# Flang drivers ```eval_rst .. contents:: :local: ``` There are two main drivers in Flang: * the compiler driver, `flang-new` * the frontend driver, `flang-new -fc1` > **_NOTE:_** The diagrams in this document refer to `flang` as opposed to > `flang-new`. Eventually, `flang-new` will be renamed as `flang` and the > diagrams reflect the final design that we are still working towards. The **compiler driver** will allow you to control all compilation phases (e.g. preprocessing, semantic checks, code-generation, code-optimisation, lowering and linking). For frontend specific tasks, the compiler driver creates a Fortran compilation job and delegates it to `flang-new -fc1`, the frontend driver. For linking, it creates a linker job and calls an external linker (e.g. LLVM's [`lld`](https://lld.llvm.org/)). It can also call other tools such as external assemblers (e.g. [`as`](https://www.gnu.org/software/binutils/)). In Clang, the compiler driver can also link the generated binaries with LLVM's static analysis/sanitizer libraries (e.g. [MemorySanitizer](https://clang.llvm.org/docs/MemorySanitizer.html)). This is not yet available in Flang, but will be relatively easy to support once such libraries become available. Flang's compiler driver is intended for Flang's end-users - its interface needs to remain stable. Otherwise, Flang's users will have to adjust their build scripts every time a compiler flag is changed. |  | |:--:| | *Flang’s compiler driver and the **tools** that it runs* | The **frontend driver** glues together and drives all of the Flang's frontend libraries. As such, it provides an easy-to-use and intuitive interface to the frontend. It uses MLIR and LLVM for code-generation and can be viewed as a driver for Flang, LLVM and MLIR libraries. Contrary to the compiler driver, it is not capable of calling any external tools (including linkers). It is aware of all the frontend internals that are "hidden" from the compiler driver. It accepts many frontend-specific options not available in `flang-new` and as such it provides a finer control over the frontend. Note that this tool is mostly intended for Flang developers. In particular, there are no guarantees about the stability of its interface and compiler developers can use it to experiment with new flags. |  | |:-:| | *Flang's frontend driver and the **libraries** that it drives* | Note that similarly to `-Xclang` in `clang`, you can use `-Xflang` to forward a frontend specific flag from the _compiler_ directly to the _frontend_ driver, e.g.: ```lang=bash flang-new -Xflang -fdebug-dump-parse-tree input.f95 ``` In the invocation above, `-fdebug-dump-parse-tree` is forwarded to `flang-new -fc1`. Without the forwarding flag, `-Xflang`, you would see the following warning: ```lang=bash flang-new: warning: argument unused during compilation: ``` As `-fdebug-dump-parse-tree` is only supported by `flang-new -fc1`, `flang-new` will ignore it when used without `Xflang`. ## Why Do We Need Two Drivers? As hinted above, `flang-new` and `flang-new -fc1` are two separate tools. The fact that these tools are accessed through one binary, `flang-new`, is just an implementation detail. Each tool has a separate list of options, albeit defined in the same file: `clang/include/clang/Driver/Options.td`. The separation helps us split various tasks and allows us to implement more specialised tools. In particular, `flang-new` is not aware of various compilation phases within the frontend (e.g. scanning, parsing or semantic checks). It does not have to be. Conversely, the frontend driver, `flang-new -fc1`, needs not to be concerned with linkers or other external tools like assemblers. Nor does it need to know where to look for various systems libraries, which is usually OS and platform specific. One helpful way of differentiating these tools is to keep in mind that: * the compiler driver is an end-user tool * frontend driver is a compiler developer tool with many additional options, Also, Since the compiler driver can call external tools, e.g. linkers, it can be used to generate **executables**. The frontend driver cannot call external tools and hence can only generate **object files**. A similar model is implemented in Clang (`clang` vs `clang -cc1` vs `clang -cc1as`), which is based on the [architecture of GCC](https://en.wikibooks.org/wiki/GNU_C_Compiler_Internals/GNU_C_Compiler_Architecture). In fact, Flang needs to adhere to this model in order to be able to re-use Clang's driver library. If you are more familiar with the [architecture of GFortran](https://gcc.gnu.org/onlinedocs/gcc-4.7.4/gfortran/About-GNU-Fortran.html) than Clang, then `flang-new` corresponds to `gfortran` and `flang-new -fc1` to `f951`. ## Compiler Driver The main entry point