Add warning when using CompareFunction::*Equal without an invariant Attribute

CHANGELOG.md

@@ -40,13 +40,32 @@ Bottom level categories:
 
 ## Unreleased
 
+### Major Changes
+
+#### @invariant Warning
+
+When using CompareFunction::Equal or CompareFunction::NotEqual on a pipeline, there is now a warning logged if the vertex
+shader does not have a @invariant tag on it. On some machines, rendering the same triangles multiple times without an
+@invariant tag will result in slightly different depths for every pixel. Because the *Equal functions rely on depth being
+the same every time it is rendered, we now warn if it is missing.
+
+```diff
+-@vertex 
+-fn vert_main(v_in: VertexInput) -> @builtin(position) vec4<f32> {...}
++@vertex 
++fn vert_main(v_in: VertexInput) -> @builtin(position) @invariant vec4<f32> {...}
+```
+
 ### Bug Fixes
 
 #### General
 - Improve the validation and error reporting of buffer mappings by @nical in [#2848](https://github.com/gfx-rs/wgpu/pull/2848)
 
 ### Changes
 
+#### General
+- Add warning when using CompareFunction::*Equal with vertex shader that is missing @invariant tag by @cwfitzgerald in [#2887](https://github.com/gfx-rs/wgpu/pull/2887)
+
 #### Metal
 - Extract the generic code into `get_metal_layer` by @jinleili in [#2826](https://github.com/gfx-rs/wgpu/pull/2826)
 

wgpu-core/src/device/mod.rs

@@ -2371,6 +2371,7 @@ impl<A: HalApi> Device<A> {
                     &desc.stage.entry_point,
                     flag,
                     io,
+                    None,
                 )?;
             }
         }
@@ -2696,6 +2697,7 @@ impl<A: HalApi> Device<A> {
                         &stage.entry_point,
                         flag,
                         io,
+                        desc.depth_stencil.as_ref().map(|d| d.depth_compare),
                     )
                     .map_err(|error| pipeline::CreateRenderPipelineError::Stage {
                         stage: flag,
@@ -2742,6 +2744,7 @@ impl<A: HalApi> Device<A> {
                                 &fragment.stage.entry_point,
                                 flag,
                                 io,
+                                desc.depth_stencil.as_ref().map(|d| d.depth_compare),
                             )
                             .map_err(|error| pipeline::CreateRenderPipelineError::Stage {
                                 stage: flag,

wgpu-core/src/validation.rs

@@ -963,6 +963,7 @@ impl Interface {
         entry_point_name: &str,
         stage_bit: wgt::ShaderStages,
         inputs: StageIo,
+        compare_function: Option<wgt::CompareFunction>,
     ) -> Result<StageIo, StageError> {
         // Since a shader module can have multiple entry points with the same name,
         // we need to look for one with the right execution model.
@@ -1183,6 +1184,21 @@ impl Interface {
                     Varying::Local { ref iv, .. } => iv.ty.dim.num_components(),
                     Varying::BuiltIn(_) => 0,
                 };
+
+                if let Some(
+                    cmp @ wgt::CompareFunction::Equal | cmp @ wgt::CompareFunction::NotEqual,
+                ) = compare_function
+                {
+                    if let Varying::BuiltIn(naga::BuiltIn::Position { invariant: false }) = *output
+                    {
+                        log::warn!(
+                            "Vertex shader with entry point {entry_point_name} outputs a @builtin(position) without the @invariant \
+                            attribute and is used in a pipeline with {cmp:?}. On some machines, this can cause bad artifacting as {cmp:?} assumes \
+                            the values output from the vertex shader exactly match the value in the depth buffer. The @invariant attribute on the \
+                            @builtin(position) vertex output ensures that the exact same pixel depths are used every render."
+                        );
+                    }
+                }
             }
         }
 

wgpu-types/src/lib.rs

@@ -2602,13 +2602,17 @@ pub enum CompareFunction {
     Never = 1,
     /// Function passes if new value less than existing value
     Less = 2,
-    /// Function passes if new value is equal to existing value
+    /// Function passes if new value is equal to existing value. When using
+    /// this compare function, make sure to mark your Vertex Shader's `@builtin(position)`
+    /// output as `@invariant` to prevent artifacting.
     Equal = 3,
     /// Function passes if new value is less than or equal to existing value
     LessEqual = 4,
     /// Function passes if new value is greater than existing value
     Greater = 5,
-    /// Function passes if new value is not equal to existing value
+    /// Function passes if new value is not equal to existing value. When using
+    /// this compare function, make sure to mark your Vertex Shader's `@builtin(position)`
+    /// output as `@invariant` to prevent artifacting.
     NotEqual = 6,
     /// Function passes if new value is greater than or equal to existing value
     GreaterEqual = 7,