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[Flang][Driver] Emit module summary for Full LTO #164302

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Merged
anchuraj merged 5 commits into from
Oct 23, 2025
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[Flang][Driver] Emit module summary for Full LTO #164302

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3309893
[Flang][Driver] Emit module summary for LTO
anchuraj Oct 17, 2025
1ecbeb9
R2: Addressing review comments
anchuraj Oct 21, 2025
51e02c1
Removing the changes from ThinLTO
anchuraj Oct 21, 2025
c050671
R3: Addressing review comments
anchuraj Oct 22, 2025
ee40244
R4: Addressing review comments
anchuraj Oct 22, 2025
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36 changes: 26 additions & 10 deletions flang/lib/Frontend/FrontendActions.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1019,24 +1019,40 @@ void CodeGenAction::runOptimizationPipeline(llvm::raw_pwrite_stream &os) {

// Create the pass manager.
llvm::ModulePassManager mpm;
if (opts.PrepareForFatLTO) {
// The module summary should be emitted by default for regular LTO
// except for ld64 targets.
bool emitSummary = opts.PrepareForThinLTO || opts.PrepareForFullLTO ||
triple.getVendor() != llvm::Triple::Apple;
// The module summary should be emitted by default for regular LTO
// except for ld64 targets.
bool emitSummary = (opts.PrepareForThinLTO || opts.PrepareForFullLTO) &&
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@tarunprabhu tarunprabhu Oct 21, 2025

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Since ThinLTO support is not being enabled here, should emitSummary only be true if opt.PrepareForFullLTO is set? Should PrepareForThinLTO be ignored here?

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@anchuraj anchuraj Oct 22, 2025 •
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Summary should be emitted for thinLTO. I will work on this as a follow upThe flag set logic is correct. However it is not used for thin LTO right now.

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@tarunprabhu tarunprabhu Oct 22, 2025

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I was thinking that since emitSummary is ignored in the if (opts.PrepareForThinLTO) branch on line 1038, the code should look like this for now:

bool emitSummary = opts.PrepareForFullLTO &&
                       triple.getVendor() != llvm::Triple::Apple;

When support for summaries with ThinLTO is implemented, it can become:

bool emitSummary = (opts.PrepareForThinLTO || opts.PrepareForFullLTO) &&
                       triple.getVendor() != llvm::Triple::Apple;

However, since you intend to work on enabling summaries for thin LTO right away (if I understand you correctly), it should be ok to leave this code as it is. Otherwise, it can be a bit confusing.

What do you think?

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@anchuraj anchuraj Oct 22, 2025 •
edited
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Updated. Agree that it can be confusing.

(triple.getVendor() != llvm::Triple::Apple);
if (opts.PrepareForFatLTO)
mpm = pb.buildFatLTODefaultPipeline(level, opts.PrepareForThinLTO,
emitSummary);
} else if (opts.PrepareForFullLTO)
else if (opts.PrepareForFullLTO)
mpm = pb.buildLTOPreLinkDefaultPipeline(level);
else if (opts.PrepareForThinLTO)
mpm = pb.buildThinLTOPreLinkDefaultPipeline(level);
else
mpm = pb.buildPerModuleDefaultPipeline(level);

if (action == BackendActionTy::Backend_EmitBC)
mpm.addPass(llvm::BitcodeWriterPass(os));
else if (action == BackendActionTy::Backend_EmitLL)
mpm.addPass(llvm::PrintModulePass(os));
if (action == BackendActionTy::Backend_EmitBC ||
action == BackendActionTy::Backend_EmitLL || opts.PrepareForFatLTO) {
if (opts.PrepareForThinLTO) {
// TODO: ThinLTO module summary support is yet to be enabled.
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@tarunprabhu tarunprabhu Oct 21, 2025

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Does -flto=thin imply that module summaries are enabled? If so, it may be a good idea to issue a diagnostic saying that thin-lto support is incomplete. Do you intend to implement module summaries for thin-lto? If that is likely to land relatively soon, it may be ok not to emit a diagnostic.

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@anchuraj anchuraj Oct 21, 2025

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The warning exists: https://github.com/llvm/llvm-project/blob/main/flang/test/Driver/lto-flags.f90#L33 I will work on this.

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@tarunprabhu tarunprabhu Oct 22, 2025

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Thanks for working on this

if (action == BackendActionTy::Backend_EmitBC)
mpm.addPass(llvm::BitcodeWriterPass(os));
else if (action == BackendActionTy::Backend_EmitLL)
mpm.addPass(llvm::PrintModulePass(os));
} else {
if (emitSummary && !llvmModule->getModuleFlag("ThinLTO"))
llvmModule->addModuleFlag(llvm::Module::Error, "ThinLTO", uint32_t(0));
if (action == BackendActionTy::Backend_EmitBC)
mpm.addPass(llvm::BitcodeWriterPass(
os, /*ShouldPreserveUseListOrder=*/false, emitSummary));
else if (action == BackendActionTy::Backend_EmitLL)
mpm.addPass(llvm::PrintModulePass(os, /*Banner=*/"",
/*ShouldPreserveUseListOrder=*/false,
emitSummary));
}
}