for Flang's compiler driver is implemented in `flang/tools/flang-driver/driver.cpp`. Flang's compiler driver is implemented in terms of Clang's driver library, `clangDriver`. This approach allows us to: * benefit from Clang's support for various targets, platforms and operating systems * leverage Clang's ability to drive various backends available in LLVM, as well as linkers and assemblers. One implication of this dependency on Clang is that all of Flang's compiler options are defined alongside Clang's options in `clang/include/clang/Driver/Options.td`. For options that are common for both Flang and Clang, the corresponding definitions are shared. Internally, a `clangDriver` based compiler driver works by creating actions that correspond to various compilation phases, e.g. `PreprocessJobClass`, `CompileJobClass`, `BackendJobClass` or `LinkJobClass` from the `clang::driver::Action::ActionClass` enum. There are also other, more specialised actions, e.g. `MigrateJobClass` or `InputClass`, that do not map directly to common compilation steps. The actions to run are determined from the supplied compiler flags, e.g. * `-E` for `PreprocessJobClass`, * `-c` for `CompileJobClass`. In most cases, the driver creates a chain of actions/jobs/phases where the output from one action is the input for the subsequent one. You can use the `-ccc-print-phases` flag to see the sequence of actions that the driver will create for your compiler invocation: ```bash flang-new -ccc-print-phases -E file.f +- 0: input, "file.f", f95-cpp-input 1: preprocessor, {0}, f95 ``` As you can see, for `-E` the driver creates only two jobs and stops immediately after preprocessing. The first job simply prepares the input. For `-c`, the pipeline of the created jobs is more complex: ```bash flang-new -ccc-print-phases -c file.f +- 0: input, "file.f", f95-cpp-input +- 1: preprocessor, {0}, f95 +- 2: compiler, {1}, ir +- 3: backend, {2}, assembler 4: assembler, {3}, object ``` Note that currently Flang does not support code-generation and `flang-new` will fail during the second step above with the following error: ```bash error: code-generation is not available yet ``` The other phases are printed nonetheless when using `-ccc-print-phases`, as that reflects what `clangDriver`, the library, will try to create and run. For actions specific to the frontend (e.g. preprocessing or code generation), a command to call the frontend driver is generated (more specifically, an instance of `clang::driver::Command`). Every command is bound to an instance of `clang::driver::Tool`. For Flang we introduced a specialisation of this class: `clang::driver::Flang`. This class implements the logic to either translate or forward compiler options to the frontend driver, `flang-new -fc1`. You can read more on the design of `clangDriver` in Clang's [Driver Design & Internals](https://clang.llvm.org/docs/DriverInternals.html). ## Frontend Driver Flang's frontend driver is the main interface between compiler developers and the Flang frontend. The high-level design is similar to Clang's frontend driver, `clang -cc1` and consists of the following classes: * `CompilerInstance`, which is a helper class that encapsulates and manages various objects that are always required by the frontend (e.g. `AllSources`, `AllCookedSources, `Parsing`, `CompilerInvocation`, etc.). In most cases `CompilerInstance` owns these objects, but it also can share them with its clients when required. It also implements utility methods to construct and manipulate them. * `CompilerInvocation` encapsulates the configuration of the current invocation of the compiler as derived from the command-line options and the input files (in particular, file extensions). Among other things, it holds an instance of `FrontendOptions`. Like `CompilerInstance`, it owns the objects that it manages. It can share them with its clients that want to access them even after the corresponding `CompilerInvocation` has been destructed. * `FrontendOptions` holds options that control the behaviour of the frontend, as well as e.g. the list of the input files. These options come either directly from the users (through command-line flags) or are derived from e.g. the host system configuration. * `FrontendAction` and `FrontendActions` (the former being the base class for the latter) implement the actual actions to perform by the frontend. Usually there is one specialisation of `FrontendActions` for every compiler action flag (e.g. `-E`, `-fdebug-unparse`). These classes also contain various hooks that allow you to e.g. fine-tune the configuration of the frontend based on the input. This list is not exhaustive and only covers the main classes that implement the driver. The main entry point for the frontend