[llvm-dev] libLTO Codegen options issue (original) (raw)

Wael Yehia via llvm-dev llvm-dev at lists.llvm.org
Wed Mar 17 15:22:48 PDT 2021

Thanks Steven.

libLTO has nothing to do with lld. libLTO provides the c API to be used by other linkers which are not in llvm-project.

Our use case is we have a linker that we want to teach to do LTO on llvm files and basically be as close to a replacement (in terms of the user/options inteface) to lld and ld+LLVMGold as possible. So I'm mentioning lld's behaviour for reference and comparison, since both lld and the external linker are invoked by clang, and it wud be ideal that clang emits the same set of linker flags (when possible).

For function section, if not used a attribute, just add a new API to libLTO that set function section in the target machine. Yea, that's the obvious fix, but I'm not sure its scalable (if other flags are needed).

Would it be useful to have a function/class that takes in a list of plugin-opt options (input format TBD) and either updates or creates a new llvm::Config object. This function/class could be shared by gold-plugin and lld, and can be made available in the libLTO api for external linkers? We don't have it ready, just brainstorming..

Wael Yehia Compiler Development IBM Canada Lab wyehia at ca.ibm.com

-----Steven Wu <stevenwu at apple.com> wrote: ----- To: Wael Yehia <wyehia at ca.ibm.com> From: Steven Wu <stevenwu at apple.com> Date: 03/17/2021 05:46PM Cc: llvm-dev <llvm-dev at lists.llvm.org> Subject: [EXTERNAL] Re: libLTO Codegen options issue

On Mar 16, 2021, at 9:00 PM, Wael Yehia <wyehia at ca.ibm.com> wrote:

Hi Steven,

Do you have a specific option in TargetOption in mind or it is something only in your downstream? function-sections is one example. - It's a codegen flag that has no corresponding attribute in the IR (in https://urldefense.proofpoint.com/v2/url?u=https-3A_reviews.llvm.orgD24644&d=DwIFAg&c=jfiaSHvJObTbx-siA1ZOg&r=z6IPFss9EigepE3DNRkfA15nrHJhAw6dnvEXzGHvU&m=cUqAFFmPGgCYTQgZBLBUtmiJv58OpvaYYs0cBmEpUng&s=IDvWMTWhsxSw4PATJ4EDBLqkJj6I6uk2plcu8bhmr80&e= there was consideration to add an attribute). - Clang currently passes -plugin-opt=-function-sections to the linker with -flto and -ffunction-sections. - There's no libLTO API to control/set the flag. - the option is respected by LLVMgold.so (via the cl::opt options queried in codegen::InitTargetOptionsFromCodeGenFlags), and is ignore by lld.

libLTO has nothing to do with lld. libLTO provides the c API to be used by other linkers which are not in llvm-project.

1. what is the recommended approach to fix this in libLTO?

For function section, if not used a attribute, just add a new API to libLTO that set function section in the target machine.

Steven

2. The timing problem I described in my initial email is still present, and fixing it would aid in debugging libLTO codegen issues at the least. I'll update https://urldefense.proofpoint.com/v2/url?u=https-3A_reviews.llvm.orgD92611&d=DwIFAg&c=jfiaSHvJObTbx-siA1ZOg&r=z6IPFss9EigepE3DNRkfA15nrHJhAw6dnvEXzGHvU&m=cUqAFFmPGgCYTQgZBLBUtmiJv58OpvaYYs0cBmEpUng&s=0vnozJ-rzGAKKUfbCz5TIPOlIAZ5lXpX4p6f9cAdBY0&e= with a fix that adds a new API for "early" debug option processing.

Wael Yehia Compiler Development IBM Canada Lab wyehia at ca.ibm.com -----Steven Wu <stevenwu at apple.com> wrote: ----- To: Wael Yehia <wyehia at ca.ibm.com> From: Steven Wu <stevenwu at apple.com> Date: 12/04/2020 12:17PM Cc: llvm-dev <llvm-dev at lists.llvm.org>, joker.eph at gmail.com Subject: [EXTERNAL] Re: libLTO Codegen options issue

On Dec 4, 2020, at 6:17 AM, Wael Yehia <wyehia at ca.ibm.com> wrote: Hi Steven and Mehdi,

Module doesn't have TargetOption. All it carries is the target triple and all the feature flags in the metadata. I was talking about LTOModule, whose factory method (LTOModule::makeLTOModule) takes a TargetOptions object. It uses it to create a TargetMachine.

