[vulkan] Fixed dual-source blending to correctly map shader outputs. (#3637)

Fixes Skyward Sword HD eye gitch and a related MoltenVK crash due to the incorrect output mapping. Verified working on mac and android.

The test in vk_pipeline_cache.cpp is a bit ugly, but it didn't seem worth it to go lambda/macro just to make it look cleaner. Could change if necessary.

Co-authored-by: tarako <none@none.com>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3637
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>
Co-authored-by: tarako <r76036296@gmail.com>
Co-committed-by: tarako <r76036296@gmail.com>

4 files changed, 58 insertions, 4 deletions

diff --git a/src/shader_recompiler/backend/spirv/emit_spirv_special.cpp b/src/shader_recompiler/backend/spirv/emit_spirv_special.cpp
index fe5e70a63b..8eca1fac29 100644
--- a/src/shader_recompiler/backend/spirv/emit_spirv_special.cpp
+++ b/src/shader_recompiler/backend/spirv/emit_spirv_special.cpp
@@ -1,3 +1,6 @@
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 // SPDX-FileCopyrightText: Copyright 2021 yuzu Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
@@ -91,6 +94,17 @@ void AlphaTest(EmitContext& ctx) {
 } // Anonymous namespace
 
 void EmitPrologue(EmitContext& ctx) {
+    if (ctx.stage == Stage::Fragment && ctx.runtime_info.dual_source_blend) {
+        // Initialize dual-source blending outputs - prevents MoltenVK crash.
+        const Id zero{ctx.Const(0.0f)};
+        const Id one{ctx.Const(1.0f)};
+        const Id default_color{ctx.ConstantComposite(ctx.F32[4], zero, zero, zero, one)};
+        for (u32 i = 0; i < 2; ++i) {
+            if (Sirit::ValidId(ctx.frag_color[i])) {
+                ctx.OpStore(ctx.frag_color[i], default_color);
+            }
+        }
+    }
     if (ctx.stage == Stage::VertexB) {
         const Id zero{ctx.Const(0.0f)};
         const Id one{ctx.Const(1.0f)};
diff --git a/src/shader_recompiler/backend/spirv/spirv_emit_context.cpp b/src/shader_recompiler/backend/spirv/spirv_emit_context.cpp
index cd3394bdf0..b9a24496c9 100644
--- a/src/shader_recompiler/backend/spirv/spirv_emit_context.cpp
+++ b/src/shader_recompiler/backend/spirv/spirv_emit_context.cpp
@@ -1670,13 +1670,22 @@ void EmitContext::DefineOutputs(const IR::Program& program) {
         break;
     case Stage::Fragment:
         for (u32 index = 0; index < 8; ++index) {
-            if (!info.stores_frag_color[index] && !profile.need_declared_frag_colors) {
+            const bool need_dual_source = runtime_info.dual_source_blend && index <= 1;
+            if (!need_dual_source && !info.stores_frag_color[index] &&
+                !profile.need_declared_frag_colors) {
                 continue;
             }
             const Id type{GetAttributeType(*this, runtime_info.color_output_types[index])};
             frag_color[index] = DefineOutput(*this, type, std::nullopt);
-            Decorate(frag_color[index], spv::Decoration::Location, index);
-            Name(frag_color[index], fmt::format("frag_color{}", index));
+            // Correct mapping for dual-source blending
+            if (runtime_info.dual_source_blend && index <= 1) {
+                Decorate(frag_color[index], spv::Decoration::Location, 0u);
+                Decorate(frag_color[index], spv::Decoration::Index, index);
+                Name(frag_color[index], index == 0 ? "frag_color0" : "frag_color0_secondary");
+            } else {
+                Decorate(frag_color[index], spv::Decoration::Location, index);
+                Name(frag_color[index], fmt::format("frag_color{}", index));
+            }
         }
         if (info.stores_frag_depth) {
             frag_depth = DefineOutput(*this, F32[1], std::nullopt);
diff --git a/src/shader_recompiler/runtime_info.h b/src/shader_recompiler/runtime_info.h
index 613c598d0c..be10a9bb08 100644
--- a/src/shader_recompiler/runtime_info.h
+++ b/src/shader_recompiler/runtime_info.h
@@ -110,6 +110,9 @@ struct RuntimeInfo {
 
