From 2a1121a43b9013f529875a98d03d35f4ff2ec89b Mon Sep 17 00:00:00 2001
From: dan sinclair <dsinclair@chromium.org>
Date: Tue, 09 Jun 2026 05:01:46 -0700
Subject: [PATCH] Reland "Fix location selection for pixel center polyfill."

This reverts commit b85548b0d317e71ea6034ceedf0bf1ed6fd352bf.

There was an issue with the fuzzer setting the position center location
which is now fixed.

Fixed: 517522769
Change-Id: I85a63e65e7ea2ff5543e04b87009f4112afae1d1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/314935
Auto-Submit: dan sinclair <dsinclair@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
---

diff --git a/src/dawn/native/vulkan/RenderPipelineVk.cpp b/src/dawn/native/vulkan/RenderPipelineVk.cpp
index 184cf10..9de4480 100644
--- a/src/dawn/native/vulkan/RenderPipelineVk.cpp
+++ b/src/dawn/native/vulkan/RenderPipelineVk.cpp
@@ -27,7 +27,6 @@
 
 #include "src/dawn/native/vulkan/RenderPipelineVk.h"
 
-#include <memory>
 #include <string>
 #include <utility>
 #include <vector>
@@ -514,14 +513,37 @@
         return {};
     };
 
+    std::optional<uint32_t> pixelCenterPolyfillLocation = std::nullopt;
+    if (NeedsPixelCenterPolyfill()) {
+        const EntryPointMetadata* vtx = GetStage(SingleShaderStage::Vertex).metadata;
+        for (size_t i = 0; i < vtx->usedInterStageVariables.size(); ++i) {
+            if (vtx->usedInterStageVariables[i] == false) {
+                pixelCenterPolyfillLocation = uint32_t(i);
+                break;
+            }
+        }
+        if (!pixelCenterPolyfillLocation.has_value()) {
+            return DAWN_INTERNAL_ERROR(
+                "unable to find a free vertex location for the pixel center polyfill");
+        }
+
+        if (HasStage(SingleShaderStage::Fragment)) {
+            const EntryPointMetadata* frag = GetStage(SingleShaderStage::Fragment).metadata;
+            // Because the fragment stage must be a subset of the vertex stage, if the value was
+            // free in the vertex stage it _must_ be free in the fragment stage.
+            DAWN_ASSERT(frag->usedInterStageVariables[pixelCenterPolyfillLocation.value()] ==
+                        false);
+        }
+    }
+
     // Add the vertex stage that's always present.
     DAWN_TRY(AddShaderStage({
         .stage = &GetStage(SingleShaderStage::Vertex),
         .layout = layout,
         .immediateMask = GetImmediateMask(),
         .ycbcrExternalTextures = &specialization.ycbcrExternalTextures,
+        .polyfillPixelCenter = pixelCenterPolyfillLocation,
         .emitPointSize = GetPrimitiveTopology() == wgpu::PrimitiveTopology::PointList,
-        .polyfillPixelCenter = NeedsPixelCenterPolyfill(),
         .pipelineUsesFramebufferFetch = UsesFramebufferFetch(),
     }));
 
@@ -532,7 +554,7 @@
             .layout = layout,
             .immediateMask = GetImmediateMask(),
             .ycbcrExternalTextures = &specialization.ycbcrExternalTextures,
-            .polyfillPixelCenter = NeedsPixelCenterPolyfill(),
+            .polyfillPixelCenter = pixelCenterPolyfillLocation,
             .pipelineUsesFramebufferFetch = UsesFramebufferFetch(),
             .needsMultisampledFramebufferFetch = UseSampleRateShading() && UsesFramebufferFetch(),
         }));
diff --git a/src/dawn/native/vulkan/ShaderModuleVk.h b/src/dawn/native/vulkan/ShaderModuleVk.h
index 337af93..693d9fb 100644
--- a/src/dawn/native/vulkan/ShaderModuleVk.h
+++ b/src/dawn/native/vulkan/ShaderModuleVk.h
@@ -72,8 +72,8 @@
         ImmediateMask immediateMask;
         raw_ptr<const absl::flat_hash_set<APIBindPoint>> ycbcrExternalTextures;
 
