Add an enable_e setting for the RV32E instruction set.

This limited RISC-V version only has registers %x0 - %x15.

Make sure the ABI lowering code doesn't use the banned registers for
arguments.

filetests/isa/riscv/abi-e.cton

@@ -0,0 +1,14 @@
+; Test the legalization of function signatures for RV32E.
+test legalizer
+isa riscv enable_e
+
+; regex: V=v\d+
+
+function f() {
+    ; Spilling into the stack args after %x15 since %16 and up are not
+    ; available in RV32E.
+    sig0 = signature(i64, i64, i64, i64) -> i64
+    ; check: sig0 = signature(i32 [%x10], i32 [%x11], i32 [%x12], i32 [%x13], i32 [%x14], i32 [%x15], i32 [0], i32 [4]) -> i32 [%x10], i32 [%x11]
+ebb0:
+    return
+}

lib/cretonne/meta/isa/riscv/settings.py

@@ -18,6 +18,9 @@ enable_m = BoolSetting(
         "Enable the use of 'M' instructions if available",
         default=True)
 
+enable_e = BoolSetting(
+        "Enable the 'RV32E' instruction set with only 16 registers")
+
 use_m = And(supports_m, enable_m)
 use_a = And(supports_a, shared.enable_atomics)
 use_f = And(supports_f, shared.enable_float)

lib/cretonne/src/isa/riscv/abi.rs

@@ -8,24 +8,27 @@
 use abi::{ArgAction, ValueConversion, ArgAssigner, legalize_args};
 use ir::{Signature, Type, ArgumentType, ArgumentLoc, ArgumentExtension, ArgumentPurpose};
 use isa::RegClass;
-use isa::riscv::registers::{GPR, FPR};
 use settings as shared_settings;
+use super::registers::{GPR, FPR};
+use super::settings;
 
 struct Args {
     pointer_bits: u16,
     pointer_bytes: u32,
     pointer_type: Type,
     regs: u32,
+    reg_limit: u32,
     offset: u32,
 }
 
 impl Args {
-    fn new(bits: u16) -> Args {
+    fn new(bits: u16, enable_e: bool) -> Args {
         Args {
             pointer_bits: bits,
             pointer_bytes: bits as u32 / 8,
             pointer_type: Type::int(bits).unwrap(),
             regs: 0,
+            reg_limit: if enable_e { 6 } else { 8 },
             offset: 0,
         }
     }
@@ -62,7 +65,7 @@ impl ArgAssigner for Args {
             }
         }
 
-        if self.regs < 8 {
+        if self.regs < self.reg_limit {
             // Assign to a register.
             let reg = if ty.is_float() {
                 FPR.unit(10 + self.regs as usize)
@@ -81,13 +84,16 @@ impl ArgAssigner for Args {
 }
 
 /// Legalize `sig` for RISC-V.
-pub fn legalize_signature(sig: &mut Signature, flags: &shared_settings::Flags, current: bool) {
+pub fn legalize_signature(sig: &mut Signature,
+                          flags: &shared_settings::Flags,
+                          isa_flags: &settings::Flags,
+                          current: bool) {
     let bits = if flags.is_64bit() { 64 } else { 32 };
 
-    let mut args = Args::new(bits);
+    let mut args = Args::new(bits, isa_flags.enable_e());
     legalize_args(&mut sig.argument_types, &mut args);
 
-    let mut rets = Args::new(bits);
+    let mut rets = Args::new(bits, isa_flags.enable_e());
     legalize_args(&mut sig.return_types, &mut rets);
 
     if current {

lib/cretonne/src/isa/riscv/mod.rs

@@ -79,8 +79,7 @@ impl TargetIsa for Isa {
     }
 
     fn legalize_signature(&self, sig: &mut Signature, current: bool) {
-        // We can pass in `self.isa_flags` too, if we need it.
-        abi::legalize_signature(sig, &self.shared_flags, current)
+        abi::legalize_signature(sig, &self.shared_flags, &self.isa_flags, current)
     }
 
     fn regclass_for_abi_type(&self, ty: Type) -> RegClass {

lib/cretonne/src/isa/riscv/settings.rs

@@ -24,7 +24,8 @@ mod tests {
                     supports_a = false\n\
                     supports_f = false\n\
                     supports_d = false\n\
-                    enable_m = true\n");
+                    enable_m = true\n\
+                    enable_e = false\n");
         // Predicates are not part of the Display output.
         assert_eq!(f.full_float(), false);
     }