Dmitry Polukhin via llvm-dev
2015-Dec-18 18:44 UTC
[llvm-dev] RFC: __attribute__((ifunc("resolver")) representation in LLVM IR
Hi Everyone, I would like to implement GCC ifunc attribute support in Clang and LLVM. At the moment Clang ignores the attribute with a warning. On LLVM IR level there is no support for ifunc. But there is some support for ELF symbol type `@gnu_indirect_function` in ELF reader/writer/target asm parser. This RFC is looking for thoughts and suggestions about representation of ifunc in LLVM IR. Alternatives for ifunc representation: 1. Add a boolean flag isIFunc to`GlobalAlias`>From implementation perspective ifunc is very close to a function aliasthat points to resolver function as a target. During asm emissions `@gnu_indirect_function` symbol attribute is added. In printed LLVM it could look like this: `@foo = alias ifunc i32 (i32), bitcast (i64 ()* @foo_ifunc to i32 (i32)*)` Pros: - minimal changes LLVM code base Cons: - ifunc is not really an alias, if some code passes through alias and starts using resolver function instead of ifunc, it is always an error - in particular in my prototype I had to add weak linkage to inhibit optimizations (it prevents them because LLVM assumes that linker could change the symbol but it could be fixed without changing linkage) 2. Extract common parts for alias and ifunc into a base class Both alias and ifunc will be derived classes. Similar to first proposal add `@gnu_indirect_function` symbol attribute. In printed LLVM it could look like: `@foo = ifunc i32 (i32), i64 ()* @foo_ifunc` Pros: - no confusion for existing code - cleaner from design perspective Cons: - Add new type of Global i.e. more textual changes - Some potential code duplication (need prototyping to estimate) 3. Emit global asm statement from Clang Generate global asm statement like `__asm__ (".type resolver_alias_name, @gnu_indirect_function")` in Clang for alias generated for resolver function. Pros: - (almost?) no changes in LLVM Cons: - ifunc is not marked in LLVM IR at all so some hacks are required to detect and inhibit optimizations in LLVM for such alias (it is always wrong to use resolver instead of ifunc they even have different function type) - asm statement in general not very reliable mechanism in general, highly depends on expected asm generated by compiler - using low-level platform dependent information in Clang I prototyped first approach, changes in http://reviews.llvm.org/D15525. But got feedback that I should use second approach instead + request to write RFC about this IR extension for broader discussion. I'm convinced that the second approach is cleaner and if there is no better suggestion or push back, I'm going to prepare such patch. Third approach mostly added for completeness. References: - GCC ifunc documentation https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html - IFUNC low-level technical details http://www.airs.com/blog/archives/403 Thanks, Dmitry Polukhin -- Software Engineer Intel Compiler Team -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20151218/b5c390b9/attachment.html>
Rafael EspĂndola via llvm-dev
2015-Dec-18 19:44 UTC
[llvm-dev] RFC: __attribute__((ifunc("resolver")) representation in LLVM IR
I would probably go with 2. 3 is particularly undesirable as it make the ifunc really hard to differentiate from an alias. Cheers, Rafael On 18 December 2015 at 13:44, Dmitry Polukhin via llvm-dev <llvm-dev at lists.llvm.org> wrote:> Hi Everyone, > > I would like to implement GCC ifunc attribute support in Clang and LLVM. At > the moment Clang ignores the attribute with a warning. On LLVM IR level > there is no support for ifunc. But there is some support for ELF symbol type > `@gnu_indirect_function` in ELF reader/writer/target asm parser. This RFC is > looking for thoughts and suggestions about representation of ifunc in LLVM > IR. > > Alternatives for ifunc representation: > > 1. Add a boolean flag isIFunc to`GlobalAlias` > From implementation perspective ifunc is very close to a function alias that > points to resolver function as a target. During asm emissions > `@gnu_indirect_function` symbol attribute is added. In printed LLVM it could > look like this: > > `@foo = alias ifunc i32 (i32), bitcast (i64 ()* @foo_ifunc to i32 (i32)*)` > > Pros: > - minimal changes LLVM code base > > Cons: > - ifunc is not really an alias, if some code passes through alias and starts > using resolver function instead of ifunc, it is always an error > - in particular in my prototype I had to add weak linkage to inhibit > optimizations (it prevents them because LLVM assumes that linker could > change the symbol but it could be fixed without changing linkage) > > 2. Extract common parts for alias and ifunc into a base class > Both alias and ifunc will be derived classes. Similar to first proposal add > `@gnu_indirect_function` symbol attribute. In printed LLVM it could look > like: > > `@foo = ifunc i32 (i32), i64 ()* @foo_ifunc` > > Pros: > - no confusion for existing code > - cleaner from design perspective > > Cons: > - Add new type of Global i.e. more textual changes > - Some potential code duplication (need prototyping to estimate) > > 3. Emit global asm statement from Clang > Generate global asm statement like `__asm__ (".type resolver_alias_name, > @gnu_indirect_function")` in Clang for alias generated for resolver > function. > > Pros: > - (almost?) no changes in LLVM > > Cons: > - ifunc is not marked in LLVM IR at all so some hacks are required to detect > and inhibit optimizations in LLVM