Implement ddx, ddy, fwidth (#256)

Author: khyperia

crates/spirv-std/src/lib.rs

@@ -1,5 +1,6 @@
 #![no_std]
 #![feature(register_attr, repr_simd, core_intrinsics)]
+#![cfg_attr(target_arch = "spirv", feature(asm))]
 #![register_attr(spirv)]
 // Our standard Clippy lints that we use in Embark projects, we opt out of a few that are not appropriate for the specific crate (yet)
 #![warn(
@@ -97,6 +98,68 @@ pointer_addrspace!(incoming_ray_payload_khr, IncomingRayPayloadKHR, true);
 pointer_addrspace!(shader_record_buffer_khr, ShaderRecordBufferKHR, true);
 pointer_addrspace!(physical_storage_buffer, PhysicalStorageBuffer, true);
 
+pub trait Derivative {
+    fn ddx(self) -> Self;
+    fn ddx_fine(self) -> Self;
+    fn ddx_coarse(self) -> Self;
+    fn ddy(self) -> Self;
+    fn ddy_fine(self) -> Self;
+    fn ddy_coarse(self) -> Self;
+    fn fwidth(self) -> Self;
+    fn fwidth_fine(self) -> Self;
+    fn fwidth_coarse(self) -> Self;
+}
+
+#[cfg(target_arch = "spirv")]
+macro_rules! deriv_caps {
+    (true) => {
+        asm!("OpCapability DerivativeControl")
+    };
+    (false) => {};
+}
+
+macro_rules! deriv_fn {
+    ($name:ident, $inst:ident, $needs_caps:tt) => {
+        fn $name(self) -> Self {
+            #[cfg(not(target_arch = "spirv"))]
+            panic!(concat!(stringify!($name), " is not supported on the CPU"));
+            #[cfg(target_arch = "spirv")]
+            unsafe {
+                let o;
+                deriv_caps!($needs_caps);
+                asm!(
+                    concat!("{1} = ", stringify!($inst), " typeof{0} {0}"),
+                    in(reg) self,
+                    out(reg) o,
+                );
+                o
+            }
+        }
+    };
+}
+macro_rules! deriv_impl {
+    ($ty:ty) => {
+        impl Derivative for $ty {
+            deriv_fn!(ddx, OpDPdx, false);
+            deriv_fn!(ddx_fine, OpDPdxFine, true);
+            deriv_fn!(ddx_coarse, OpDPdxCoarse, true);
+            deriv_fn!(ddy, OpDPdy, false);
+            deriv_fn!(ddy_fine, OpDPdyFine, true);
+            deriv_fn!(ddy_coarse, OpDPdyCoarse, true);
+            deriv_fn!(fwidth, OpFwidth, false);
+            deriv_fn!(fwidth_fine, OpFwidthFine, true);
+            deriv_fn!(fwidth_coarse, OpFwidthCoarse, true);
+        }
+    };
+}
+
+// "must be a scalar or vector of floating-point type. The component width must be 32 bits."
+deriv_impl!(f32);
+// TODO: Fix rustc to support these
+// deriv_impl!(glam::Vec2);
+// deriv_impl!(glam::Vec3);
+// deriv_impl!(glam::Vec4);
+
 /// libcore requires a few external symbols to be defined:
 /// <https://github.com/rust-lang/rust/blob/c2bc344eb23d8c1d18e803b3f1e631cf99926fbb/library/core/src/lib.rs#L23-L27>
 /// TODO: This is copied from `compiler_builtins/mem.rs`. Can we use that one instead? The note in the above link says