driver, `fc1_main`, is implemented in `flang/tools/flang-driver/driver.cpp`. It can be accessed by invoking the compiler driver, `flang-new`, with the `-fc1` flag. The frontend driver will only run one action at a time. If you specify multiple action flags, only the last one will be taken into account. The default action is `ParseSyntaxOnlyAction`, which corresponds to `-fsyntax-only`. In other words, `flang-new -fc1 ` is equivalent to `flang-new -fc1 -fsyntax-only `. ## The `flang-to-external-fc` script The `flang-to-external-fc` wrapper script for `flang-new` was introduced as a development tool and to facilitate testing. The `flang-to-external-fc` wrapper script will: * use `flang-new` to unparse the input source file (i.e. it will run `flang-new -fc1 -fdebug-unparse `), and then * call a host Fortran compiler, e.g. `gfortran`, to compile the unparsed file. Here's a basic breakdown of what happens inside `flang-to-external-fc` when you run `flang-to-external-fc file.f90`: ```bash flang-new -fc1 -fdebug-unparse file.f90 -o file-unparsed.f90 gfortran file-unparsed.f90 ``` This is a simplified version for illustration purposes only. In practice, `flang-to-external-fc` adds a few more frontend options and it also supports various other use cases (e.g. compiling C files, linking existing object files). `gfortran` is the default host compiler used by `flang-to-external-fc`. You can change it by setting the `FLANG_FC` environment variable. ## Adding new Compiler Options Adding a new compiler option in Flang consists of two steps: * define the new option in a dedicated TableGen file, * parse and implement the option in the relevant drivers that support it. ### Option Definition All of Flang's compiler and frontend driver options are defined in `clang/include/clang/Driver/Options.td` in Clang. When adding a new option to Flang, you will either: * extend the existing definition for an option that is already available in one of Clang's drivers (e.g. `clang`), but not yet available in Flang, or * add a completely new definition if the option that you are adding has not been defined yet. There are many predefined TableGen classes and records that you can use to fine tune your new option. The list of available configurations can be overwhelming at times. Sometimes the easiest approach is to find an existing option that has similar semantics to your new option and start by copying that. For every new option, you will also have to define the visibility of the new option. This is controlled through the `Flags` field. You can use the following Flang specific option flags to control this: * `FlangOption` - this option will be available in the `flang-new` compiler driver, * `FC1Option` - this option will be available in the `flang-new -fc1` frontend driver, * `FlangOnlyOption` - this option will not be visible in Clang drivers. Please make sure that options that you add are only visible in drivers that can support it. For example, options that only make sense for Fortran input files (e.g. `-ffree-form`) should not be visible in Clang and be marked as `FlangOnlyOption`. When deciding what `OptionGroup` to use when defining a new option in the `Options.td` file, many new options fall into one of the following two categories: * `Action_Group` - options that define an action to run (e.g. `-fsyntax-only`, `-E`) * `f_Group` - target independent compiler flags (e.g. `-ffixed-form`, `-fopenmp`) There are also other groups and occasionally you will use them instead of the groups listed above. ### Option Implementation First, every option needs to be parsed. Flang compiler options are parsed in two different places, depending on which driver they belong to: * frontend driver: `flang/lib/Frontend/CompilerInvocation.cpp`, * compiler driver: `clang/lib/Driver/ToolChains/Flang.cpp`. The parsing will depend on the semantics encoded in the TableGen definition. When adding a compiler driver option (i.e. an option that contains `FlangOption` among its `Flags`) that you also intend to be understood by the frontend, make sure that it is either forwarded to `flang-new -fc1` or translated into some other option that is accepted by the frontend driver. In the case of options that contain both `FlangOption` and `FC1Option` among its flags, we usually just forward from `flang-new` to `flang-new -fc1`. This is then tested in `flang/test/Driver/frontend-forward.F90`. What follows is usually very dependant on the meaning of the corresponding option. In general, regular compiler flags (e.g. `-ffree-form`) are mapped to some state within the driver. A lot of this state is stored within an instance of `FrontendOptions`, but there are other more specialised classes too. Action flags (e.g. `-fsyntax-only`) are usually