Rework riscv -march and -mabi detection

Instead of adding cases for all the operating systems, Use target
architecture directly as ISA string, replacing "riscv" with "rv", and
detect floating point ABI based on support for the D and F extensions.

Fixes #795

src/lib.rs

@@ -1958,25 +1958,37 @@ impl Build {
                     let mut parts = target.split('-');
                     if let Some(arch) = parts.next() {
                         let arch = &arch[5..];
-                        if target.contains("linux") && arch.starts_with("64") {
-                            cmd.args.push(("-march=rv64gc").into());
-                            cmd.args.push("-mabi=lp64d".into());
-                        } else if target.contains("freebsd") && arch.starts_with("64") {
-                            cmd.args.push(("-march=rv64gc").into());
-                            cmd.args.push("-mabi=lp64d".into());
-                        } else if target.contains("openbsd") && arch.starts_with("64") {
-                            cmd.args.push(("-march=rv64gc").into());
-                            cmd.args.push("-mabi=lp64d".into());
-                        } else if target.contains("linux") && arch.starts_with("32") {
-                            cmd.args.push(("-march=rv32gc").into());
-                            cmd.args.push("-mabi=ilp32d".into());
-                        } else if arch.starts_with("64") {
-                            cmd.args.push(("-march=rv".to_owned() + arch).into());
-                            cmd.args.push("-mabi=lp64".into());
+
+                        // Assume that "rv{arch}" is a valid RISC-V ISA string.
+                        // The compiler would error out otherwise, and we fix
+                        // that later.
+                        cmd.args.push(("-march=rv".to_owned() + arch).into());
+
+                        // Detect single-letter extensions from `arch`, assuming
+                        // no version numbers and canonical order
+                        let riscv_implements = |ext: &str| -> bool {
+                            let pattern = |c| ['_', 'z', 's'].contains(&c);
+                            let single_letter = arch.split(pattern).next().expect("Empty arch string?");
+                            single_letter.contains(ext)
+                        };
+
+                        let float_abi = if riscv_implements("g") || riscv_implements("d") {
+                            // Implements "d" (double-float), use double-float ABI
+                            "d"
+                        } else if riscv_implements("f") {
+                            // Implements "f" (single-float), use single-float ABI
+                            "f"
+                        } else {
+                            // No floating support, use soft-float ABI
+                            ""
+                        };
+
+                        if arch.starts_with("64") {
+                            cmd.args.push(("-mabi=lp64".to_owned() + float_abi).into());
                         } else {
-                            cmd.args.push(("-march=rv".to_owned() + arch).into());
-                            cmd.args.push("-mabi=ilp32".into());
+                            cmd.args.push(("-mabi=ilp32".to_owned() + float_abi).into());
                         }
+
                         cmd.args.push("-mcmodel=medany".into());
                     }
                 }