add "Fast" option to LinkTimeOptimization API which enables LTCG:incremental for vs2010, and behaves as "On" for other toolsets by withmorten · Pull Request #2549 · premake/premake-core

withmorten · 2025-12-01T20:57:40Z

add "Fast" option to LinkTimeOptimization API which enables LTCG:incremental for vs2010, and behaves as "On" for other toolsets

What does this PR do?

^ see commit description

How does this PR change Premake's behavior?

This adds a "Fast" option to the LinkTimeOptimiation for vs2010 projects.

Anything else we should know?

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Focus on a single fix or feature; remove any unrelated formatting or code changes
Add unit tests showing fix or feature works; all tests pass
Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
Follow our coding conventions
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Align documentation to your changes

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samsinsane

Default is supposed to do nothing so that you get the default of the tool you're using. The linked PR didn't change that behaviour, so I'm inclined to reject this PR.

nickclark2016 · 2025-12-07T23:57:08Z

When Premake specifies a "default" value, we are using the toolset default (i.e. if you used cl.exe, what is the behavior), not the default generated project from VS. I agree with @samsinsane and would agree that the behavior as implemented is the correct behavior.

withmorten · 2025-12-08T09:10:59Z

I see - the cl.exe default is WholeProgramOptimization being off.

Would it be acceptable to create another PR that adds an "incremental" or "fast" option? The API changed the setting from incremental/fast to "full" when enabling it.

nickclark2016 · 2025-12-11T14:14:37Z

Would this be a new API for Link Time Code Generation? (Fast, Full, etc)

withmorten · 2025-12-11T19:24:37Z

I think? I'm not well versed in the internal terms of premake :)

Currently you can do

linktimeoptimization "on"
linktimeoptimization "off"
linktimeoptimization "default"

I would add either

linktimeoptimization "full"

or

linktimeoptimization "fast"

and keep the other as "on".

The previous premake behaviour with

flags { "LinkTimeOptimization" }

was essentially "fast", with WholeProgramOptimization set to TRUE and LinkTimeCodeGeneration for the linker options unset - which Visual Studio defaults to as /LTCG:INCREMENTAL.

The behaviour now with "on" is LinkTimeCodeGeneration set to UseLinkTimeCodeGeneration, which would be "full".

So my question is - should "on" revert the behaviour back to the previous flags behaviour and add the new behaviour as "full", or should it add the old flags behaviour as "fast" and keep the new behaviour as "on" ?

nickclark2016 · 2025-12-11T19:35:05Z

I would argue that we should have:

linktimeoptimization "On" -> `/LTCG`
linktimeoptimization "Fast" -> `/LTCG:INCREMENTAL`

In GCC and Clang, "On" should stay mapped to -flto
On GCC, "Fast" should be -flto=auto
On Clang, "Fast" should be -flto=thin

samsinsane · 2025-12-12T11:36:17Z

should "on" revert the behaviour back to the previous flags behaviour

The API didn't introduce new behaviour for "on", it's exactly as it was with the flags. So I agree with @nickclark2016 adding "Fast" as an option makes the most sense.

withmorten · 2025-12-12T13:00:00Z

Ah I see. The version I have been using was so old, it didn't have this change yet:

490686c

This is what changed the default on behaviour to full from fast by adding the missing xml element.

Ok, I will add a new "fast" option then. I will rename and force push this PR, so don't close it yet :)

withmorten · 2025-12-13T13:54:57Z

I've force pushed and renamed the PR. I just searched for "linktimeoptimization" and hopefully fixed up all the required places, and added tests.

modules/gmakelegacy/tests/cpp/test_clang.lua

website/docs/linktimeoptimization.md

modules/gmakelegacy/tests/cpp/test_clang.lua

src/tools/gcc.lua

nickclark2016

See prior comments. Hit approve on accident.

nickclark2016

Minor changes, but overall happy with the code. Suggest merging in master, since there's a decent amount of changes that landed in the last 48 hours.

nickclark2016 · 2025-12-14T15:29:23Z

tests/tools/test_clang.lua

 		test.isequal("windres", clang.gettoolname(cfg, "rc"))
 	end

+	function suite.tools_onLinkTimeOptimizationViaFlag()


The LTO flag was actually removed. The "via flag" test is no longer required. (Same comment on Clang, GCC, and MSC).

Ah - I see. it would have been nice to keep that as deprecated option so I don't need to use different versions of premake for git bisecting old project files, but oh well :D

All flags are being deprecated and removed ahead of the 5.0 full release. We'll be rolling out a new release soon with them all deprecated in favor of a dedicated API for each.

nickclark2016 · 2025-12-14T15:32:53Z

website/docs/linktimeoptimization.md

-Premake 5.0-beta4 and later
+Premake 5.0-beta4 and later
+
+Fast is available from Premake 5.0-beta8 or later. Applies to Visual Studio 2010 and clang, will behave as On for other toolsets.


Given it's slightly more complex than just "Exists starting here", leave the "Premake 5.0-beta4 and later", then add a column to the table called Notes (see flags.md)

Done - or get rid of the "available from Premake 5.0-beta8 or later" ?

…emental for vs2010, ftlo=thin for clang and behaves as "On" for other toolsets add missing clang toolset tests for LinkTimeOptimization

withmorten · 2025-12-14T19:25:27Z

Minor changes, but overall happy with the code. Suggest merging in master, since there's a decent amount of changes that landed in the last 48 hours.

Done.

I think I just accidentally started a rewiew and deleted the pending comment, oh well. What a mess this interface is.

withmorten · 2025-12-15T18:41:10Z

Thanks!

samsinsane requested changes Dec 6, 2025

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withmorten force-pushed the fix-lto-default-msvc branch from b1eff90 to 6189c3d Compare December 13, 2025 13:51

withmorten changed the title ~~fix visual studio LinkTimeOptimization default not setting WholeProgramOptimization to true~~ add "Fast" option to LinkTimeOptimization API which enables LTCG:incremental for vs2010, and behaves as "On" for other toolsets Dec 13, 2025

withmorten requested a review from samsinsane December 13, 2025 13:55

Jarod42 reviewed Dec 13, 2025

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withmorten force-pushed the fix-lto-default-msvc branch from 6189c3d to 36b7608 Compare December 13, 2025 14:56

nickclark2016 approved these changes Dec 13, 2025

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nickclark2016 requested changes Dec 13, 2025

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withmorten force-pushed the fix-lto-default-msvc branch 2 times, most recently from 417dcac to 4fd78b8 Compare December 14, 2025 14:50

withmorten requested review from Jarod42 and nickclark2016 December 14, 2025 14:50

nickclark2016 requested changes Dec 14, 2025

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add "Fast" option to LinkTimeOptimization API which enables LTCG:incr…

df234e9

…emental for vs2010, ftlo=thin for clang and behaves as "On" for other toolsets add missing clang toolset tests for LinkTimeOptimization

withmorten force-pushed the fix-lto-default-msvc branch from 4fd78b8 to df234e9 Compare December 14, 2025 19:17

withmorten requested a review from nickclark2016 December 14, 2025 19:23

nickclark2016 approved these changes Dec 14, 2025

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samsinsane approved these changes Dec 15, 2025

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samsinsane merged commit 47cd240 into premake:master Dec 15, 2025
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