From 1f135f709ba63609e4eee6776a23a5fa50e66156 Mon Sep 17 00:00:00 2001
From: treeform <starplant@gmail.com>
Date: Wed, 12 Aug 2026 20:41:45 -0700
Subject: [PATCH] add portable shader instance IDs

---
 src/shady/backends/dx12.nim   | 13 +++++++--
 src/shady/backends/glsl.nim   |  2 +-
 src/shady/backends/metal4.nim |  7 ++++-
 src/shady/backends/shared.nim | 14 +++++++--
 tests/test_backends.nim       | 54 +++++++++++++++++++++++++++++++++++
 tests/test_metal.nim          |  8 +++++-
 6 files changed, 90 insertions(+), 8 deletions(-)

diff --git a/src/shady/backends/dx12.nim b/src/shady/backends/dx12.nim
index 1c90264..1ddf958 100644
--- a/src/shady/backends/dx12.nim
+++ b/src/shady/backends/dx12.nim
@@ -121,6 +121,8 @@ proc hlslSemantic*(name: string, index: int, isOutput: bool): string =
     " : SV_IsFrontFace"
   of "gl_VertexID":
     " : SV_VertexID"
+  of "gl_InstanceID":
+    " : SV_InstanceID"
   else:
     let base = semanticBase(name)
     if base.startsWith("SV_"):
@@ -133,8 +135,8 @@ proc nextSemanticIndex(
   counts: var Table[string, int]
 ): int =
   case name
-  of "fragColor", "gl_FragColor", "gl_Position", "gl_FragCoord", "gl_VertexID",
-      "gl_FrontFacing":
+  of "fragColor", "gl_FragColor", "gl_Position", "gl_FragCoord",
+      "gl_VertexID", "gl_InstanceID", "gl_FrontFacing":
     return 0
   else:
     let base = semanticBase(name)
@@ -291,6 +293,13 @@ proc emitHlslEntry*(
       else:
         result.add ",\n"
       result.add "  uint gl_VertexID : SV_VertexID"
+    if not params.hasParam("gl_InstanceID") and "gl_InstanceID" in bodyCode:
+      if first:
+        result.add "\n"
+        first = false
+      else:
+        result.add ",\n"
+      result.add "  uint gl_InstanceID : SV_InstanceID"
     if not first:
       result.add "\n"
     result.add ") {\n"
diff --git a/src/shady/backends/glsl.nim b/src/shady/backends/glsl.nim
index bc1d662..34a5d0a 100644
--- a/src/shady/backends/glsl.nim
+++ b/src/shady/backends/glsl.nim
@@ -75,7 +75,7 @@ proc glslTypeDefault*(t: string): string =
 
 const glslGlobals* = [
   "gl_Position", "gl_FragCoord", "gl_GlobalInvocationID",
-  "gl_VertexID", "gl_FrontFacing",
+  "gl_VertexID", "gl_InstanceID", "gl_FrontFacing",
 ]
 
 const glslFunctions* = [
diff --git a/src/shady/backends/metal4.nim b/src/shady/backends/metal4.nim
index 71b6292..1bb39da 100644
--- a/src/shady/backends/metal4.nim
+++ b/src/shady/backends/metal4.nim
@@ -102,6 +102,8 @@ proc metalAttribute*(name: string, index: int, isOutput: bool): string =
     " [[position]]"
   of "gl_VertexID":
     " [[vertex_id]]"
+  of "gl_InstanceID":
+    " [[instance_id]]"
   of "gl_FrontFacing":
     " [[front_facing]]"
   else:
@@ -196,7 +198,7 @@ proc hasParam(params: openArray[EntryParam], name: string): bool =
       return true
 
 proc isSpecialInput(p: EntryParam): bool =
-  p.name in ["gl_VertexID", "gl_FrontFacing"]
+  p.name in ["gl_VertexID", "gl_InstanceID", "gl_FrontFacing"]
 
 proc outputLocals(params: openArray[EntryParam], level: int): string =
   let indent = repeat("  ", level)
@@ -322,6 +324,9 @@ proc emitMetalEntry*(
     if not params.hasParam("gl_VertexID") and "gl_VertexID" in bodyCode:
       result.addComma(first)
       result.add "  uint gl_VertexID [[vertex_id]]"
+    if not params.hasParam("gl_InstanceID") and "gl_InstanceID" in bodyCode:
+      result.addComma(first)
+      result.add "  uint gl_InstanceID [[instance_id]]"
     for resource in resourceParams(uniforms, textures, stage):
       result.addComma(first)
       result.add "  "
diff --git a/src/shady/backends/shared.nim b/src/shady/backends/shared.nim
index 7a9b165..8d0fd7f 100644
--- a/src/shady/backends/shared.nim
+++ b/src/shady/backends/shared.nim
@@ -177,7 +177,7 @@ proc typeDefault(t: string, n: NimNode): string =
       hlslTypeDefault(t)
     of langMetal:
       metalTypeDefault(t)
-  if result.len == 0:
+  if result.len == 0 and (t.len == 0 or not t[0].isUpperAscii):
     err "no typeDefault " & t, n
 
