shaderir: Change the param order

internal/shaderir/glsl.go

@@ -249,10 +249,10 @@ func (p *Program) glslBlock(b *Block, level int, localVarIndex int) []string {
 				switch {
 				case idx < na:
 					return fmt.Sprintf("A%d", idx)
-				case idx < na+nv:
-					return fmt.Sprintf("V%d", idx-na)
-				case idx == na+nv:
+				case idx == na:
 					return "gl_Position"
+				case idx < na+nv+1:
+					return fmt.Sprintf("V%d", idx-na-1)
 				default:
 					return fmt.Sprintf("l%d", idx-(na+nv+1))
 				}

internal/shaderir/ir_test.go

@@ -646,15 +646,15 @@ varying vec3 V0;`,
 					Block: block(
 						nil,
 						assignStmt(
-							localVariableExpr(5),
+							localVariableExpr(3),
 							localVariableExpr(0),
 						),
 						assignStmt(
-							localVariableExpr(3),
+							localVariableExpr(4),
 							localVariableExpr(1),
 						),
 						assignStmt(
-							localVariableExpr(4),
+							localVariableExpr(5),
 							localVariableExpr(2),
 						),
 					),
@@ -695,15 +695,15 @@ varying vec2 V1;`,
 					Block: block(
 						nil,
 						assignStmt(
-							localVariableExpr(5),
+							localVariableExpr(3),
 							localVariableExpr(0),
 						),
 						assignStmt(
-							localVariableExpr(3),
+							localVariableExpr(4),
 							localVariableExpr(1),
 						),
 						assignStmt(
-							localVariableExpr(4),
+							localVariableExpr(5),
 							localVariableExpr(2),
 						),
 					),

internal/shaderir/program.go

@@ -37,10 +37,10 @@ type Func struct {
 }
 
 // VertexFunc takes pseudo params, and the number if len(attributes) + len(varyings) + 1.
-// If 0 <= index < len(attributes), the params are in-params and treated as attribute variables.
-// If len(attributes) <= index < len(attributes) + len(varyings), the params are out-params and treated as varying
-// variables.
-// The last param represents the position in vec4 (gl_Position in GLSL).
+// If 0 <= index < len(attributes), the params are in-params and represent attribute variables.
+// If index == len(attributes), the param is an out-param and repesents the position in vec4 (gl_Position in GLSL)
+// If len(attributes) + 1 <= index < len(attributes) + len(varyings) + 1, the params are out-params and represent
+// varying variables.
 type VertexFunc struct {
 	Block Block
 }

internal/testing/shader.go

@@ -159,7 +159,7 @@ var (
 					Exprs: []shaderir.Expr{
 						{
 							Type:  shaderir.LocalVariable,
-							Index: 4, // the varying variable
+							Index: 5, // the varying variable
 						},
 						{
 							Type:  shaderir.LocalVariable,
@@ -172,7 +172,7 @@ var (
 					Exprs: []shaderir.Expr{
 						{
 							Type:  shaderir.LocalVariable,
-							Index: 5, // gl_Position in GLSL
+							Index: 4, // gl_Position in GLSL
 						},
 						{
 							Type: shaderir.Binary,