     /// Output types for each color attachment
     std::array<AttributeType, 8> color_output_types{};
+
+    /// Dual source blending
+    bool dual_source_blend{};
 };
 
 } // namespace Shader
diff --git a/src/video_core/renderer_vulkan/vk_pipeline_cache.cpp b/src/video_core/renderer_vulkan/vk_pipeline_cache.cpp
index f3dd0f90d8..77a4e8616a 100644
--- a/src/video_core/renderer_vulkan/vk_pipeline_cache.cpp
+++ b/src/video_core/renderer_vulkan/vk_pipeline_cache.cpp
@@ -237,11 +237,38 @@ Shader::RuntimeInfo MakeRuntimeInfo(std::span<const Shader::IR::Program> program
         }
         info.convert_depth_mode = gl_ndc;
         break;
-    case Shader::Stage::Fragment:
+    case Shader::Stage::Fragment: {
         info.alpha_test_func = MaxwellToCompareFunction(
             key.state.UnpackComparisonOp(key.state.alpha_test_func.Value()));
         info.alpha_test_reference = std::bit_cast<float>(key.state.alpha_test_ref);
 
+        // Check for dual source blending
+        const auto& blend0 = key.state.attachments[0];
+        if (blend0.enable != 0) {
+            using F = Maxwell::Blend::Factor;
+            const auto src_rgb = blend0.SourceRGBFactor();
+            const auto dst_rgb = blend0.DestRGBFactor();
+            const auto src_a = blend0.SourceAlphaFactor();
+            const auto dst_a = blend0.DestAlphaFactor();
+            info.dual_source_blend =
+                src_rgb == F::Source1Color_D3D || src_rgb == F::OneMinusSource1Color_D3D ||
+                src_rgb == F::Source1Alpha_D3D || src_rgb == F::OneMinusSource1Alpha_D3D ||
+                src_rgb == F::Source1Color_GL || src_rgb == F::OneMinusSource1Color_GL ||
+                src_rgb == F::Source1Alpha_GL || src_rgb == F::OneMinusSource1Alpha_GL ||
+                dst_rgb == F::Source1Color_D3D || dst_rgb == F::OneMinusSource1Color_D3D ||
+                dst_rgb == F::Source1Alpha_D3D || dst_rgb == F::OneMinusSource1Alpha_D3D ||
+                dst_rgb == F::Source1Color_GL || dst_rgb == F::OneMinusSource1Color_GL ||
+                dst_rgb == F::Source1Alpha_GL || dst_rgb == F::OneMinusSource1Alpha_GL ||
+                src_a == F::Source1Color_D3D || src_a == F::OneMinusSource1Color_D3D ||
+                src_a == F::Source1Alpha_D3D || src_a == F::OneMinusSource1Alpha_D3D ||
+                src_a == F::Source1Color_GL || src_a == F::OneMinusSource1Color_GL ||
+                src_a == F::Source1Alpha_GL || src_a == F::OneMinusSource1Alpha_GL ||
+                dst_a == F::Source1Color_D3D || dst_a == F::OneMinusSource1Color_D3D ||
+                dst_a == F::Source1Alpha_D3D || dst_a == F::OneMinusSource1Alpha_D3D ||
+                dst_a == F::Source1Color_GL || dst_a == F::OneMinusSource1Color_GL ||
+                dst_a == F::Source1Alpha_GL || dst_a == F::OneMinusSource1Alpha_GL;
+        }
+
         if (device.IsMoltenVK()) {
             for (size_t i = 0; i < 8; ++i) {
                 const auto format = static_cast<Tegra::RenderTargetFormat>(key.state.color_formats[i]);
@@ -258,6 +285,7 @@ Shader::RuntimeInfo MakeRuntimeInfo(std::span<const Shader::IR::Program> program
             }
         }
         break;
+    }
     default:
         break;
     }