Add SampledImage::sample_by_lod (#755)

* Add 'SampledImage::sample_by_lod'

* Add a compiletest

* Format the compiletest, but manually because running cargo fmt doesn't seem to do anything 🤷

* Run rustfmt

crates/spirv-std/src/image.rs

@@ -951,7 +951,7 @@ impl<
         Image<SampledType, DIM, DEPTH, ARRAYED, { Multisampled::False as u32 }, SAMPLED, FORMAT>,
     >
 {
-    /// Sample texels at `coord` from the sampled image.
+    /// Sample texels at `coord` from the sampled image with an implicit lod.
     ///
     /// # Safety
     /// Sampling with a type (`S`) that doesn't match the image's image format
@@ -974,6 +974,36 @@ impl<
         );
         result
     }
+
+    /// Sample texels at `coord` from the sampled image with an explicit lod.
+    ///
+    /// # Safety
+    /// Sampling with a type (`S`) that doesn't match the image's image format
+    /// will result in undefined behaviour.
+    #[crate::macros::gpu_only]
+    pub unsafe fn sample_by_lod<F, V>(
+        &self,
+        coord: impl ImageCoordinate<F, DIM, ARRAYED>,
+        lod: f32,
+    ) -> V
+    where
+        F: Float,
+        V: Vector<SampledType, 4>,
+    {
+        let mut result = Default::default();
+        asm!(
+            "%sampledImage = OpLoad typeof*{1} {1}",
+            "%coord = OpLoad typeof*{2} {2}",
+            "%lod = OpLoad typeof*{3} {3}",
+            "%result = OpImageSampleExplicitLod typeof*{0} %sampledImage %coord Lod %lod",
+            "OpStore {0} %result",
+            in(reg) &mut result,
+            in(reg) self,
+            in(reg) &coord,
+            in(reg) &lod,
+        );
+        result
+    }
 }
 
 /// This is a marker trait to represent the constraints on `OpImageGather` too complex to be

tests/ui/image/sample_lod.rs

@@ -1,20 +1,24 @@
 // Test `OpImageSampleExplicitLod` Lod
 // build-pass
 
-use spirv_std::{arch, Image, Sampler};
+use spirv_std::{arch, image::SampledImage, Image, Sampler};
 
 #[spirv(fragment)]
 pub fn main(
     #[spirv(descriptor_set = 0, binding = 0)] image2d: &Image!(2D, type=f32, sampled),
     #[spirv(descriptor_set = 1, binding = 1)] image2d_array: &Image!(2D, type=f32, arrayed, sampled),
     #[spirv(descriptor_set = 2, binding = 2)] cubemap: &Image!(3D, type=f32, sampled),
     #[spirv(descriptor_set = 3, binding = 3)] sampler: &Sampler,
+    #[spirv(descriptor_set = 4, binding = 4)] sampled_image: &SampledImage<
+        Image!(2D, type=f32, sampled),
+    >,
     output: &mut glam::Vec4,
 ) {
     let v2 = glam::Vec2::new(0.0, 1.0);
     let v3 = glam::Vec3::new(0.0, 1.0, 0.5);
     let r1: glam::Vec4 = image2d.sample_by_lod(*sampler, v2, 0.0);
     let r2: glam::Vec4 = image2d_array.sample_by_lod(*sampler, v3, 0.0);
     let r3: glam::Vec4 = cubemap.sample_by_lod(*sampler, v3, 0.0);
-    *output = r1 + r2 + r3;
+    let r4: glam::Vec4 = unsafe { sampled_image.sample_by_lod(v2, 0.0) };
+    *output = r1 + r2 + r3 + r4;
 }