MSL: Adjust FragCoord for sample-rate shading.

In Metal, the `[[position]]` input to a fragment shader remains at
fragment center, even at sample rate, like OpenGL and Direct3D. In
Vulkan, however, when the fragment shader runs at sample rate, the
`FragCoord` builtin moves to the sample position in the framebuffer,
instead of the fragment center. To account for this difference, adjust
the `FragCoord`, if present, by the sample position. The -0.5 offset is
because the fragment center is at (0.5, 0.5).

Also, add an option to force sample-rate shading in a fragment shader.
Since Metal has no explicit control for this, this is done by adding a
dummy `[[sample_id]]` which is otherwise unused, if none is already
present. This is intended to be used from e.g. MoltenVK when a
pipeline's `minSampleShading` value is nonzero.

Instead of checking if any `Input` variables have `Sample`
interpolation, I've elected to check that the `SampleRateShading`
capability is present. Since `SampleId`, `SamplePosition`, and the
`Sample` interpolation decoration require this cap, this should be
equivalent for any valid SPIR-V module. If this isn't acceptable, let me
know.

8 files changed, 106 insertions, 0 deletions

diff --git a/reference/opt/shaders-msl/frag/basic.force-sample.frag b/reference/opt/shaders-msl/frag/basic.force-sample.frag
new file mode 100644
index 00000000..b9706b73
--- /dev/null
+++ b/reference/opt/shaders-msl/frag/basic.force-sample.frag
@@ -0,0 +1,23 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct main0_out
+{
+    float4 FragColor [[color(0)]];
+};
+
+struct main0_in
+{
+    float4 vColor [[user(locn0)]];
+    float2 vTex [[user(locn1)]];
+};
+
+fragment main0_out main0(main0_in in [[stage_in]], texture2d<float> uTex [[texture(0)]], sampler uTexSmplr [[sampler(0)]], uint gl_SampleID [[sample_id]])
+{
+    main0_out out = {};
+    out.FragColor = in.vColor * uTex.sample(uTexSmplr, in.vTex);
+    return out;
+}
+
diff --git a/reference/opt/shaders-msl/frag/input-attachment-ms.arrayed-subpass.msl21.frag b/reference/opt/shaders-msl/frag/input-attachment-ms.arrayed-subpass.msl21.frag
index 5f137b04..52a78cf9 100644
--- a/reference/opt/shaders-msl/frag/input-attachment-ms.arrayed-subpass.msl21.frag
+++ b/reference/opt/shaders-msl/frag/input-attachment-ms.arrayed-subpass.msl21.frag
@@ -11,6 +11,7 @@ struct main0_out
 fragment main0_out main0(texture2d_ms_array<float> uSubpass0 [[texture(0)]], texture2d_ms_array<float> uSubpass1 [[texture(1)]], uint gl_SampleID [[sample_id]], float4 gl_FragCoord [[position]], uint gl_Layer [[render_target_array_index]])
 {
     main0_out out = {};
+    gl_FragCoord.xy += get_sample_position(gl_SampleID) - 0.5;
     out.FragColor = (uSubpass0.read(uint2(gl_FragCoord.xy), gl_Layer, 1) + uSubpass1.read(uint2(gl_FragCoord.xy), gl_Layer, 2)) + uSubpass0.read(uint2(gl_FragCoord.xy), gl_Layer, gl_SampleID);
     return out;
 }
diff --git a/reference/opt/shaders-msl/frag/input-attachment-ms.frag b/reference/opt/shaders-msl/frag/input-attachment-ms.frag
index 906cabbf..0c47348d 100644
--- a/reference/opt/shaders-msl/frag/input-attachment-ms.frag
+++ b/reference/opt/shaders-msl/frag/input-attachment-ms.frag
@@ -11,6 +11,7 @@ struct main0_out
 fragment main0_out main0(texture2d_ms<float> uSubpass0 [[texture(0)]], texture2d_ms<float> uSubpass1 [[texture(1)]], uint gl_SampleID [[sample_id]], float4 gl_FragCoord [[position]])
 {
     main0_out out = {};
+    gl_FragCoord.xy += get_sample_position(gl_SampleID) - 0.5;
     out.FragColor = (uSubpass0.read(uint2(gl_FragCoord.xy), 1) + uSubpass1.read(uint2(gl_FragCoord.xy), 2)) + uSubpass0.read(uint2(gl_FragCoord.xy), gl_SampleID);
     return out;
 }
diff --git a/reference/opt/shaders-msl/frag/input-attachment-ms.multiview.msl21.frag b/reference/opt/shaders-msl/frag/input-attachment-ms.multiview.msl21.frag
index 2e4ca8a7..e27b24ad 100644
--- a/reference/opt/shaders-msl/frag/input-attachment-ms.multiview.msl21.frag
+++ b/reference/opt/shaders-msl/frag/input-attachment-ms.multiview.msl21.frag