+        std::optional<uint32_t> polyfillPixelCenter = std::nullopt;
         bool emitPointSize = false;
-        bool polyfillPixelCenter = false;
         bool pipelineUsesFramebufferFetch = false;
         bool needsMultisampledFramebufferFetch = false;
     };
diff --git a/src/dawn/tests/BUILD.gn b/src/dawn/tests/BUILD.gn
index 5e533fc..73d213a 100644
--- a/src/dawn/tests/BUILD.gn
+++ b/src/dawn/tests/BUILD.gn
@@ -648,6 +648,7 @@
     "end2end/ShaderAtomicTests.cpp",
     "end2end/ShaderBuiltinPartialConstArgsErrorTests.cpp",
     "end2end/ShaderF16Tests.cpp",
+    "end2end/ShaderIOPolyfillTests.cpp",
     "end2end/ShaderModuleCachingTests.cpp",
     "end2end/ShaderPrintTests.cpp",
     "end2end/ShaderTests.cpp",
diff --git a/src/dawn/tests/CMakeLists.txt b/src/dawn/tests/CMakeLists.txt
index ec77cff..9b64d0b 100644
--- a/src/dawn/tests/CMakeLists.txt
+++ b/src/dawn/tests/CMakeLists.txt
@@ -160,6 +160,7 @@
     "end2end/ShaderAtomicTests.cpp"
     "end2end/ShaderBuiltinPartialConstArgsErrorTests.cpp"
     "end2end/ShaderF16Tests.cpp"
+    "end2end/ShaderIOPolyfillTests.cpp"
     "end2end/ShaderModuleCachingTests.cpp"
     "end2end/ShaderPrintTests.cpp"
     "end2end/ShaderTests.cpp"
diff --git a/src/dawn/tests/end2end/ShaderIOPolyfillTests.cpp b/src/dawn/tests/end2end/ShaderIOPolyfillTests.cpp
new file mode 100644
index 0000000..20501ff
--- /dev/null
+++ b/src/dawn/tests/end2end/ShaderIOPolyfillTests.cpp
@@ -0,0 +1,111 @@
+// Copyright 2026 The Dawn & Tint Authors
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this
+//    list of conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice,
+//    this list of conditions and the following disclaimer in the documentation
+//    and/or other materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its
+//    contributors may be used to endorse or promote products derived from
+//    this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+// SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+// CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+// OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include <utility>
+
+#include "src/dawn/tests/DawnTest.h"
+#include "src/dawn/utils/ComboRenderPipelineDescriptor.h"
+#include "src/dawn/utils/WGPUHelpers.h"
+
+namespace dawn {
+namespace {
+
+using ShaderIOPolyfillTests = DawnTest;
+
+// https://crbug.com/517522769
+TEST_P(ShaderIOPolyfillTests, DivergentLocations) {
+    wgpu::ShaderModule vsModule = utils::CreateShaderModule(device, R"(
+        struct VOut {
+            @builtin(position) pos : vec4f,
+            @location(0) a : vec4f,
+            @location(1) @interpolate(flat) b : i32,
+        };
+        @vertex fn vs(@builtin(vertex_index) vi : u32) -> VOut {
+            var p = array<vec2f, 3>(
+                vec2f(-1, -1),
+                vec2f( 3, -1),
+                vec2f(-1,  3));
+            var o : VOut;
+            o.pos = vec4f(p[vi], 0, 1);
+            o.a   = vec4f(1, 0, 0, 1);
+            o.b   = 0x7eadbeef;
+            return o;
+        }
+    )");
+
+    wgpu::ShaderModule fsModule = utils::CreateShaderModule(device, R"(
+        @fragment fn fs(
+            @location(0) a : vec4f,
+            @builtin(sample_index) s : u32,
+            @builtin(position) p : vec4f) -> @location(0) vec4f {
+            return a + vec4f(p.z, f32(s) * 0, 0, 0);
+        }
+    )");
+
+    utils::ComboRenderPipelineDescriptor pipelineDesc;
+    pipelineDesc.vertex.module = std::move(vsModule);
+    pipelineDesc.cFragment.module = std::move(fsModule);
+    pipelineDesc.multisample.count = 4;
+    pipelineDesc.cTargets[0].format = wgpu::TextureFormat::RGBA8Unorm;
+
+    wgpu::RenderPipeline pipeline = device.CreateRenderPipeline(&pipelineDesc);
+
+    wgpu::TextureDescriptor msaaDesc;
+    msaaDesc.size = {64, 64, 1};

