Implement legalize_signature for RISC-V.

Add an abi module with code that is probably useful to all ISAs when
implementing this function.

Add a unit() method to RegClassData which can be used to index the
register units in a class.

filetests/isa/riscv/abi.cton

@@ -0,0 +1,23 @@
+; Test the legalization of function signatures.
+test legalizer
+isa riscv
+
+function f(i32) {
+    sig0 = signature(i32) -> i32
+
+; check: sig0 = signature(i32 [%x10]) -> i32 [%x10]
+
+    sig1 = signature(i64) -> b1
+; check: sig1 = signature(i32 [%x10], i32 [%x11]) -> b1 [%x10]
+
+; The i64 argument must go in an even-odd register pair.
+    sig2 = signature(f32, i64) -> f64
+; check: sig2 = signature(f32 [%f10], i32 [%x12], i32 [%x13]) -> f64 [%f10]
+
+; Spilling into the stack args.
+    sig3 = signature(f64, f64, f64, f64, f64, f64, f64, i64) -> f64
+; check: sig3 = signature(f64 [%f10], f64 [%f11], f64 [%f12], f64 [%f13], f64 [%f14], f64 [%f15], f64 [%f16], i32 [0], i32 [4]) -> f64 [%f10]
+
+ebb0(v0: i32):
+    return_reg v0
+}

lib/cretonne/meta/gen_registers.py

@@ -37,6 +37,7 @@ def gen_regclass(rc, fmt):
         fmt.line('name: "{}",'.format(rc.name))
         fmt.line('index: {},'.format(rc.index))
         fmt.line('width: {},'.format(rc.width))
+        fmt.line('first: {},'.format(rc.bank.first_unit + rc.start))
         fmt.line('subclasses: 0x{:x},'.format(rc.subclass_mask()))
         mask = ', '.join('0x{:08x}'.format(x) for x in rc.mask())
         fmt.line('mask: [{}],'.format(mask))

lib/cretonne/src/abi.rs

@@ -0,0 +1,72 @@
+//! Common helper code for ABI lowering.
+//!
+//! This module provides functions and data structures that are useful for implementing the
+//! `TargetIsa::legalize_signature()` method.
+
+use ir::{ArgumentLoc, ArgumentType, Type};
+
+/// Legalization action to perform on a single argument or return value.
+///
+/// An argument may go through a sequence of legalization steps before it reaches the final
+/// `Assign` action.
+pub enum ArgAction {
+    /// Assign the argument to the given location.
+    Assign(ArgumentLoc),
+
+    /// Split the argument into smaller parts, then call again.
+    ///
+    /// This action can split an integer type into two smaller integer arguments, or it can split a
+    /// SIMD vector into halves.
+    ///
+    /// Floating point scalar types can't be split.
+    Split,
+}
+
+/// Common trait for assigning arguments to registers or stack locations.
+///
+/// This will be implemented by individual ISAs.
+pub trait ArgAssigner {
+    /// Pick an assignment action for function argument (or return value) `arg`.
+    fn assign(&mut self, arg: &ArgumentType) -> ArgAction;
+}
+
+/// Legalize the arguments in `args` using the given argument assigner.
+///
+/// This function can be used for both arguments and return values.
+pub fn legalize_args<AA: ArgAssigner>(args: &mut Vec<ArgumentType>, aa: &mut AA) {
+    // Iterate over the arguments.
+    // We may need to mutate the vector in place, so don't use a normal iterator, and clone the
+    // argument to avoid holding a reference.
+    let mut argno = 0;
+    while let Some(arg) = args.get(argno).cloned() {
+        // Leave the pre-assigned arguments alone.
+        // We'll assume that they don't interfere with our assignments.
+        if arg.location.is_assigned() {
+            argno += 1;
+            continue;
+        }
+
+        match aa.assign(&arg) {
+            // Assign argument to a location and move on to the next one.
+            ArgAction::Assign(loc) => {
+                args[argno].location = loc;
+                argno += 1;
+            }
+            // Split this argument into two smaller ones. Then revisit both.
+            ArgAction::Split => {
+                let new_arg = ArgumentType { value_type: split_type(arg.value_type), ..arg };
+                args[argno].value_type = new_arg.value_type;
+                args.insert(argno + 1, new_arg);
+            }
+        }
+    }
+}
+
+/// Given a value type that isn't legal, compute a replacement type half the size.
+fn split_type(ty: Type) -> Type {
+    if ty.is_int() {
+        ty.half_width().expect("Integer type too small to split")
+    } else {
+        ty.half_vector().expect("Can only split integers and vectors")
+    }
+}

lib/cretonne/src/isa/registers.rs

@@ -108,6 +108,9 @@ pub struct RegClassData {
     /// How many register units to allocate per register.
     pub width: u8,
 