for such alias (it is always wrong to use > resolver instead of ifunc they even have different function type) > - asm statement in general not very reliable mechanism in general, highly > depends on expected asm generated by compiler > - using low-level platform dependent information in Clang > > I prototyped first approach, changes in http://reviews.llvm.org/D15525. But > got feedback that I should use second approach instead + request to write > RFC about this IR extension for broader discussion. I'm convinced that the > second approach is cleaner and if there is no better suggestion or push > back, I'm going to prepare such patch. Third approach mostly added for > completeness. > > References: > - GCC ifunc documentation > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html > - IFUNC low-level technical details http://www.airs.com/blog/archives/403 > > Thanks, > Dmitry Polukhin > -- > Software Engineer > Intel Compiler Team > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >
Reid Kleckner via llvm-dev
2015-Dec-19 01:47 UTC
[llvm-dev] RFC: __attribute__((ifunc("resolver")) representation in LLVM IR
Are you going to have to teach LLVM how to look through ifuncs, or is it OK if they are totally opaque? For example, with __attribute__((target)), you might have one target-specialized function call another. Is it important that we be able to optimize away the dynamic dispatch there or not? Either way, I think a new IR construct is the way to go. On Fri, Dec 18, 2015 at 11:44 AM, Rafael EspĂndola <llvm-dev at lists.llvm.org> wrote:> I would probably go with 2. > > 3 is particularly undesirable as it make the ifunc really hard to > differentiate from an alias. > > Cheers, > Rafael > > > On 18 December 2015 at 13:44, Dmitry Polukhin via llvm-dev > <llvm-dev at lists.llvm.org> wrote: > > Hi Everyone, > > > > I would like to implement GCC ifunc attribute support in Clang and LLVM. > At > > the moment Clang ignores the attribute with a warning. On LLVM IR level > > there is no support for ifunc. But there is some support for ELF symbol > type > > `@gnu_indirect_function` in ELF reader/writer/target asm parser. This > RFC is > > looking for thoughts and suggestions about representation of ifunc in > LLVM > > IR. > > > > Alternatives for ifunc representation: > > > > 1. Add a boolean flag isIFunc to`GlobalAlias` > > From implementation perspective ifunc is very close to a function alias > that > > points to resolver function as a target. During asm emissions > > `@gnu_indirect_function` symbol attribute is added. In printed LLVM it > could > > look like this: > > > > `@foo = alias ifunc i32 (i32), bitcast (i64 ()* @foo_ifunc to i32 > (i32)*)` > > > > Pros: > > - minimal changes LLVM code base > > > > Cons: > > - ifunc is not really an alias, if some code passes through alias and > starts > > using resolver function instead of ifunc, it is always an error > > - in particular in my prototype I had to add weak linkage to inhibit > > optimizations (it prevents them because LLVM assumes that linker could > > change the symbol but it could be fixed without changing linkage) > > > > 2. Extract common parts for alias and ifunc into a base class > > Both alias and ifunc will be derived classes. Similar to first proposal > add > > `@gnu_indirect_function` symbol attribute. In printed LLVM it could look > > like: > > > > `@foo = ifunc i32 (i32), i64 ()* @foo_ifunc` > > > > Pros: > > - no confusion for existing code > > - cleaner from design perspective > > > > Cons: > > - Add new type of Global i.e. more textual changes > > - Some potential code duplication (need prototyping to estimate) > > > > 3. Emit global asm statement from Clang > > Generate global asm statement like `__asm__ (".type resolver_alias_name, > > @gnu_indirect_function")` in Clang for alias generated for resolver > > function. > > > > Pros: > > - (almost?) no changes in LLVM > > > > Cons: > > - ifunc is not marked in LLVM IR at all so some hacks are required to > detect > > and inhibit optimizations in LLVM for such alias (it is always wrong to > use > > resolver instead of ifunc they even have different function type) > > - asm statement in general not very reliable mechanism in general, highly > > depends on expected asm generated by compiler > > - using low-level platform dependent information in Clang > > > > I prototyped first approach, changes in http://reviews.llvm.org/D15525. > But > > got feedback that I should use second approach instead + request to write > > RFC about this IR extension for broader discussion. I'm convinced that > the > > second approach is cleaner and if there is no better suggestion or push > > back, I'm going to prepare such patch. Third approach mostly added for > > completeness. > > > > References: > > - GCC ifunc documentation > > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html > > - IFUNC low-level technical details > http://www.airs.com/blog/archives/403 > > > > Thanks, > > Dmitry Polukhin > > -- > > Software Engineer > > Intel Compiler Team > > > > > > _______________________________________________ > > LLVM Developers mailing list > > llvm-dev at lists.llvm.org > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20151218/0dc02fb7/attachment.html>
Seemingly Similar Threads
- RFC: __attribute__((ifunc("resolver")) representation in LLVM IR
- RFC: __attribute__((ifunc("resolver")) representation in LLVM IR
- RFC: __attribute__((ifunc("resolver")) representation in LLVM IR
- [IR] Modelling of GlobalIFunc
- resolve the name of an ifunc resolver function