 ## Simply SKIP these functions.
@@ -200,6 +200,11 @@ proc isVectorAccess(s: string): bool =
 
 proc procRename(t: string): string =
   ## Some shader proc names don't match Nim names, rename here.
+  if isVulkan():
+    if t == "gl_VertexID":
+      return "gl_VertexIndex"
+    if t == "gl_InstanceID":
+      return "gl_InstanceIndex"
   case shaderTarget.language
   of langGlsl:
     glslProcRename(t)
@@ -681,8 +686,10 @@ proc toCode(n: NimNode, res: var string, level = 0) =
           res.add " = "
           n[j + 2].toCode(res)
         else:
-          res.add " = "
-          res.add typeDefault(typeStr, n[j])
+          let defaultValue = typeDefault(typeStr, n[j])
+          if defaultValue.len > 0:
+            res.add " = "
+            res.add defaultValue
 
   of nnkReturnStmt:
     res.addIndent level
@@ -1551,6 +1558,7 @@ var
   ## GLSL globals.
   gl_Position*: Vec4
   gl_VertexID*: int32
+  gl_InstanceID*: int32
   gl_FrontFacing*: bool
 
 proc mat3*(m: Mat4): Mat3 =
diff --git a/tests/test_backends.nim b/tests/test_backends.nim
index acc1453..7d48884 100644
--- a/tests/test_backends.nim
+++ b/tests/test_backends.nim
@@ -54,6 +54,60 @@ block:
   doAssert "float4 position [[position]];" in msl
   doAssert "output.position = gl_Position;" in msl
 
+block:
+  proc instancedVertex(gl_Position: var Vec4) =
+    ## Exercises portable vertex and instance identifiers.
+    let
+      vertex = float32(gl_VertexID)
+      instance = float32(gl_InstanceID)
+    gl_Position = vec4(vertex, instance, 0.0, 1.0)
+
+  let glsl = toShader(instancedVertex, glsl4Desktop, shaderVertex)
+  doAssert "gl_VertexID" in glsl
+  doAssert "gl_InstanceID" in glsl
+
+  let web = toShader(instancedVertex, glsl3WebGL, shaderVertex)
+  doAssert "gl_VertexID" in web
+  doAssert "gl_InstanceID" in web
+
+  let vulkan = toShader(instancedVertex, vulkanGlsl450, shaderVertex)
+  doAssert "gl_VertexIndex" in vulkan
+  doAssert "gl_InstanceIndex" in vulkan
+
+  let hlsl = toHLSL(instancedVertex, shaderVertex)
+  doAssert "uint gl_VertexID : SV_VertexID" in hlsl
+  doAssert "uint gl_InstanceID : SV_InstanceID" in hlsl
+
+  let msl = toMSL(instancedVertex, shaderVertex)
+  doAssert "uint gl_VertexID [[vertex_id]]" in msl
+  doAssert "uint gl_InstanceID [[instance_id]]" in msl
+
+block:
+  type Params = object
+    offset: float32
+
+  proc readOffset(params: Params): float32 =
+    ## Returns a field from a shader-local parameter structure.
+    params.offset
+
+  proc structuredVertex(gl_Position: var Vec4) =
+    ## Exercises an uninitialized shader-local parameter structure.
+    var params: Params
+    params.offset = gl_InstanceID.float32
+    gl_Position = vec4(readOffset(params), 0.0, 0.0, 1.0)
+
+  let glsl = toShader(structuredVertex, glsl4Desktop, shaderVertex)
+  doAssert "struct Params" in glsl
+  doAssert "Params params;" in glsl
+
+  let hlsl = toHLSL(structuredVertex, shaderVertex)
+  doAssert "struct Params" in hlsl
+  doAssert "Params params;" in hlsl
+
+  let msl = toMSL(structuredVertex, shaderVertex)
+  doAssert "struct Params" in msl
+  doAssert "Params params;" in msl
+
 block:
   var atlas: Uniform[Sampler2d]
 
diff --git a/tests/test_metal.nim b/tests/test_metal.nim
index 37693d8..6d5b499 100644
--- a/tests/test_metal.nim
+++ b/tests/test_metal.nim
@@ -8,7 +8,13 @@ when defined(macosx) and defined(shadyRunMetal):
     gl_Position: var Vec4,
     vertColor: var Vec3
   ) =
-    gl_Position = vec4(vPos.x, vPos.y, vPos.z, 1.0)
+    let instanceOffset = gl_InstanceID.float32 * 0.001
+    gl_Position = vec4(
+      vPos.x + instanceOffset,
+      vPos.y,
+      vPos.z,
+      1.0
+    )
     vertColor = vCol
 
   proc fragmentMain(fragColor: var Vec4) =