[ThinLTO/WPD] Enable -wholeprogramdevirt-skip in ThinLTO backends

[tejohnson]

Summary

Previously this option could be used to skip devirtualizations of the
given functions in regular LTO and in the ThinLTO indexing step. This
change allows them to be skipped in the backend as well, which is useful
when debugging WPD in a distributed ThinLTO backend.

[evgeny777]

Thanks, LGTM

clang/test/CodeGen/thinlto-distributed-cfi-devirt.ll

 ; RUN: llvm-dis %t.o.thinlto.bc -o - | FileCheck %s --check-prefix=CHECK-DIS
 ; Round trip it through llvm-as
 ; RUN: llvm-dis %t.o.thinlto.bc -o - | llvm-as -o - | llvm-dis -o - | FileCheck %s --check-prefix=CHECK-DIS
 ; CHECK-DIS: ^0 = module: (path: "{{.*}}thinlto-distributed-cfi-devirt.ll.tmp.o", hash: ({{.*}}, {{.*}}, {{.*}}, {{.*}}, {{.*}}))
 ; CHECK-DIS: ^1 = gv: (guid: 8346051122425466633, summaries: (function: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 1, dsoLocal: 0, canAutoHide: 0), insts: 18, typeIdInfo: (typeTests: (^2), typeCheckedLoadVCalls: (vFuncId: (^2, offset: 8), vFuncId: (^2, offset: 0))))))
 ; CHECK-DIS: ^2 = typeid: (name: "_ZTS1A", summary: (typeTestRes: (kind: allOnes, sizeM1BitWidth: 7), wpdResolutions: ((offset: 0, wpdRes: (kind: branchFunnel)), (offset: 8, wpdRes: (kind: singleImpl, singleImplName: "_ZN1A1nEi"))))) ; guid = 7004155349499253778
 
 ; RUN: %clang_cc1 -triple x86_64-grtev4-linux-gnu \
-; RUN:   -emit-obj -fthinlto-index=%t.o.thinlto.bc -O2 \
-; RUN:   -emit-llvm -o - -x ir %t.o | FileCheck %s --check-prefixes=CHECK-IR
+; RUN:   -emit-obj -fthinlto-index=%t.o.thinlto.bc -O2 -Rpass=wholeprogramdevirt \
+; RUN:   -emit-llvm -o - -x ir %t.o 2>&1 | FileCheck %s --check-prefixes=CHECK-IR --check-prefixes=REMARKS
+
+; Check that the devirtualization is suppressed via -wholeprogramdevirt-skip
+; RUN: %clang_cc1 -triple x86_64-grtev4-linux-gnu -mllvm -wholeprogramdevirt-skip=_ZN1A1nEi \
+; RUN:   -emit-obj -fthinlto-index=%t.o.thinlto.bc -O2 -Rpass=wholeprogramdevirt \
+; RUN:   -emit-llvm -o - -x ir %t.o 2>&1 | FileCheck %s --check-prefixes=SKIP-IR --check-prefixes=SKIP-REMARKS
+
+; REMARKS: single-impl: devirtualized a call to _ZN1A1nEi
+; SKIP-REMARKS-NOT: single-impl
 
 ; Check that backend does not fail generating native code.
 ; RUN: %clang_cc1 -triple x86_64-grtev4-linux-gnu \
 ; RUN:   -emit-obj -fthinlto-index=%t.o.thinlto.bc -O2 \
 ; RUN:   -o %t.native.o -x ir %t.o
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-grtev4-linux-gnu"
trap:
   unreachable
 
 cont:
   %3 = extractvalue { i8*, i1 } %1, 0
   %4 = bitcast i8* %3 to i32 (%struct.A*, i32)*
 
   ; Check that the call was devirtualized.
   ; CHECK-IR: %call = tail call i32 @_ZN1A1nEi
+  ; SKIP-IR-NOT: %call = tail call i32 @_ZN1A1nEi
   %call = tail call i32 %4(%struct.A* nonnull %obj, i32 %a)
   %vtable16 = load i8*, i8** %0
   %5 = tail call { i8*, i1 } @llvm.type.checked.load(i8* %vtable16, i32 0, metadata !"_ZTS1A")
   %6 = extractvalue { i8*, i1 } %5, 1
   br i1 %6, label %cont2, label %trap
 
 cont2:
   %7 = extractvalue { i8*, i1 } %5, 0

llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp

bool DevirtIndex::tryFindVirtualCallTargets(
   }
 
   // Give up if we couldn't find any targets.
   return !TargetsForSlot.empty();
 }
 
 void DevirtModule::applySingleImplDevirt(VTableSlotInfo &SlotInfo,
                                          Constant *TheFn, bool &IsExported) {
+  // Don't devirtualize function if we're told to skip it
+  // in -wholeprogramdevirt-skip.
+  if (FunctionsToSkip.match(TheFn->stripPointerCasts()->getName()))
+    return;
   auto Apply = [&](CallSiteInfo &CSInfo) {
     for (auto &&VCallSite : CSInfo.CallSites) {
       if (RemarksEnabled)
         VCallSite.emitRemark("single-impl",
                              TheFn->stripPointerCasts()->getName(), OREGetter);
       VCallSite.CB.setCalledOperand(ConstantExpr::getBitCast(
           TheFn, VCallSite.CB.getCalledOperand()->getType()));
       // This use is no longer unsafe.