Peng Yu via llvm-dev
2019-Jan-19 06:42 UTC
[llvm-dev] With -flto, is manual inline unnecessary?
Hi, With LTO, it seems that compilers should have the information to decide whether to inline a function or not. In this case, is it unnecessary to specify functions as inline in C/C++ code? -- Regards, Peng
Craig Topper via llvm-dev
2019-Jan-19 06:51 UTC
[llvm-dev] With -flto, is manual inline unnecessary?
The inline keyword is a hint. The inliner calculates the cost of inlining a function using various heuristics. If the cost is too high it won't be inlined. The inline keyword increases the threshold that is considered too costly. ~Craig On Fri, Jan 18, 2019 at 10:42 PM Peng Yu via llvm-dev < llvm-dev at lists.llvm.org> wrote:> Hi, > > With LTO, it seems that compilers should have the information to > decide whether to inline a function or not. In this case, is it > unnecessary to specify functions as inline in C/C++ code? > > -- > Regards, > Peng > _______________________________________________ > 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/20190118/25a72080/attachment.html>
Teresa Johnson via llvm-dev
2019-Jan-19 15:32 UTC
[llvm-dev] With -flto, is manual inline unnecessary?
To follow on to Craig's note about it being a hint, note that the inline keyword is largely there to adjust the linkage type of the symbol so that it can be defined in a header without getting multiply defined symbol errors, and to allow it to be elided by the compiler when it isn't used in a translation unit that included that header. Without LTO, defining an inline function in a header can aid in inlining simply by making the definition available to all callers. But there is a cost (compile time, memory overhead) for defining a function in the header and therefore in all translation units that include it. With LTO (or ThinLTO), you can move these definitions to a source file (in which case the inline keyword *must* be removed), and it will still be available to inline as needed. LTO does this by merging all IR, which of course comes with a potentially big cost to memory and loss of incremental compiles. With ThinLTO (-flto=thin) the IR will be temporarily imported into other modules where it is thought to be profitable to inline. To really force inlining, you would instead need to use the attribute "__attribute__((always_inline))". However, with or without LTO, it is typically better to let the compiler use its heuristics to determine whether inlining is profitable. Teresa On Fri, Jan 18, 2019 at 10:52 PM Craig Topper via llvm-dev < llvm-dev at lists.llvm.org> wrote:> The inline keyword is a hint. The inliner calculates the cost of inlining > a function using various heuristics. If the cost is too high it won't be > inlined. The inline keyword increases the threshold that is considered too > costly. > > ~Craig > > > On Fri, Jan 18, 2019 at 10:42 PM Peng Yu via llvm-dev < > llvm-dev at lists.llvm.org> wrote: > >> Hi, >> >> With LTO, it seems that compilers should have the information to >> decide whether to inline a function or not. In this case, is it >> unnecessary to specify functions as inline in C/C++ code? >> >> -- >> Regards, >> Peng >> _______________________________________________ >> 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 >-- Teresa Johnson | Software Engineer | tejohnson at google.com | -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190119/fc58a39a/attachment.html>