[clang] [llvm] [RISCV] Support __builtin_cpu_init and __builtin_cpu_supports (PR #99700)

[Craig Topper via llvm-commits]

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@@ -1020,3 +1020,43 @@ std::string RISCVISAInfo::getTargetFeatureForExtension(StringRef Ext) {
   return isExperimentalExtension(Name) ? "experimental-" + Name.str()
                                        : Name.str();
 }
+
+struct RISCVExtBit {
+  const StringRef ext;
+  uint8_t bitpos;
+};
+
+/// Maps extensions with assigned bit positions within group 0 of
+/// __riscv_features_bits to their respective bit position.  At the
+/// moment all extensions are within group 0.
+static RISCVExtBit RISCVGroup0BitPositions[] = {
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topperc wrote:

Make this `constexpr` to make sure the strlen for the strings actually happens at compile time.

https://github.com/llvm/llvm-project/pull/99700