[LTO] Make detection of WPD remark enablement more robust

[tejohnson]

Summary

Currently only the first function in the module is checked to
see if it has remarks enabled. If that first function is a declaration,
remarks will be incorrectly skipped. Change to look for the first
non-empty function.

[tejohnson]

ping

llvm/trunk/lib/Transforms/IPO/WholeProgramDevirt.cpp

void DevirtModule::rebuildGlobal(VTableBits &B) {
   Alias->takeName(B.GV);
 
   B.GV->replaceAllUsesWith(Alias);
   B.GV->eraseFromParent();
 }
 
 bool DevirtModule::areRemarksEnabled() {
   const auto &FL = M.getFunctionList();
-  if (FL.empty())
-    return false;
-  const Function &Fn = FL.front();
-
+  for (const Function &Fn : FL) {
     const auto &BBL = Fn.getBasicBlockList();
     if (BBL.empty())
-    return false;
+      continue;
     auto DI = OptimizationRemark(DEBUG_TYPE, "", DebugLoc(), &BBL.front());
     return DI.isEnabled();
   }
+  return false;
+}
 
 void DevirtModule::scanTypeTestUsers(Function *TypeTestFunc,
                                      Function *AssumeFunc) {
   // Find all virtual calls via a virtual table pointer %p under an assumption
   // of the form llvm.assume(llvm.type.test(%p, %md)). This indicates that %p
   // points to a member of the type identifier %md. Group calls by (type ID,
   // offset) pair (effectively the identity of the virtual function) and store
   // to CallSlots.

llvm/trunk/test/ThinLTO/X86/cfi-devirt.ll

 ; REQUIRES: x86-registered-target
 
 ; Test CFI devirtualization through the thin link and backend.
 
 ; RUN: opt -thinlto-bc -o %t.o %s
 
 ; Legacy PM
 ; RUN: llvm-lto2 run %t.o -save-temps -pass-remarks=. \
 ; RUN:   -o %t3 \
 ; RUN:   -r=%t.o,test,px \
 ; RUN:   -r=%t.o,_ZN1A1nEi,p \
 ; RUN:   -r=%t.o,_ZN1B1fEi,p \
 ; RUN:   -r=%t.o,_ZN1C1fEi,p \
+; RUN:   -r=%t.o,empty,p \
 ; RUN:   -r=%t.o,_ZTV1B, \
 ; RUN:   -r=%t.o,_ZTV1C, \
 ; RUN:   -r=%t.o,_ZN1A1nEi, \
 ; RUN:   -r=%t.o,_ZN1B1fEi, \
 ; RUN:   -r=%t.o,_ZN1C1fEi, \
 ; RUN:   -r=%t.o,_ZTV1B,px \
 ; RUN:   -r=%t.o,_ZTV1C,px 2>&1 | FileCheck %s --check-prefix=REMARK
 ; RUN: llvm-dis %t3.1.4.opt.bc -o - | FileCheck %s --check-prefix=CHECK-IR
 
 ; New PM
 ; RUN: llvm-lto2 run %t.o -save-temps -use-new-pm -pass-remarks=. \
 ; RUN:   -o %t3 \
 ; RUN:   -r=%t.o,test,px \
 ; RUN:   -r=%t.o,_ZN1A1nEi,p \
 ; RUN:   -r=%t.o,_ZN1B1fEi,p \
 ; RUN:   -r=%t.o,_ZN1C1fEi,p \
+; RUN:   -r=%t.o,empty,p \
 ; RUN:   -r=%t.o,_ZTV1B, \
 ; RUN:   -r=%t.o,_ZTV1C, \
 ; RUN:   -r=%t.o,_ZN1A1nEi, \
 ; RUN:   -r=%t.o,_ZN1B1fEi, \
 ; RUN:   -r=%t.o,_ZN1C1fEi, \
 ; RUN:   -r=%t.o,_ZTV1B,px \
 ; RUN:   -r=%t.o,_ZTV1C,px 2>&1 | FileCheck %s --check-prefix=REMARK
 ; RUN: llvm-dis %t3.1.4.opt.bc -o - | FileCheck %s --check-prefix=CHECK-IR
 
 ; REMARK: single-impl: devirtualized a call to _ZN1A1nEi
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-grtev4-linux-gnu"
 
 %struct.A = type { i32 (...)** }
 %struct.B = type { %struct.A }
 %struct.C = type { %struct.A }
 
 @_ZTV1B = constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* undef, i8* bitcast (i32 (%struct.B*, i32)* @_ZN1B1fEi to i8*), i8* bitcast (i32 (%struct.A*, i32)* @_ZN1A1nEi to i8*)] }, !type !0, !type !1
 @_ZTV1C = constant { [4 x i8*] } { [4 x i8*] [i8* null, i8* undef, i8* bitcast (i32 (%struct.C*, i32)* @_ZN1C1fEi to i8*), i8* bitcast (i32 (%struct.A*, i32)* @_ZN1A1nEi to i8*)] }, !type !0, !type !2
 
+; Put declaration first to test handling of remarks when the first
+; function has no basic blocks.
+declare void @empty()
+
 ; CHECK-IR-LABEL: define i32 @test
 define i32 @test(%struct.A* %obj, i32 %a) {
 entry:
   %0 = bitcast %struct.A* %obj to i8**
   %vtable5 = load i8*, i8** %0
 
   %1 = tail call { i8*, i1 } @llvm.type.checked.load(i8* %vtable5, i32 8, metadata !"_ZTS1A")
   %2 = extractvalue { i8*, i1 } %1, 1