// FIXME: This should eventually be replaced by a first-class driver option.
// This should be done for both flang and clang simultaneously.
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1 change: 1 addition & 0 deletions flang/test/CMakeLists.txt
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Expand Up @@ -72,6 +72,7 @@ if (NOT FLANG_STANDALONE_BUILD)
FileCheck
count
not
llvm-bcanalyzer
llvm-dis
llvm-objcopy
llvm-objdump
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30 changes: 19 additions & 11 deletions flang/test/Driver/lto-bc.f90
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@@ -1,21 +1,29 @@
! Test that the output is LLVM bitcode for LTO and not a native objectfile by
! disassembling it to LLVM IR.
! Right now there is nothing special about it and it is similar to non-lto IR,
! more work is needed to add things like module summaries.
! disassembling it to LLVM IR. Also tests that module summaries are emitted for LTO

! RUN: %flang %s -c -o - | not llvm-dis -o %t
! RUN: %flang_fc1 %s -emit-llvm-bc -o - | llvm-dis -o - | FileCheck %s

! RUN: %flang -flto %s -c -o - | llvm-dis -o - | FileCheck %s
! RUN: %flang -flto=thin %s -c -o - | llvm-dis -o - | FileCheck %s

! CHECK: define void @_QQmain()
! CHECK-NEXT: ret void
! CHECK-NEXT: }
! CHECK-NOT: !{{.*}} = !{i32 1, !"ThinLTO", i32 0}
! CHECK-NOT: ^{{.*}} = module:
! CHECK-NOT: ^{{.*}} = gv: (name:
! CHECK-NOT: ^{{.*}} = blockcount:

! RUN: %flang -flto=thin %s -c -o - | llvm-dis -o - | FileCheck %s --check-prefix=THIN
! THIN-NOT: !{{.*}} = !{i32 1, !"ThinLTO", i32 0}
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@tarunprabhu tarunprabhu Oct 21, 2025

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When setting -flto=full, metadata named ThinLTO is present in the module (line 27), but when -flto=thin, it is not? Does the 0 value for "ThinLTO" mean that the ThinLTO is off?

Also, it looks like the output in both the thin and full LTO cases are the same - or at least, what is being checked in this test is the same. What is the difference between the two schemes?

I apologize for my ignorance - as I said, I am not very familiar with LLVM's LTO implementation. Do you mind clarifying some of this? Thanks.

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@tarunprabhu tarunprabhu Oct 21, 2025

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Thanks for the clarification (in the response to a separate comment)

To be clear, if testing the output of full vs. thin is too much, we need not do it - presumably that will have been tested elsewhere, perhaps in llvm/LTO. But if we can test something other than the presence/absence of "ThinLTO" metadata, we probably should.

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@anchuraj anchuraj Oct 21, 2025

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Thank you! Working on these.

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@anchuraj anchuraj Oct 21, 2025

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Hi, ThinLTO requires more work to enable module summaries. I was wrong to test only the emitted module summary. At least https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/BackendUtil.cpp#L1168C9-L1182C8 needs to be enabled. I have removed the tests for thinLTO. This PR enables only module summary for Full LTO by default (same as https://reviews.llvm.org/D34156). Thank you for pointing out this

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@tarunprabhu tarunprabhu Oct 21, 2025

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In this, will -flto=thin and -flto=full produce the same output? Or will the output of -flto=thin be the same as the output if a -flto= option was not given at all?

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@anchuraj anchuraj Oct 22, 2025 •
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Right now, flto = thin does not emit the summary, only flto = full emits. Thin LTO summary emission logic does not change - it is same as previous:

if (action == BackendActionTy::Backend_EmitBC)
    mpm.addPass(llvm::BitcodeWriterPass(os));
  else if (action == BackendActionTy::Backend_EmitLL)
    mpm.addPass(llvm::PrintModulePass(os));

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@anchuraj anchuraj Oct 22, 2025

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flang hello.f90 -c -emit-llvm -o -| llvm-dis -o - and flang -flto=thin hello.f90 -c -o -| llvm-dis -o - will produce the same output in ll files.

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@tarunprabhu tarunprabhu Oct 22, 2025

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Sorry, I got a bit confused and asked a strange question. Let me try again.

In this test, when -flto=full, we are checking for the "ThinLTO = 0" metadata and the presence of "module:, gv:, blockcount:" (I assume that this is the summary, is that right?).

However, when flto=thin, we are only checking for the absence of the "ThinLTO" metadata. We are not checking for the absence of the summaries. Should we be doing that as well?

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@anchuraj anchuraj Oct 22, 2025

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Yes, it is the module summary. Thanks a lot for reviewing this and helping me to improve my change. Since I am learning through this as well, there are misses which I try to correct. As this is a work in progress and as is a feature not yet implemented, I did not have preferences for checking or not checking this. I have added it back since it is more consistent with the existed test case.


! CHECK-NOT: ^0 = module:
! CHECK-NOT: ^1 = gv: (name:
! CHECK-NOT: ^2 = flags:
! CHECK-NOT: ^3 = blockcount:
! RUN: %flang -flto %s -c -o - | llvm-dis -o - | FileCheck %s --check-prefix=FULL
! FULL: define void @_QQmain()
! FULL-NEXT: ret void
! FULL-NEXT: }
! FULL: !{{.*}} = !{i32 1, !"ThinLTO", i32 0}
! FULL: ^{{.*}} = module:
! FULL: ^{{.*}} = gv: (name:
! FULL: ^{{.*}} = blockcount:

! RUN: %flang_fc1 -flto -emit-llvm-bc %s -o - | llvm-bcanalyzer -dump| FileCheck --check-prefix=MOD-SUMM %s
! MOD-SUMM: FULL_LTO_GLOBALVAL_SUMMARY_BLOCK
program main
end program