Add interfaces and tests based on function domains

Create a type representing a function's domain and a test that does a
logarithmic sweep of points within the domain.

library/compiler-builtins/libm/crates/libm-test/src/gen/domain_logspace.rs

@@ -0,0 +1,43 @@
+//! A generator that produces logarithmically spaced values within domain bounds.
+
+use libm::support::{IntTy, MinInt};
+
+use crate::domain::HasDomain;
+use crate::op::OpITy;
+use crate::{MathOp, logspace};
+
+/// Number of tests to run.
+// FIXME(ntests): replace this with a more logical algorithm
+const NTESTS: usize = {
+    if cfg!(optimizations_enabled) {
+        if crate::emulated()
+            || !cfg!(target_pointer_width = "64")
+            || cfg!(all(target_arch = "x86_64", target_vendor = "apple"))
+        {
+            // Tests are pretty slow on non-64-bit targets, x86 MacOS, and targets that run
+            // in QEMU.
+            100_000
+        } else {
+            5_000_000
+        }
+    } else {
+        // Without optimizations just run a quick check
+        800
+    }
+};
+
+/// Create a range of logarithmically spaced inputs within a function's domain.
+///
+/// This allows us to get reasonably thorough coverage without wasting time on values that are
+/// NaN or out of range. Random tests will still cover values that are excluded here.
+pub fn get_test_cases<Op>() -> impl Iterator<Item = (Op::FTy,)>
+where
+    Op: MathOp + HasDomain<Op::FTy>,
+    IntTy<Op::FTy>: TryFrom<usize>,
+{
+    let domain = Op::DOMAIN;
+    let start = domain.range_start();
+    let end = domain.range_end();
+    let steps = OpITy::<Op>::try_from(NTESTS).unwrap_or(OpITy::<Op>::MAX);
+    logspace(start, end, steps).map(|v| (v,))
+}