Can we add a test for this?

Thanks for the review David. I've added the addresses to the test for now. Extending the verbose output for dwarfdump seems like a good idea, we will look into adding that in the future.

Check addresses as well, simplify triple.

Thanks for the review all. I've updated the test to use llvm-dwarfdump to check that the line table is as expected for the hot and cold parts.

Updates based on review comments.

Thanks for the quick review @saugustine , I'll wait for @tmsriram take a look.

Add UNSUPPORTED tag for windows and mac os.

lgtm, thanks for adding the test.

Rebase.

Thanks for the review.

Add comments, rebase.

Thanks for the review.

Update prefix to .text.split.

PTAL, thanks!

Spell out TTI as TargetTransformInfo, rebase.

Update comment and rebase.

This pass is only enabled on X86 platforms (D87047 for clang options and platform check). Longer term it does make sense to move it to TTI so I've added a FIXME to get this change off the critical path and I'll follow up with a refactoring change.

Add a FIXME to move defaults to TTI.

Updated tests to use .text.split. prefix.

lgtm with some minor comments.

Let me look into module flags a little bit and I'll come back to this patch once I have a better understanding. Thanks for the comments!

Thanks for the review.

Update comment, test.

Updated description and tests, PTAL thanks!

Update description, add test.

Update git commit message to specify option.

PTAL, thanks!

Add another check for the test.

Document flag, tighten test, rename var and option for clarity.

ping @tmsriram
Lets push this if it's good to go?

In D88041#2287900, @tejohnson wrote:

Is it expected that the user will want to specify this only at link time and not during compile time? If it is normally specified in the compile step, you should consider adding it to the IR in some fashion (e.g. function attributes). The advantage is that the user doesn't need to pass different options for the LTO and non-LTO cases.

Ideally we should only have to specify this once. However, using function attributes doesn't seem ideal since the pass will be scheduled and repeatedly invoked only to return without actually running the pass. It would be cleaner to marshal the codegen specific options from the compile invocation and restore them for the LTO step. There are a couple of other codegen options which would also benefit from this approach --lto-unique-basic-block-section-names, --lto-basic-block-sections=<value>. @mtrofin pointed out that -fembed-bitcode saves the invocation in .llvmcmd. A similar approach to stash codegen specific options always for LTO to pick up and enable might be less intrusive. However, this is a larger effort and for current LTO builds it would be nice to have a command line option to enable it. WDYT about this alternative?

ping @tmsriram @MaskRay

Rebase and update git commit message.

Thanks for the quick review!

Drop braces, add a test.

Carrying over the discussion from D87813 since it's more appropriate here:

Drop lld/ELF/Writer.cpp changes.

Thanks for the review.

Update the test.

Remove unnecessary includes, update doc text.

That makes sense. I moved the check to lib/Driver/ToolChains/Clang.cpp and updated the test. Seems cleaner to have all the checks in one place.
PTAL, thanks.

Check profile flag in Driver, update test.

Remove extra -c from test command line.

@MaskRay ping, let me know if you have any further comments. Thanks!

Rebased patch.

It feels wrong that the assembly+llvm-profdata test is in clang/test

I agree with @dblaikie and your assessment that it feels wrong to add such a test to clang. In the first version of this patch, the test served the purpose of representing the canonical usage with profdata as well as check for the plumbing of the profile along with the presence of the flag. Enhancing the driver test removed the need for the former while adding a diagnostic warning message (and a check for it) removes the need for the latter. I've removed the clang codegen test.

Remove clang/CodeGen test, update arg render logic.

Fix test formatting.

PTAL, thanks!

Use OptInFFlag, split-file and update tests.

Check warning, specify target to avoid failures on windows.

Updated test and warning type.

Thanks for explaining the rationale, PTAL.

Update test to use not tool and -c flag.

Thanks for the quick review @MaskRay, PTAL.

Update test based on review comments.