
       if (SI->isVolatile())
         return nullptr;
 
+    // A value may still have uses before we process it here (for example, in
+    // another unreachable block), so convert those to undef.
+    replaceInstUsesWith(*Prev, UndefValue::get(Prev->getType()));
     eraseInstFromFunction(*Prev);
     return &I;
   }


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