@@ -11,6 +11,7 @@ struct main0_out
 fragment main0_out main0(constant uint* spvViewMask [[buffer(24)]], texture2d_ms_array<float> uSubpass0 [[texture(0)]], texture2d_ms_array<float> uSubpass1 [[texture(1)]], uint gl_SampleID [[sample_id]], float4 gl_FragCoord [[position]], uint gl_ViewIndex [[render_target_array_index]])
 {
     main0_out out = {};
+    gl_FragCoord.xy += get_sample_position(gl_SampleID) - 0.5;
     gl_ViewIndex += spvViewMask[0];
     out.FragColor = (uSubpass0.read(uint2(gl_FragCoord.xy), gl_ViewIndex, 1) + uSubpass1.read(uint2(gl_FragCoord.xy), gl_ViewIndex, 2)) + uSubpass0.read(uint2(gl_FragCoord.xy), gl_ViewIndex, gl_SampleID);
     return out;
diff --git a/reference/opt/shaders-msl/frag/sample-rate-frag-coord-sample-id.frag b/reference/opt/shaders-msl/frag/sample-rate-frag-coord-sample-id.frag
new file mode 100644
index 00000000..5df60f90
--- /dev/null
+++ b/reference/opt/shaders-msl/frag/sample-rate-frag-coord-sample-id.frag
@@ -0,0 +1,19 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct main0_out
+{
+    float4 FragColor [[color(0)]];
+};
+
+fragment main0_out main0(texture2d_array<float> tex [[texture(0)]], sampler texSmplr [[sampler(0)]], float4 gl_FragCoord [[position]], uint gl_SampleID [[sample_id]])
+{
+    main0_out out = {};
+    gl_FragCoord.xy += get_sample_position(gl_SampleID) - 0.5;
+    float3 _28 = float3(gl_FragCoord.xy, float(gl_SampleID));
+    out.FragColor = tex.sample(texSmplr, _28.xy, uint(round(_28.z)));
+    return out;
+}
+
diff --git a/reference/opt/shaders-msl/frag/sample-rate-frag-coord-sample-input.frag b/reference/opt/shaders-msl/frag/sample-rate-frag-coord-sample-input.frag
new file mode 100644
index 00000000..386230ef
--- /dev/null
+++ b/reference/opt/shaders-msl/frag/sample-rate-frag-coord-sample-input.frag
@@ -0,0 +1,24 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct main0_out
+{
+    float4 FragColor [[color(0)]];
+};
+
+struct main0_in
+{
+    float foo [[user(locn0), sample_perspective]];
+};
+
+fragment main0_out main0(main0_in in [[stage_in]], texture2d_array<float> tex [[texture(0)]], sampler texSmplr [[sampler(0)]], float4 gl_FragCoord [[position]], uint gl_SampleID [[sample_id]])
+{
+    main0_out out = {};
+    gl_FragCoord.xy += get_sample_position(gl_SampleID) - 0.5;
+    float3 _26 = float3(gl_FragCoord.xy, in.foo);
+    out.FragColor = tex.sample(texSmplr, _26.xy, uint(round(_26.z)));
+    return out;
+}
+
diff --git a/reference/opt/shaders-msl/frag/sample-rate-frag-coord-sample-pos.frag b/reference/opt/shaders-msl/frag/sample-rate-frag-coord-sample-pos.frag
new file mode 100644
index 00000000..f8f357fe
--- /dev/null
+++ b/reference/opt/shaders-msl/frag/sample-rate-frag-coord-sample-pos.frag
@@ -0,0 +1,19 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct main0_out
+{
+    float4 FragColor [[color(0)]];
+};
+
+fragment main0_out main0(texture2d<float> tex [[texture(0)]], sampler texSmplr [[sampler(0)]], float4 gl_FragCoord [[position]], uint gl_SampleID [[sample_id]])
+{
+    main0_out out = {};
+    gl_FragCoord.xy += get_sample_position(gl_SampleID) - 0.5;
+    float2 gl_SamplePosition = get_sample_position(gl_SampleID);
+    out.FragColor = tex.sample(texSmplr, (gl_FragCoord.xy - gl_SamplePosition));
+    return out;
+}
+
diff --git a/reference/opt/shaders-msl/frag/sample-rate-frag-coord.force-sample.frag b/reference/opt/shaders-msl/frag/sample-rate-frag-coord.force-sample.frag
new file mode 100644
index 00000000..1ed8148d
--- /dev/null
+++ b/reference/opt/shaders-msl/frag/sample-rate-frag-coord.force-sample.frag
@@ -0,0 +1,18 @@
+#include <metal_stdlib>
+#include <simd/simd.h>
+
+using namespace metal;
+
+struct main0_out
+{
+    float4 FragColor [[color(0)]];
+};
+
+fragment main0_out main0(texture2d<float> tex [[texture(0)]], sampler texSmplr [[sampler(0)]], float4 gl_FragCoord [[position]], uint gl_SampleID [[sample_id]])
+{
+    main0_out out = {};
+    gl_FragCoord.xy += get_sample_position(gl_SampleID) - 0.5;
+    out.FragColor = tex.sample(texSmplr, gl_FragCoord.xy);
+    return out;
+}
+