From ab54f683bee5d0480e6c7830cb0e3b683f6a2ff6 Mon Sep 17 00:00:00 2001
From: Dzmitry Malyshau <kvarkus@gmail.com>
Date: Thu, 1 Apr 2021 01:25:34 -0400
Subject: [PATCH] [spv-in] flip Y axis

---
 src/front/spv/function.rs    | 37 ++++++++++++++++++++++++++++++++----
 src/front/spv/mod.rs         |  1 +
 tests/out/quad-vert.msl.snap |  1 +
 3 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/src/front/spv/function.rs b/src/front/spv/function.rs
index 082c9ff170..8a71b4037a 100644
--- a/src/front/spv/function.rs
+++ b/src/front/spv/function.rs
@@ -279,12 +279,41 @@ impl<I: Iterator<Item = u32>> super::Parser<I> {
                 }
             }
 
-            let old_len = function.expressions.len();
-            for component in components.iter_mut() {
-                *component = function.expressions.append(crate::Expression::Load {
-                    pointer: *component,
+            let position_index = members.iter().position(|member| match member.binding {
+                Some(crate::Binding::BuiltIn(crate::BuiltIn::Position)) => true,
+                _ => false,
+            });
+            if let Some(component_index) = position_index {
+                let old_len = function.expressions.len();
+                let global_expr = components[component_index];
+                let access_expr = function.expressions.append(crate::Expression::AccessIndex {
+                    base: global_expr,
+                    index: 1,
+                });
+                let load_expr = function.expressions.append(crate::Expression::Load {
+                    pointer: access_expr,
+                });
+                let neg_expr = function.expressions.append(crate::Expression::Unary {
+                    op: crate::UnaryOperator::Negate,
+                    expr: load_expr,
+                });
+                function.body.push(crate::Statement::Emit(
+                    function.expressions.range_from(old_len),
+                ));
+                function.body.push(crate::Statement::Store {
+                    pointer: access_expr,
+                    value: neg_expr,
                 });
             }
+
+            let old_len = function.expressions.len();
+            for component in components.iter_mut() {
+                let load_expr = crate::Expression::Load {
+                    pointer: *component,
+                };
+                *component = function.expressions.append(load_expr);
+            }
+
             match members.len() {
                 0 => {}
                 1 => {
diff --git a/src/front/spv/mod.rs b/src/front/spv/mod.rs
index 651fc11a62..1fe4e51daa 100644
--- a/src/front/spv/mod.rs
+++ b/src/front/spv/mod.rs
@@ -1825,6 +1825,7 @@ impl<I: Iterator<Item = u32>> Parser<I> {
             crate::Module::default()
         };
 
+        // register indexing constants
         self.index_constants.clear();
         for i in 0..4 {
             let handle = module.constants.append(crate::Constant {
diff --git a/tests/out/quad-vert.msl.snap b/tests/out/quad-vert.msl.snap
index 16b3529771..79d91a3cd4 100644
--- a/tests/out/quad-vert.msl.snap
+++ b/tests/out/quad-vert.msl.snap
@@ -69,6 +69,7 @@ vertex main2Output main2(
     a_uv = a_uv1;
     a_pos = a_pos1;
     main1(v_uv, a_uv, _, a_pos);
+    _.gl_Position.y = -_.gl_Position.y;
     const auto _tmp = type10 {v_uv, _.gl_Position, _.gl_PointSize, {_.gl_ClipDistance[0]}};
     return main2Output { _tmp.member, _tmp.gl_Position1, _tmp.gl_PointSize1, {_tmp.gl_ClipDistance1[0]} };
 }