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

[dwightguth]

Summary

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

[dwightguth]

fix copy-paste error

[aeubanks]

could you add a test for the comdat part?

[dwightguth]

add test for comdat

[dwightguth]

@aeubanks this is done

[dwightguth]

fix commit message

[dwightguth]

Should be ready to commit. You should double check that the commit message is in the right format though because one of my updates to this diff seems to have messed with it by accident. I think I fixed it, but I'm not super familiar with Phabricator.

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

+; 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

+; 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

for (auto &BB : F) {
         replaceBranchTerminator(BB, BBsToKeep);
     }
 
   // Replace out-of-chunk switch uses
   for (auto &BB : BBsToDelete) {
     // Instructions might be referenced in other BBs
     for (auto &I : *BB)
       I.replaceAllUsesWith(UndefValue::get(I.getType()));
+    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();
     }
   }
+}
 
 /// Counts the amount of basic blocks and prints their name & respective index
 static int countBasicBlocks(Module &Program) {
   // TODO: Silence index with --quiet flag
   outs() << "----------------------------\n";
   int BBCount = 0;
   for (auto &F : Program)
     for (auto &BB : F) {