From 0fdc229b8b43f1897de4611672ac65b4db954be0 Mon Sep 17 00:00:00 2001
From: Mikko Rasa <tdb@tdb.fi>
Date: Sat, 1 Oct 2022 23:05:45 +0300
Subject: [PATCH] Hack to work around an issue with the flat interpolation
 qualifier

It appears that at least on my current system (GeForce 980 Ti, driver
460.91.03) vertex attributes somehow block the flat qualifier from
working on the same location.  It feels like a driver bug but that
remains to be confirmed.
---
 source/glsl/finalize.cpp | 6 +++++-
 source/glsl/finalize.h   | 4 ++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/source/glsl/finalize.cpp b/source/glsl/finalize.cpp
index d840648b..469cb92d 100644
--- a/source/glsl/finalize.cpp
+++ b/source/glsl/finalize.cpp
@@ -91,6 +91,8 @@ void LocationAllocator::apply(Stage &stage)
 	stage.content.visit(*this);
 
 	allocate_locations("in");
+	if(stage.type==Stage::VERTEX)
+		swap(used_locations["in"], used_vertex_attribs);
 	allocate_locations("out");
 }
 
@@ -121,6 +123,8 @@ void LocationAllocator::allocate_locations(const string &iface)
 		if(!alloc_new)
 			continue;
 
+		bool flat = ((*i)->interpolation=="flat" || ((*i)->linked_declaration && (*i)->linked_declaration->interpolation=="flat"));
+
 		set<unsigned> &used = used_locations[(*i)->interface];
 
 		unsigned size = LocationCounter().apply(**i);
@@ -128,7 +132,7 @@ void LocationAllocator::allocate_locations(const string &iface)
 		{
 			int blocking = -1;
 			for(unsigned j=0; j<size; ++j)
-				if(used.count(next+j))
+				if(used.count(next+j) || (flat && used_vertex_attribs.count(next+j)))
 					blocking = next+j;
 			if(blocking<0)
 				break;
diff --git a/source/glsl/finalize.h b/source/glsl/finalize.h
index ec5f459a..4fc2e1b0 100644
--- a/source/glsl/finalize.h
+++ b/source/glsl/finalize.h
@@ -43,6 +43,10 @@ private:
 	std::vector<VariableDeclaration *> unbound_textures;
 	std::vector<VariableDeclaration *> unbound_blocks;
 
+	/* Dirty hack to work around an issue where vertex attributes prevent the
+	flat qualifier from working on the same location. */
+	std::set<unsigned> used_vertex_attribs;
+
 public:
 	void apply(Module &, const Features &, bool = true);
 private:
-- 
2.43.0