[llvm-reduce] Fix invalid reduction in basic-blocks delta pass

Previously, if the basic-blocks delta pass tried to remove a basic block
that was the last basic block in a function that did not have external
or weak linkage, the resulting IR would become invalid. Since removing
the last basic block in a function is effectively identical to removing
the function body itself, we check explicitly for this case and if we
detect it, we run the same logic as in ReduceFunctionBodies.cpp

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D113486

Author: Dwight Guth

llvm/test/tools/llvm-reduce/remove-bbs-comdat.ll

@@ -0,0 +1,21 @@
+; RUN: llvm-reduce --delta-passes=basic-blocks --abort-on-invalid-reduction --test FileCheck --test-arg --check-prefixes=CHECK-ALL,CHECK-INTERESTINGNESS --test-arg %s --test-arg --input-file %s -o %t
+; RUN: cat %t | FileCheck --check-prefixes=CHECK-ALL,CHECK-FINAL %s
+
+; CHECK-FINAL-NOT: = comdat
+; CHECK-INTERESTINGNESS: @callee(
+; CHECK-FINAL: declare void @callee()
+
+$foo = comdat any
+
+define void @callee() comdat($foo) {
+  ret void
+}
+
+; CHECK-ALL: define void @caller()
+define void @caller() {
+entry:
+; CHECK-ALL: call void @callee()
+; CHECK-ALL: ret void
+  call void @callee()
+  ret void
+}

llvm/test/tools/llvm-reduce/remove-bbs-entry.ll

@@ -0,0 +1,18 @@
+; Test that llvm-reduce correctly removes the entry block of functions for
+; linkages other than external and weak.
+;
+; RUN: llvm-reduce --abort-on-invalid-reduction --delta-passes=basic-blocks --test FileCheck --test-arg --check-prefixes=CHECK-INTERESTINGNESS --test-arg %s --test-arg --input-file %s -o %t
+; RUN: cat %t | FileCheck %s
+
+; CHECK-INTERESTINGNESS: interesting1:
+
+; CHECK-NOT: uninteresting
+define linkonce_odr i32 @foo() {
+uninteresting:
+  ret i32 0
+}
+
+define i32 @main(i1 %c) {
+interesting1:
+  ret i32 0
+}

llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.cpp

@@ -126,7 +126,15 @@ static void extractBasicBlocksFromModule(Oracle &O, Module &Program) {
     // Instructions might be referenced in other BBs
     for (auto &I : *BB)
       I.replaceAllUsesWith(UndefValue::get(I.getType()));
-    BB->eraseFromParent();
+    if (BB->getParent()->size() == 1) {
+      // this is the last basic block of the function, thus we must also make
+      // sure to remove comdat and set linkage to external
+      auto F = BB->getParent();
+      F->deleteBody();
+      F->setComdat(nullptr);
+    } else {
+      BB->eraseFromParent();
+    }
   }
 }