[LTO] Always mark regular LTO units with EnableSplitLTOUnit=1

[tejohnson]

Summary

Regular LTO modules do not need LTO Unit splitting, only ThinLTO does
(they must be consistently split into regular and Thin units for
optimizations such as whole program devirtualization and lower type
tests). In order to avoid spurious errors from LTO when combining with
split ThinLTO modules, always set this flag for regular LTO modules.

[pcc]

Sorry, just realized this. If I do

clang++ -c -flto a.cpp # "split"
clang++ -c -flto=thin b.cpp -fwhole-program-vtables # non-split
clang++ a.o b.o

this should fail, right? If I'm not mistaken, this patch series will cause this to succeed. I think we need to change this patch so that it always sets EnableSplitLTOUnit to 1 for regular LTO, then you can drop the other patch.

[tejohnson]

In D65009#1594009, @pcc wrote:

Sorry, just realized this. If I do

clang++ -c -flto a.cpp # "split"
clang++ -c -flto=thin b.cpp -fwhole-program-vtables # non-split
clang++ a.o b.o

this should fail, right? If I'm not mistaken, this patch series will cause this to succeed. I think we need to change this patch so that it always sets EnableSplitLTOUnit to 1 for regular LTO, then you can drop the other patch.

This is a good point. We could detect and handle this during the LTO merging here, but I don't think it is worth it (it would essentially be treating regular LTO as split in any case). The change you suggested fixes the original bug, re running the llvm/clang tests now and will update the patches afterwards.

[tejohnson]

Address comments

[pcc]

LGTM

lib/CodeGen/BackendUtil.cpp

if (CodeGenOpts.PrepareForThinLTO && !CodeGenOpts.DisableLLVMPasses) {
       bool EmitLTOSummary =
           (CodeGenOpts.PrepareForLTO &&
            !CodeGenOpts.DisableLLVMPasses &&
            llvm::Triple(TheModule->getTargetTriple()).getVendor() !=
                llvm::Triple::Apple);
       if (EmitLTOSummary) {
         if (!TheModule->getModuleFlag("ThinLTO"))
           TheModule->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
-        TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
-                                 CodeGenOpts.EnableSplitLTOUnit);
       }
 
       PerModulePasses.add(createBitcodeWriterPass(
           *OS, CodeGenOpts.EmitLLVMUseLists, EmitLTOSummary));
     }
     break;
 
   case Backend_EmitLL:

test/CodeGen/split-lto-unit.c

+// ; Check that -flto=thin without -fsplit-lto-unit has EnableSplitLTOUnit = 0
+// RUN: %clang_cc1 -flto=thin -emit-llvm-bc < %s | llvm-dis -o - | FileCheck %s
+// CHECK: !{i32 1, !"EnableSplitLTOUnit", i32 0}
+//
+// ; Check that -flto=thin with -fsplit-lto-unit has EnableSplitLTOUnit = 1
+// RUN: %clang_cc1 -flto=thin -fsplit-lto-unit -emit-llvm-bc < %s | llvm-dis -o - | FileCheck %s --check-prefix=SPLIT
+// SPLIT: !{i32 1, !"EnableSplitLTOUnit", i32 1}
+//
+// ; Check that regular LTO has no EnableSplitLTOUnit flag
+// RUN: %clang_cc1 -flto -triple x86_64-pc-linux-gnu -emit-llvm-bc < %s | llvm-dis -o - | FileCheck %s --implicit-check-not="EnableSplitLTOUnit" --check-prefix=THINLTOOFF
+// THINLTOOFF: !{i32 1, !"ThinLTO", i32 0}
+
+int main() {}