diff --git a/src/dawn/tests/BUILD.gn b/src/dawn/tests/BUILD.gn
index 5e533fc..73d213a 100644
--- a/src/dawn/tests/BUILD.gn
+++ b/src/dawn/tests/BUILD.gn
@@ -648,6 +648,7 @@
     "end2end/ShaderAtomicTests.cpp",
     "end2end/ShaderBuiltinPartialConstArgsErrorTests.cpp",
     "end2end/ShaderF16Tests.cpp",
+    "end2end/ShaderIOPolyfillTests.cpp",
     "end2end/ShaderModuleCachingTests.cpp",
     "end2end/ShaderPrintTests.cpp",
     "end2end/ShaderTests.cpp",
diff --git a/src/dawn/tests/CMakeLists.txt b/src/dawn/tests/CMakeLists.txt
index ec77cff..9b64d0b 100644
--- a/src/dawn/tests/CMakeLists.txt
+++ b/src/dawn/tests/CMakeLists.txt
@@ -160,6 +160,7 @@
     "end2end/ShaderAtomicTests.cpp"
     "end2end/ShaderBuiltinPartialConstArgsErrorTests.cpp"
     "end2end/ShaderF16Tests.cpp"
+    "end2end/ShaderIOPolyfillTests.cpp"
     "end2end/ShaderModuleCachingTests.cpp"
     "end2end/ShaderPrintTests.cpp"
     "end2end/ShaderTests.cpp"
diff --git a/src/dawn/tests/end2end/ShaderIOPolyfillTests.cpp b/src/dawn/tests/end2end/ShaderIOPolyfillTests.cpp
new file mode 100644
index 0000000..20501ff
--- /dev/null
+++ b/src/dawn/tests/end2end/ShaderIOPolyfillTests.cpp
@@ -0,0 +1,111 @@
+// Copyright 2026 The Dawn & Tint Authors
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this
+//    list of conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice,
+//    this list of conditions and the following disclaimer in the documentation
+//    and/or other materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its
+//    contributors may be used to endorse or promote products derived from
+//    this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+// SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+// CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+// OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include <utility>
+
+#include "src/dawn/tests/DawnTest.h"
+#include "src/dawn/utils/ComboRenderPipelineDescriptor.h"
+#include "src/dawn/utils/WGPUHelpers.h"
+
+namespace dawn {
+namespace {
+
+using ShaderIOPolyfillTests = DawnTest;
+
+// https://crbug.com/517522769
+TEST_P(ShaderIOPolyfillTests, DivergentLocations) {
+    wgpu::ShaderModule vsModule = utils::CreateShaderModule(device, R"(
+        struct VOut {
+            @builtin(position) pos : vec4f,
+            @location(0) a : vec4f,
+            @location(1) @interpolate(flat) b : i32,
+        };
+        @vertex fn vs(@builtin(vertex_index) vi : u32) -> VOut {
+            var p = array<vec2f, 3>(
+                vec2f(-1, -1),
+                vec2f( 3, -1),
+                vec2f(-1,  3));
+            var o : VOut;
+            o.pos = vec4f(p[vi], 0, 1);
+            o.a   = vec4f(1, 0, 0, 1);
+            o.b   = 0x7eadbeef;
+            return o;
+        }
+    )");
+
+    wgpu::ShaderModule fsModule = utils::CreateShaderModule(device, R"(
+        @fragment fn fs(
+            @location(0) a : vec4f,
+            @builtin(sample_index) s : u32,
+            @builtin(position) p : vec4f) -> @location(0) vec4f {
+            return a + vec4f(p.z, f32(s) * 0, 0, 0);
+        }
+    )");
+
+    utils::ComboRenderPipelineDescriptor pipelineDesc;
+    pipelineDesc.vertex.module = std::move(vsModule);
+    pipelineDesc.cFragment.module = std::move(fsModule);
+    pipelineDesc.multisample.count = 4;
+    pipelineDesc.cTargets[0].format = wgpu::TextureFormat::RGBA8Unorm;
+
+    wgpu::RenderPipeline pipeline = device.CreateRenderPipeline(&pipelineDesc);
+
+    wgpu::TextureDescriptor msaaDesc;
+    msaaDesc.size = {64, 64, 1};
+    msaaDesc.sampleCount = 4;
+    msaaDesc.format = wgpu::TextureFormat::RGBA8Unorm;
+    msaaDesc.usage = wgpu::TextureUsage::RenderAttachment;
+    wgpu::Texture msaa = device.CreateTexture(&msaaDesc);
+
+    wgpu::TextureDescriptor resolveDesc;
+    resolveDesc.size = {64, 64, 1};
+    resolveDesc.format = wgpu::TextureFormat::RGBA8Unorm;
+    resolveDesc.usage = wgpu::TextureUsage::RenderAttachment | wgpu::TextureUsage::CopySrc;
+    wgpu::Texture resolve = device.CreateTexture(&resolveDesc);
+
+    wgpu::CommandEncoder encoder = device.CreateCommandEncoder();
+    utils::ComboRenderPassDescriptor renderPass({msaa.CreateView()});
+    renderPass.cColorAttachments[0].resolveTarget = resolve.CreateView();
+    renderPass.cColorAttachments[0].clearValue = {0.0, 0.0, 0.0, 0.0};
+    renderPass.cColorAttachments[0].loadOp = wgpu::LoadOp::Clear;
+    renderPass.cColorAttachments[0].storeOp = wgpu::StoreOp::Discard;
+
+    wgpu::RenderPassEncoder pass = encoder.BeginRenderPass(&renderPass);
+    pass.SetPipeline(pipeline);
+    pass.Draw(3);
+    pass.End();
+
+    wgpu::CommandBuffer commands = encoder.Finish();
+    queue.Submit(1, &commands);
+
+    EXPECT_PIXEL_RGBA8_EQ(utils::RGBA8::kRed, resolve, 32, 32);
+}
+
+DAWN_INSTANTIATE_TEST(ShaderIOPolyfillTests, VulkanBackend());
+
+}  // namespace
+}  // namespace dawn
diff --git a/src/tint/lang/spirv/writer/raise/shader_io_test.cc b/src/tint/lang/spirv/writer/raise/shader_io_test.cc
index 08928cc..0b79059 100644
--- a/src/tint/lang/spirv/writer/raise/shader_io_test.cc
+++ b/src/tint/lang/spirv/writer/raise/shader_io_test.cc
@@ -1422,7 +1422,7 @@
 