I would suggest you come with solid example about which cl::opt you are trying to overwrite and let's see if we can make an API out of it. please see my reasoning in my next paragraph. In general, I see any cl::opt as a debugging tool. Clients should use programmatic API and not rely on string-based CL opt to control what's happening behind the APIs. I see, makes sense. But what if the cl::opt, that is used for debugging only, is not respected when you're actually trying to use it during debugging? This is the issue here, the mechanism by which you can pass "debugging" options to libLTO has a flaw where as a result, some cl::opt objects are read too soon, before option parsing has occurred. To be specific, we were adding a cl::opt option to control the value of a field in a TargetOptions object when the object is created via InitTargetOptionsFromCodeGenFlags. This is nothing new, as there are already a bunch of options (for example -march, -mcpu, -relocation-model) in CommandFlags.cpp that have an effect on TargetOptions objects created via InitTargetOptionsFromCodeGenFlags (which libLTO currently calls) Surely, we don't wanna add a new function to the libLTO API for every option in CommandFlag.cpp? Do you have a specific option in TargetOption in mind or it is something only in your downstream? The idea is that modules need to be standalone and contain all the information needed to construct the target it is optimized for. Most of those target specific options cannot be overwrite during link time because the previous optimization pipeline might have assumed otherwise. The preferred way to set a target related option is to put that into a function attribute. E.g.: attributes #0 = { "target-features"="+foo" } You just need to teach code generator to correctly understand them. Steven Also please note, that we as the client of libLTO are an intermediary between the real user and LTO. And the real user when debugging his application will try passing options to the LTO component that he sees are listed with --help, and so options defined in CommandFlag.cpp won't behave as expected. Thanks everyone. Wael -----Steven Wu <stevenwu at apple.com> wrote: ----- To: Wael Yehia <wyehia at ca.ibm.com> From: Steven Wu <stevenwu at apple.com> Date: 12/03/2020 07:28PM Cc: llvm-dev at lists.llvm.org, joker.eph at gmail.com Subject: [EXTERNAL] Re: libLTO Codegen options issue On Dec 3, 2020, at 4:14 PM, Wael Yehia wrote:...