+    /// The first register unit in this class.
+    pub first: RegUnit,
+
     /// Bit-mask of sub-classes of this register class, including itself.
     ///
     /// Bits correspond to RC indexes.
@@ -142,6 +145,12 @@ impl RegClassData {
     pub fn has_subclass<RCI: Into<RegClassIndex>>(&self, other: RCI) -> bool {
         self.subclasses & (1 << other.into().0) != 0
     }
+
+    /// Get a specific register unit in this class.
+    pub fn unit(&self, offset: usize) -> RegUnit {
+        let uoffset = offset * self.width as usize;
+        self.first + uoffset as RegUnit
+    }
 }
 
 /// A small reference to a register class.

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

@@ -2,11 +2,79 @@
 //!
 //! This module implements the RISC-V calling convention through the primary `legalize_signature()`
 //! entry point.
+//!
+//! This doesn't support the soft-float ABI at the moment.
 
-use ir::Signature;
+use abi::{ArgAction, ArgAssigner, legalize_args};
+use ir::{Signature, ArgumentType, ArgumentLoc};
+use isa::riscv::registers::{GPR, FPR};
 use settings as shared_settings;
 
-/// Legalize `sig` for RISC-V.
-pub fn legalize_signature(_sig: &mut Signature, _flags: &shared_settings::Flags) {
-    // TODO: Actually do something.
+struct Args {
+    pointer_bits: u16,
+    pointer_bytes: u32,
+    regs: u32,
+    offset: u32,
+}
+
+impl Args {
+    fn new(bits: u16) -> Args {
+        Args {
+            pointer_bits: bits,
+            pointer_bytes: bits as u32 / 8,
+            regs: 0,
+            offset: 0,
+        }
+    }
+}
+
+impl ArgAssigner for Args {
+    fn assign(&mut self, arg: &ArgumentType) -> ArgAction {
+        fn align(value: u32, to: u32) -> u32 {
+            (value + to - 1) & !(to - 1)
+        }
+
+        let ty = arg.value_type;
+
+        // Check for a legal type.
+        // RISC-V doesn't have SIMD at all, so break all vectors down.
+        if !ty.is_scalar() {
+            return ArgAction::Split;
+        }
+
+        // Large integers and booleans are broken down to fit in a register.
+        if !ty.is_float() && ty.bits() > self.pointer_bits {
+            // Align registers and stack to a multiple of two pointers.
+            self.regs = align(self.regs, 2);
+            self.offset = align(self.offset, 2 * self.pointer_bytes);
+            return ArgAction::Split;
+        }
+
+        if self.regs < 8 {
+            // Assign to a register.
+            let reg = if ty.is_float() {
+                FPR.unit(10 + self.regs as usize)
+            } else {
+                GPR.unit(10 + self.regs as usize)
+            };
+            self.regs += 1;
+            ArgAction::Assign(ArgumentLoc::Reg(reg))
+        } else {
+            // Assign a stack location.
+            let loc = ArgumentLoc::Stack(self.offset);
+            self.offset += self.pointer_bytes;
+            ArgAction::Assign(loc)
+        }
+    }
+}
+
+/// Legalize `sig` for RISC-V.
+pub fn legalize_signature(sig: &mut Signature, flags: &shared_settings::Flags) {
+    let bits = if flags.is_64bit() { 64 } else { 32 };
+
+    let mut args = Args::new(bits);
+    legalize_args(&mut sig.argument_types, &mut args);
+
+    let mut rets = Args::new(bits);
+    legalize_args(&mut sig.return_types, &mut rets);
 }

lib/cretonne/src/lib.rs

@@ -21,6 +21,7 @@ pub mod settings;
 pub mod sparse_map;
 pub mod verifier;
 
+mod abi;
 mod constant_hash;
 mod context;
 mod legalizer;

lib/cretonne/src/regalloc/allocatable_set.rs

@@ -121,6 +121,7 @@ mod tests {
         name: "GPR",
         index: 0,
         width: 1,
+        first: 28,
         subclasses: 0,
         mask: [0xf0000000, 0x0000000f, 0],
     };
@@ -128,6 +129,7 @@ mod tests {
         name: "DPR",
         index: 0,
         width: 2,
+        first: 28,
         subclasses: 0,
         mask: [0x50000000, 0x0000000a, 0],
     };