Add legalization patterns for fabs and fneg.

These sign bit manipulations need to use a -0.0 floating point constant
which we didn't have a way of materializing previously.

Add a ieee32.bits(0x...) syntax to the Python AST nodes that creates am
f32 immediate value with the exact requested bitwise representation.

cranelift/filetests/wasm/f32-arith.cton

@@ -17,8 +17,18 @@ ebb0:
 
 ; Unary operations
 
-; function %f32_abs(f32) -> f32
+function %f32_abs(f32) -> f32 {
-; function %f32_neg(f32) -> f32
+ebb0(v0: f32):
+    v1 = fabs v0
+    return v1
+}
+
+function %f32_neg(f32) -> f32 {
+ebb0(v0: f32):
+    v1 = fneg v0
+    return v1
+}
+
 ; function %f32_sqrt(f32) -> f32
 ; function %f32_ceil(f32) -> f32
 ; function %f32_floor(f32) -> f32

cranelift/filetests/wasm/f64-arith.cton

@@ -14,8 +14,18 @@ ebb0:
 
 ; Unary operations
 
-; function %f64_abs(f64) -> f64
+function %f64_abs(f64) -> f64 {
-; function %f64_neg(f64) -> f64
+ebb0(v0: f64):
+    v1 = fabs v0
+    return v1
+}
+
+function %f64_neg(f64) -> f64 {
+ebb0(v0: f64):
+    v1 = fneg v0
+    return v1
+}
+
 ; function %f64_sqrt(f64) -> f64
 ; function %f64_ceil(f64) -> f64
 ; function %f64_floor(f64) -> f64

lib/cretonne/meta/base/legalize.py

@@ -7,8 +7,9 @@ patterns that describe how base instructions can be transformed to other base
 instructions that are legal.
 """
 from __future__ import absolute_import
-from .immediates import intcc
+from .immediates import intcc, ieee32, ieee64
 from . import instructions as insts
+from . import types
 from .instructions import iadd, iadd_cout, iadd_cin, iadd_carry, iadd_imm
 from .instructions import isub, isub_bin, isub_bout, isub_borrow
 from .instructions import imul, imul_imm
@@ -18,6 +19,7 @@ from .instructions import icmp, icmp_imm
 from .instructions import iconst, bint
 from .instructions import ishl, ishl_imm, sshr, sshr_imm, ushr, ushr_imm
 from .instructions import rotl, rotl_imm, rotr, rotr_imm
+from .instructions import f32const, f64const
 from cdsl.ast import Var
 from cdsl.xform import Rtl, XFormGroup
 
@@ -207,3 +209,20 @@ for inst_not,      inst in [
                 a1 << bnot(y),
                 a << inst(x, a1)
             ))
+
+# Floating-point sign manipulations.
+for ty,             minus_zero in [
+        (types.f32, f32const(ieee32.bits(0x80000000))),
+        (types.f64, f64const(ieee64.bits(0x8000000000000000)))]:
+    expand.legalize(
+            a << insts.fabs.bind(ty)(x),
+            Rtl(
+                b << minus_zero,
+                a << band_not(x, b),
+            ))
+    expand.legalize(
+            a << insts.fneg.bind(ty)(x),
+            Rtl(
+                b << minus_zero,
+                a << bxor(x, b),
+            ))

lib/cretonne/meta/cdsl/ast.py

@@ -502,6 +502,27 @@ class ConstantInt(Literal):
         return str(self.value)
 
 
+class ConstantBits(Literal):
+    """
+    A bitwise value of an immediate operand.
+
+    This is used to create bitwise exact floating point constants using
+    `ieee32.bits(0x80000000)`.
+    """
+
+    def __init__(self, kind, bits):
+        # type: (ImmediateKind, int) -> None
+        v = '{}::with_bits({:#x})'.format(kind.rust_type, bits)
+        super(ConstantBits, self).__init__(kind, v)
+
+    def __str__(self):
+        # type: () -> str
+        """
+        Get the Rust expression form of this constant.
+        """
+        return str(self.value)
+
+
 class Enumerator(Literal):
     """
     A value of an enumerated immediate operand.

lib/cretonne/meta/cdsl/operands.py

@@ -8,7 +8,7 @@ try:
     from typing import Union, Dict, TYPE_CHECKING, Iterable  # noqa
     OperandSpec = Union['OperandKind', ValueType, TypeVar]
     if TYPE_CHECKING:
-        from .ast import Enumerator, ConstantInt, Literal  # noqa
+        from .ast import Enumerator, ConstantInt, ConstantBits, Literal  # noqa
 except ImportError:
     pass
 
@@ -123,6 +123,15 @@ class ImmediateKind(OperandKind):
                     .format(self.name, value))
         return ConstantInt(self, value)
 
+    def bits(self, bits):
+        # type: (int) -> ConstantBits
+        """
+        Create an AST literal node for the given bitwise representation of this
+        immediate operand kind.
+        """
+        from .ast import ConstantBits  # noqa
+        return ConstantBits(self, bits)
+
     def rust_enumerator(self, value):
         # type: (str) -> str
         """