more complex overall, but also more structured in terms of the implementation. ### Action Options For options that correspond to an action (i.e. marked as `Action_Group`), you will have to define a dedicated instance of `FrontendActions` in `flang/include/flang/Frontend/FrontendOptions.h`. For example, for `-fsyntax-only` we defined: ```cpp class ParseSyntaxOnlyAction : public PrescanAndSemaAction { void ExecuteAction() override; }; ``` Command line options are mapped to frontend actions through the `Fortran::frontend::ActionKind` enum. For every new action option that you add, you will have to add a dedicated entry in that enum (e.g. `ParseSyntaxOnly` for `-fsyntax-only`) and a corresponding `case` in `ParseFrontendArgs` function in the `CompilerInvocation.cpp` file, e.g.: ```cpp case clang::driver::options::OPT_fsyntax_only: opts.programAction = ParseSyntaxOnly; break; ``` Note that this simply sets the program/frontend action within the frontend driver. You still have make sure that the corresponding frontend action class is instantiated when your new action option is used. The relevant `switch` statement is implemented in `Fortran::frontend::CreatedFrontendBaseAction` in the `ExecuteCompilerInvocation.cpp` file. Here's an example for `-fsyntax-only`: ```cpp case ParseSyntaxOnly: return std::make_unique(); ``` At this point you should be able to trigger that frontend action that you have just added using your new frontend option. # CMake Support As of [#7246](https://gitlab.kitware.com/cmake/cmake/-/merge_requests/7246) (and soon to be released CMake 3.24.0), `cmake` can detect `flang-new` as a supported Fortran compiler. You can configure your CMake projects to use `flang-new` as follows: ```bash cmake -DCMAKE_Fortran_FLAGS="-flang-experimental-exec" -DCMAKE_Fortran_COMPILER= ``` You should see the following in the output: ``` -- The Fortran compiler identification is LLVMFlang ``` where `` corresponds to the LLVM Flang version. Note that while generating executables remains experimental, you will need to inform CMake to use the `-flang-experimental-exec` flag when invoking `flang-new` as in the example above. # Testing In LIT, we define two variables that you can use to invoke Flang's drivers: * `%flang` is expanded as `flang-new` (i.e. the compiler driver) * `%flang_fc1` is expanded as `flang-new -fc1` (i.e. the frontend driver) For most regression tests for the frontend, you will want to use `%flang_fc1`. In some cases, the observable behaviour will be identical regardless of whether `%flang` or `%flang_fc1` is used. However, when you are using `%flang` instead of `%flang_fc1`, the compiler driver will add extra flags to the frontend driver invocation (i.e. `flang-new -fc1 -`). In some cases that might be exactly what you want to test. In fact, you can check these additional flags by using the `-###` compiler driver command line option. Lastly, you can use `! REQUIRES: ` for tests that will only work when `` is available. For example, you can use`! REQUIRES: shell` to mark a test as only available on Unix-like systems (i.e. systems that contain a Unix shell). In practice this means that the corresponding test is skipped on Windows. # Frontend Driver Plugins Plugins are an extension to the frontend driver that make it possible to run extra user defined frontend actions, in the form of a specialization of a `PluginParseTreeAction`. These actions are run during compilation, after semantic checks. Similarly to Clang, Flang leverages `LoadLibraryPermanently` from LLVM's `llvm::sys::DynamicLibrary` to load dynamic objects that implement plugins. The process for using plugins includes: * [Creating a plugin](#creating-a-plugin) * [Loading and running a plugin](#loading-and-running-a-plugin) Flang plugins are limited to `flang-new -fc1` and are currently only available / been tested on Linux. ## Creating a Plugin There are three parts required for plugins to work: 1. [`PluginParseTreeAction` subclass](#a-pluginparsetreeaction-subclass) 1. [Implementation of `ExecuteAction`](#implementation-of-executeaction) 1. [Plugin registration](#plugin-registration) There is an example plugin located in `flang/example/PrintFlangFunctionNames` that demonstrates these points by using the `ParseTree` API to print out function and subroutine names declared in the input file. ### A `PluginParseTreeAction` Subclass This subclass will wrap everything together and represent the `FrontendAction` corresponding to your plugin. It will need to inherit from `PluginParseTreeAction` (defined in `flang/include/flang/FrontendActions.h`), in order to have access to the parse tree post semantic checks, and also so that it can be registered, e.g. ```cpp class PrintFunctionNamesAction : public PluginParseTreeAction ``` ### Implementation of `ExecuteAction` Like in other frontend actions, the driver looks for an `ExecuteAction` function to run, so in order for your plugin to do something, you will need to implement the `ExecuteAction` method in your plugin class. This method will contain the implementation of what the plugin actually does, for example: ```cpp // Forward declaration struct ParseTreeVisitor; void ExecuteAction() override { ParseTreeVisitor visitor; Fortran::parser::Walk(getParsing().parseTree(), visitor); } ``` In the example plugin, the `ExecuteAction` method first creates an instance of `visitor` struct, before passing it together with the parse tree to the `Fortran::parser::Walk` function that will traverse the parse tree. The parse tree will normally be generated by the frontend driver and can be retrieved in your plugin through the `getParsing()` member method. Implementation and details of the `Walk` function can be found in `flang/include/flang/Parser/parse-tree-visitor.h`. You will have to define your own `visitor` struct. It should define different `Pre` and `Post` functions that take the type of a specific `ParseTree` node as an argument. When the `Walk` function is traversing the parse tree, these functions will be run before/after a node of that type is visited. Template functions for `Pre`/`Post` are defined so that when a node is visited that you have not defined a function for, it will still be able to continue. `Pre` returns a `bool` indicating whether to visit that node's children or not. For example: ```cpp struct ParseTreeVisitor { template bool Pre(const A&) { return true; } template void Post(const A&) {} void Post(const Fortran::parser::FunctionStmt &f) { llvm::outs() << std::get(f.t).ToString() << "\n" ; } } ``` The different types of nodes and also what each node structure contains are defined in `flang/include/flang/Parser/parse-tree.h`. In the example, there is a `Post` function, with a line that gets the `Name` element from a tuple `t` in the `FunctionStmt` struct and prints it. This function will be run after every `FunctionStmt` node is visited in the parse tree. ### Plugin Registration A plugin registry is used to store names and descriptions of a collection of plugins. The Flang plugin registry, defined in `flang/include/flang/Frontend/FrontendPluginRegistry.h`, is an alias of `llvm::Registry` of type `PluginParseTreeAction`. The plugin will need to be registered, which will add the Plugin to the registry and allow it to be used. The format is as follows, with `print-fns` being the plugin name that is used later to call the plugin and `Print Function names` being the description: ```cpp static FrontendPluginRegistry::Add X( "print-fns", "Print Function names"); ``` ## Loading and Running a Plugin In order to use plugins, there are 2 command line options made available to the frontend driver, `flang-new -fc1`: * [`-load `](#the--load-dsopath-option) for loading the dynamic shared object of the plugin * [`-plugin `](#the--plugin-name-option) for calling the registered plugin Invocation of the example plugin is done through: ```bash flang-new -fc1 -load flangPrintFunctionNames.so -plugin print-fns file.f90 ``` Both these options are parsed in `flang/lib/Frontend/CompilerInvocation.cpp` and fulfil their actions in `flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp` ### The `-load ` option This loads the plugin shared object library, with the path given at ``, using `LoadLibraryPermantly` from LLVM's `llvm::sys::DynamicLibrary`, which itself uses `dlopen`. During this stage, the plugin is registered with the registration line from the plugin, storing the name and description. ### The `-plugin ` option This sets `frontend::ActionKind programAction` in `FrontendOptions` to `PluginAction`, through which it searches the plugin registry for the plugin name from ``. If found, it returns the instantiated plugin, otherwise it reports an error diagnostic and returns `nullptr`. ## Enabling In-Tree Plugins For in-tree plugins, there is the CMake flag `FLANG_PLUGIN_SUPPORT`, enabled by default, that controls the exporting of executable symbols from `flang-new`, which plugins need access to. Additionally, there is the CMake flag `LLVM_BUILD_EXAMPLES`, turned off by default, that is used to control if the example programs are built. This includes plugins that are in the `flang/example` directory and added as a `sub_directory` to the `flang/examples/CMakeLists.txt`, for example, the `PrintFlangFunctionNames` plugin. It is also possible to develop plugins out-of-tree. ## Limitations Note that the traversal API presented here is under active development and might change in the future. We expect it to evolve as support for new language features are