[clang] [flang] [llvm] [AArch64][Driver] Better handling of target feature dependencies (PR #78270)

[David Spickett via cfe-commits]

================
@@ -308,6 +312,104 @@ inline constexpr ExtensionInfo Extensions[] = {
 };
 // clang-format on
 
+struct ExtensionSet {
+  // Set of extensions which are currently enabled.
+  ExtensionBitset Enabled;
+  // Set of extensions which have been enabled or disabled at any point. Used
+  // to avoid cluttering the cc1 command-line with lots of unneeded features.
+  ExtensionBitset Touched;
+  // Base architecture version, which we need to know because some feature
+  // dependencies change depending on this.
+  const ArchInfo *BaseArch;
+
+  ExtensionSet() : Enabled(), Touched(), BaseArch(nullptr) {}
+
+  // Enable the given architecture extension, and any other extensions it
+  // depends on. Does not change the base architecture, or follow dependencies
+  // between features which are only related by required arcitecture versions.
+  void enable(ArchExtKind E);
+
+  // Disable the given architecture extension, and any other extensions which
+  // depend on it. Does not change the base architecture, or follow
+  // dependencies between features which are only related by required
+  // arcitecture versions.
+  void disable(ArchExtKind E);
+
+  // Add default extensions for the given CPU. Records the base architecture,
+  // to later resolve dependencies which depend on it.
+  void addCPUDefaults(const CpuInfo &CPU);
+
+  // Add default extensions for the given architecture version. Records the
+  // base architecture, to later resolve dependencies which depend on it.
+  void addArchDefaults(const ArchInfo &Arch);
+
+  // Add or remove a feature based on a modifier string. The string must be of
+  // the form "<name>" to enable a feature or "no<name>" to disable it. This
+  // will also enable or disable any features as required by the dependencies
+  // between them.
+  bool parseModifier(StringRef Modifier);
+
+  // Convert the set of enabled extension to an LLVM feature list, appending
+  // them to Features.
+  void toLLVMFeatureList(std::vector<StringRef> &Features) const;
+};
+
+// Represents a dependency between two architecture extensions. If Later is
+// enabled, then Earlier must also be enabled. If Earlier is disabled, then
+// Later must also be disabled.
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DavidSpickett wrote:

You should clarify what kind of later this is. "Where feature Earlier was added to the architecture before Later".

As opposed to later on the command line, added later in the compiler, etc.

https://github.com/llvm/llvm-project/pull/78270