From 28a1ee435463c898314a0db0d42a1eacd40e95fa Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jo=C3=A3o=20Capucho?= <jcapucho7@gmail.com>
Date: Fri, 21 May 2021 22:46:17 +0100
Subject: [PATCH] [glsl-in] Don't read builtin if it isn't available

---
 src/front/glsl/functions.rs | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/src/front/glsl/functions.rs b/src/front/glsl/functions.rs
index 6da0028a21..8838562380 100644
--- a/src/front/glsl/functions.rs
+++ b/src/front/glsl/functions.rs
@@ -439,6 +439,7 @@ impl Program<'_> {
             for (binding, input, handle) in self.entry_args.iter().cloned() {
                 match binding {
                     Binding::Location { .. } if !input => continue,
+                    Binding::BuiltIn(builtin) if !should_read(builtin, stage) => continue,
                     _ => {}
                 }
 
@@ -524,6 +525,32 @@ impl Program<'_> {
     }
 }
 
+// FIXME: Both of the functions below should be removed they are a temporary solution
+//
+// The fix should analyze the entry point and children function calls
+// (recursively) and store something like `GlobalUse` and then later only read
+// or store the globals that need to be read or written in that stage
+
+fn should_read(built_in: BuiltIn, stage: ShaderStage) -> bool {
+    match (built_in, stage) {
+        (BuiltIn::Position, ShaderStage::Fragment)
+        | (BuiltIn::BaseInstance, ShaderStage::Vertex)
+        | (BuiltIn::BaseVertex, ShaderStage::Vertex)
+        | (BuiltIn::ClipDistance, ShaderStage::Fragment)
+        | (BuiltIn::InstanceIndex, ShaderStage::Vertex)
+        | (BuiltIn::VertexIndex, ShaderStage::Vertex)
+        | (BuiltIn::FrontFacing, ShaderStage::Fragment)
+        | (BuiltIn::SampleIndex, ShaderStage::Fragment)
+        | (BuiltIn::SampleMask, ShaderStage::Fragment)
+        | (BuiltIn::GlobalInvocationId, ShaderStage::Compute)
+        | (BuiltIn::LocalInvocationId, ShaderStage::Compute)
+        | (BuiltIn::LocalInvocationIndex, ShaderStage::Compute)
+        | (BuiltIn::WorkGroupId, ShaderStage::Compute)
+        | (BuiltIn::WorkGroupSize, ShaderStage::Compute) => true,
+        _ => false,
+    }
+}
+
 fn should_write(built_in: BuiltIn, stage: ShaderStage) -> bool {
     match (built_in, stage) {
         (BuiltIn::Position, ShaderStage::Vertex)