From 30ecf75a0feb84c433e2502f16d5e667222d6a99 Mon Sep 17 00:00:00 2001
From: Elias Naur <mail@eliasnaur.com>
Date: Tue, 5 Oct 2021 08:41:12 +0200
Subject: [PATCH] gpu/internal/opengl: save and restore GL state only for
 shared contexts

Most Gio programs have exlusive access to their OpenGL contexts, where
saving and restoring of the GL state is not required.

This change applies to all OpenGL platforms, but matters most for WASM
where syscalls are slow.

Signed-off-by: Elias Naur <mail@eliasnaur.com>
---
 gpu/internal/driver/api.go    |  3 +++
 gpu/internal/opengl/opengl.go | 17 ++++++++++++++---
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/gpu/internal/driver/api.go b/gpu/internal/driver/api.go
index 12b4e1a9..9a762a67 100644
--- a/gpu/internal/driver/api.go
+++ b/gpu/internal/driver/api.go
@@ -50,6 +50,9 @@ type OpenGL struct {
 	// empty for all other platforms; an OpenGL context is assumed current when
 	// calling NewDevice.
 	Context gl.Context
+	// Shared instructs users of the context to restore the GL state after
+	// use.
+	Shared bool
 }
 
 type Direct3D11 struct {
diff --git a/gpu/internal/opengl/opengl.go b/gpu/internal/opengl/opengl.go
index af4a673c..ef891973 100644
--- a/gpu/internal/opengl/opengl.go
+++ b/gpu/internal/opengl/opengl.go
@@ -23,6 +23,7 @@ type Backend struct {
 	glstate    glState
 	state      state
 	savedState glState
+	sharedCtx  bool
 
 	glver [2]int
 	gles  bool
@@ -201,6 +202,7 @@ func newOpenGLDevice(api driver.OpenGL) (driver.Device, error) {
 		floatTriple: floatTriple,
 		alphaTriple: alphaTripleFor(ver),
 		srgbaTriple: srgbaTriple,
+		sharedCtx:   api.Shared,
 	}
 	b.feats.BottomLeftOrigin = true
 	if srgbErr == nil {
@@ -216,13 +218,20 @@ func newOpenGLDevice(api driver.OpenGL) (driver.Device, error) {
 		b.feats.Features |= driver.FeatureTimers
 	}
 	b.feats.MaxTextureSize = f.GetInteger(gl.MAX_TEXTURE_SIZE)
+	if !b.sharedCtx {
+		// We have exclusive access to the context, so query the GL state once
+		// instead of at each frame.
+		b.glstate = b.queryState()
+	}
 	return b, nil
 }
 
 func (b *Backend) BeginFrame(target driver.RenderTarget, clear bool, viewport image.Point) driver.Texture {
 	b.clear = clear
-	b.glstate = b.queryState()
-	b.savedState = b.glstate
+	if b.sharedCtx {
+		b.glstate = b.queryState()
+		b.savedState = b.glstate
+	}
 	b.state = state{}
 	var renderFBO gl.Framebuffer
 	if target != nil {
@@ -286,7 +295,9 @@ func (b *Backend) EndFrame() {
 		}
 		b.sRGBFBO.Blit()
 	}
-	b.restoreState(b.savedState)
+	if b.sharedCtx {
+		b.restoreState(b.savedState)
+	}
 }
 
 func (b *Backend) queryState() glState {