[opt] Do not replace MD refs to destroyed constants with undef

[StephenTozer]

Summary

This patch fixes some incorrect behaviour in Constants.cpp, in which as part of destroying a constant we replace any references to that constant from metadata with an undef value of the same type. This is incorrect for any non-debug-info metadata, and debug info metadata already has RAUW handlers to insert an undef value if the constant is destroyed (and replaced with nullptr).

This resolves the issue introduced in rGe5d958c45629 and mentioned in D108642.

[aprantl]

Assuming we already have a test that ensures that the desired debug info behavior keeps working after this patch, this looks good!

llvm/lib/IR/Constants.cpp

while (I != E) {
     // Since we return immediately upon finding a live user, we can always
     // restart from user_begin().
     if (RemoveDeadUsers)
       I = C->user_begin();
     else
       ++I;
   }
 
-  if (RemoveDeadUsers) {
-    // If C is only used by metadata, it should not be preserved but should
-    // have its uses replaced.
-    if (C->isUsedByMetadata()) {
-      const_cast<Constant *>(C)->replaceAllUsesWith(
-          UndefValue::get(C->getType()));
-    }
+  if (RemoveDeadUsers)
     const_cast<Constant *>(C)->destroyConstant();
-  }
 
   return true;
 }
 
 void Constant::removeDeadConstantUsers() const {
   Value::const_user_iterator I = user_begin(), E = user_end();
   Value::const_user_iterator LastNonDeadUser = E;
   while (I != E) {

llvm/test/LTO/Resolution/X86/Inputs/no-undef-type-md.ll

+; ModuleID = 'test.cpp.o'
+source_filename = "test.cpp"
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare void @a()
+
+!llvm.module.flags = !{!9, !39}
+
+!9 = !{i32 1, !"EnableSplitLTOUnit", i32 1}
+!39 = !{i32 5, !"CG Profile", !40}
+!40 = !{!41}
+!41 = distinct !{null, i32 ()* bitcast (void ()* @a to i32 ()*), i64 2594092}

llvm/test/LTO/Resolution/X86/no-undef-type-md.ll

+; RUN: opt <%s -o %t0.o -thinlto-bc -thinlto-split-lto-unit
+; RUN: llvm-as -o %t1.o %S/Inputs/no-undef-type-md.ll
+; RUN: llvm-lto2 run -o a.out \
+; RUN: %t0.o \
+; RUN: -r=%t0.o,a, \
+; RUN: -r=%t0.o,b,pl \
+; RUN: %t1.o \
+; RUN: -r=%t1.o,a,pl \
+; RUN: | FileCheck --allow-empty --check-prefix=ERROR %s
+; RUN llvm-nm a.out.0 a.out.1 -S | FileCheck %s
+
+; ERROR-NOT: expected a Function or null
+; ERROR-NOT: i32 (%0*, i32*)* undef
+
+; CHECK: a.out.0:
+; CHECK: a.out.1:
+
+; ModuleID = 'test.cpp.o'
+source_filename = "test.cpp"
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i32 @a() {
+entry:
+  ret i32 0
+}
+
+define i32 @b() {
+entry:
+  ret i32 0
+}
+
+!llvm.module.flags = !{!39}
+
+!39 = !{i32 5, !"CG Profile", !40}
+!40 = !{!41}
+!41 = !{i32 ()* @b, i32 ()* @a, i64 2594092}