This Message Is From an External Sender This message came from outside your organization. On Dec 3, 2020, at 4:14 PM, Wael Yehia <wyehia at ca.ibm.com> wrote: Thanks Steven. A quick feedback will be that (1) is not an option. libLTO APIs need to be stable and existing APIs cannot be changed. Fair point. I am curious about your motivation for the change. If you have access to InitTargetOptionsFromCodeGenFlags, then you don't need libLTO interface at all since you are building again LLVM and you are free to call any underlying API you want. InitTargetOptionsFromCodeGenFlags is used in the implementation of libLTO, and the implementation (not the interface) has access to llvm. Does that clarify things? In libLTO, parseCodeGenDebugOptions is a debug function as name suggested. People should not rely on that in production. Instead, new APIs should be created for the underlying setting you need. The function comments indicate otherwise: $ git show d5268ebe1925:llvm/include/llvm/LTO/legacy/LTOCodeGenerator.h ... 120 /// Pass options to the driver and optimization passes. 121 /// 122 /// These options are not necessarily for debugging purpose (the function 123 /// name is misleading). This function should be called before 124 /// LTOCodeGenerator::compilexxx(), and 125 /// LTOCodeGenerator::writeMergedModules(). 126 void setCodeGenDebugOptions(ArrayRef Opts); 127 128 /// Parse the options set in setCodeGenDebugOptions. 129 /// 130 /// Like \a setCodeGenDebugOptions(), this must be called before 131 /// LTOCodeGenerator::compilexxx() and 132 /// LTOCodeGenerator::writeMergedModules(). 133 void parseCodeGenDebugOptions(); We don't officially support any option that has to pick up by cl::opt. I don't see reasons why you need to parse CodeGen flags before creating code generator. As I mention in my email: 1. the construction of llvm::TargetOptions relies on having parsed the options 2. the construction of LTOModule relies on the TargetOptions, 3. the LTOCodeGenerator does not rely on TargetOptions, but the factory method createCodeGen which is used in libLTO to construct the opaque type ltocodegent (which is just a pointer to a LTOCodeGenerator), creates a the LTOCodeGenerator and then sets the target options: $ git show d5268ebe1925:llvm/tools/lto/lto.cpp ... 363 static ltocodegent createCodeGen(bool InLocalContext) { 364 ltoinitialize(); 365 366 TargetOptions Options = codegen::InitTargetOptionsFromCodeGenFlags(Triple()); 367 368 LibLTOCodeGenerator *CodeGen = 369 InLocalContext ? new LibLTOCodeGenerator(std::makeunique()) 370 : new LibLTOCodeGenerator(); 371 CodeGen->setTargetOptions(Options); 372 return wrap(CodeGen); 373 } 374 375 ltocodegent ltocodegencreate(void) { 376 return createCodeGen(/* InLocalContext */ false); 377 } I could move the creation of TargetOptions and CodeGen->setTargetOptions(Options) into: 435 static void maybeParseOptions(ltocodegent cg) { 436 if (!parsedOptions) { 437 unwrap(cg)->parseCodeGenDebugOptions(); 438 ltoaddattrs(cg); 439 parsedOptions = true; 440 } 441 } but maybeParseOptions is only called from compile and optimize functions (i.e. after we've read in all the LTO modules), so LTOModules would have inaccurate TargetOptions (because command line options have not been parsed yet). Module doesn't have TargetOption. All it carries is the target triple and all the feature flags in the metadata. I would suggest you come with solid example about which cl::opt you are trying to overwrite and let's see if we can make an API out of it. Steven Wael -----Steven Wu <stevenwu at apple.com> wrote: ----- To: Wael Yehia <wyehia at ca.ibm.com> From: Steven Wu <stevenwu at apple.com> Date: 12/03/2020 06:11PM Cc: llvm-dev at lists.llvm.org, joker.eph at gmail.com Subject: [EXTERNAL] Re: libLTO Codegen options issue A quick feedback will be that (1) is not an option. libLTO APIs need to be stable and existing APIs cannot be changed. I am curious about your motivation for the change. If you have access to InitTargetOptionsFromCodeGenFlags, then you don't need libLTO interface at all since you are building again LLVM and you are free to call any underlying API you want. In libLTO, parseCodeGenDebugOptions is a debug function as name suggested. People should not rely on that in production. Instead, new APIs should be created for the underlying setting you need. I don't see reasons why you need to parse CodeGen flags before creating code generator. Steven On Dec 3, 2020, at 2:57 PM, Wael Yehia <wyehia at ca.ibm.com> wrote: Hi, We're trying to use libLTO, and noticed an issue (more of a timing problem) with option processing where TargetOptions is created before cl::ParseCommandLineOptions is invoked. Right now, the only place where ParseCommandLineOptions is called is in LTOCodeGenerator::parseCodeGenDebugOptions, which is only called from maybeParseOptions, which is called after TargetOptions have been created. We construct TargetOptions (by calling InitTargetOptionsFromCodeGenFlags) first and then use it to we create modules or the codegen object. Assuming this is in fact a problem, one way to fix this is to decouple parseCodeGenDebugOptions from LTOCodeGenerator, so that it can be called before we create the LTOCodeGenerator. Now we can either (1) modify the signature of the ltocodegendebugoptions and ltocodegendebugoptionsarray API functions or (2) add new API functions. I prefer (1) because as it is now, the API is broken. I uploaded a patch here INVALID URI REMOVED D92611&d=DwIFAg&c=jfiaSHvJObTbx-siA1ZOg&r=z6IPFss9EigepE3DNRkfA15n rHJhAw6dnvEXzGHvU&m=nxuUhuXwX2i7vXVLKxoBKtX2GQDqBVZvkN7o0m9ltE&s=Gx m0FzQUCAbsP-vv5nMMZCURqJje6Ft2Tt1PEmnnbY&e= Any feedback is appreciated. Thank you Wael

More information about the llvm-dev mailing list