added. This document and the examples will be updated accordingly. The current `ParseTree` structure is not suitable for modifications. The copy constructors are not available and hence duplicating code might not be trivial. Please take this into consideration when designing your plugin. In particular, creating a transformation plugin will be noticeably harder than analysis plugins that just consume (rather than edit) `ParseTree`. Lastly, if `ParseTree` modifications are performed, then it might be necessary to re-analyze expressions and modify scope or symbols. You can check [Semantics.md](Semantics.md) for more details on how `ParseTree` is edited e.g. during the semantic checks. # LLVM Pass Plugins Pass plugins are dynamic shared objects that consist of one or more LLVM IR passes. The `-fpass-plugin` option enables these passes to be passed to the middle-end where they are added to the optimization pass pipeline and run after lowering to LLVM IR.The exact position of the pass in the pipeline will depend on how it has been registered with the `llvm::PassBuilder`. See the documentation for [`llvm::PassBuilder`](https://llvm.org/doxygen/classllvm_1_1PassBuilder.html) for details. The framework to enable pass plugins in `flang-new` uses the exact same machinery as that used by `clang` and thus has the same capabilities and limitations. In order to use a pass plugin, the pass(es) must be compiled into a dynamic shared object which is then loaded using the `-fpass-plugin` option. ``` flang-new -fpass-plugin=/path/to/plugin.so ``` This option is available in both the compiler driver and the frontend driver. Note that LLVM plugins are not officially supported on Windows. ## LLVM Pass Extensions Pass extensions are similar to plugins, except that they can also be linked statically. Setting `-DLLVM_${NAME}_LINK_INTO_TOOLS` to `ON` in the cmake command turns the project into a statically linked extension. An example would be Polly, e.g., using `-DLLVM_POLLY_LINK_INTO_TOOLS=ON` would link Polly passes into `flang-new` as built-in middle-end passes. See the [`WritingAnLLVMNewPMPass`](https://llvm.org/docs/WritingAnLLVMNewPMPass.html#id9) documentation for more details. ## Ofast and Fast Math `-Ofast` in Flang means `-O3 -ffast-math -fstack-arrays`. `-ffast-math` means the following: - `-fno-honor-infinities` - `-fno-honor-nans` - `-fassociative-math` - `-freciprocal-math` - `-fapprox-func` - `-fno-signed-zeros` - `-ffp-contract=fast` These correspond to LLVM IR Fast Math attributes: https://llvm.org/docs/LangRef.html#fast-math-flags When `-ffast-math` is specified, any linker steps generated by the compiler driver will also link to `crtfastmath.o`, which adds a static constructor that sets the FTZ/DAZ bits in MXCSR, affecting not only the current only the current compilation unit but all static and shared libraries included in the program. Setting these bits causes denormal floating point numbers to be flushed to zero. ### Comparison with GCC/GFortran GCC/GFortran translate `-Ofast` to `-O3 -ffast-math -fstack-arrays -fno-semantic-interposition`. `-fno-semantic-interposition` is not used because Clang does not enable this as part of `-Ofast` as the default behaviour is similar. GCC/GFortran has a wider definition of `-ffast-math`: also including `-fno-trapping-math`, `-fno-rounding-math`, and `-fsignaling-nans`; these aren't included in Flang because Flang currently has no support for strict floating point and so always acts as though these flags were specified. GCC/GFortran will also set flush-to-zero mode: linking `crtfastmath.o`, the same as Flang. The only GCC/GFortran warning option currently supported is `-Werror`. Passing any unsupported GCC/GFortran warning flags into Flang's compiler driver will result in warnings being emitted. ### Comparison with nvfortran nvfortran defines `-fast` as `-O2 -Munroll=c:1 -Mnoframe -Mlre -Mpre -Mvect=simd -Mcache_align -Mflushz -Mvect`. - `-O2 -Munroll=c:1 -Mlre -Mautoinline -Mpre -Mvect-simd` affect code optimization. `flang -O3` should enable all optimizations for execution time, similarly to `clang -O3`. The `-O3` pipeline has passes that perform transformations like inlining, vectorisation, unrolling, etc. Additionally, the GVN and LICM passes perform redundancy elimination like `Mpre` and `Mlre` - `-Mnoframe`: the equivalent flag would be `-fomit-frame-pointer`. This flag is not yet supported in Flang and so Flang follows GFortran in not including this in `-Ofast`. There is no plan to include this flag as part of `-Ofast`. - `-Mcache_align`: there is no equivalent flag in Flang or Clang. - `-Mflushz`: flush-to-zero mode - when `-ffast-math` is specified, Flang will link to `crtfastmath.o` to ensure denormal numbers are flushed to zero.