     core::ir::transform::ImmediateDataLayout immediate_data;
     ShaderIOConfig config{immediate_data};
-    config.polyfill_pixel_center = true;
+    config.polyfill_pixel_center = 1;
     Run(ShaderIO, config);
 
     EXPECT_EQ(expect, str());
@@ -1457,7 +1457,7 @@
 $B1: {  # root
   %foo_position_Input:ptr<__in, vec4<f32>, read> = var undef @builtin(position)
   %foo_sample_index_Input:ptr<__in, u32, read> = var undef @interpolate(flat) @builtin(sample_index)
-  %foo_loc0_Input:ptr<__in, vec4<f32>, read> = var undef @location(0) @interpolate(perspective, center)
+  %foo_loc1_Input:ptr<__in, vec4<f32>, read> = var undef @location(1) @interpolate(perspective, center)
 }
 
 %foo_inner = func(%position:vec4<f32>, %idx:u32):void {
@@ -1472,7 +1472,7 @@
     %10:vec2<f32> = swizzle %9, xy
     %11:vec2<f32> = floor %10
     %12:vec2<f32> = add %11, vec2<f32>(0.5f)
-    %13:vec4<f32> = spirv.interpolate_at_offset %foo_loc0_Input, vec2<f32>(0.0f)
+    %13:vec4<f32> = spirv.interpolate_at_offset %foo_loc1_Input, vec2<f32>(0.0f)
     %14:f32 = swizzle %13, z
     %15:f32 = swizzle %13, w
     %16:f32 = div %14, %15
@@ -1487,7 +1487,7 @@
 
     core::ir::transform::ImmediateDataLayout immediate_data;
     ShaderIOConfig config{immediate_data};
-    config.polyfill_pixel_center = true;
+    config.polyfill_pixel_center = 1;
     Run(ShaderIO, config);
 
     EXPECT_EQ(expect, str());
@@ -1552,7 +1552,7 @@
 
     core::ir::transform::ImmediateDataLayout immediate_data;
     ShaderIOConfig config{immediate_data};
-    config.polyfill_pixel_center = true;
+    config.polyfill_pixel_center = 1;
     Run(ShaderIO, config);
 
     EXPECT_EQ(expect, str());
@@ -1641,7 +1641,7 @@
 
     core::ir::transform::ImmediateDataLayout immediate_data;
     ShaderIOConfig config{immediate_data};
-    config.polyfill_pixel_center = true;
+    config.polyfill_pixel_center = 0;
     Run(ShaderIO, config);
 
     EXPECT_EQ(expect, str());
@@ -1725,7 +1725,7 @@
 
     core::ir::transform::ImmediateDataLayout immediate_data;
     ShaderIOConfig config{immediate_data};
-    config.polyfill_pixel_center = true;
+    config.polyfill_pixel_center = 0;
     Run(ShaderIO, config);
 
     EXPECT_EQ(expect, str());
@@ -1792,7 +1792,7 @@
 
     core::ir::transform::ImmediateDataLayout immediate_data;
     ShaderIOConfig config{immediate_data};
-    config.polyfill_pixel_center = true;
+    config.polyfill_pixel_center = 1;
     Run(ShaderIO, config);
 
     EXPECT_EQ(expect, str());
diff --git a/src/tint/lang/spirv/writer/writer_test.cc b/src/tint/lang/spirv/writer/writer_test.cc
index 9d2b1c0..0dc44b4 100644
--- a/src/tint/lang/spirv/writer/writer_test.cc
+++ b/src/tint/lang/spirv/writer/writer_test.cc
@@ -457,7 +457,7 @@
     });
 
     Options options;
-    options.polyfill_pixel_center = true;
+    options.polyfill_pixel_center = 0;
     auto result = Generate(options);
     ASSERT_EQ(result, Success) << result.Failure() << output_;
     EXPECT_INST(R"(