diff --git a/src/mesa/shader/slang/slang_codegen.c b/src/mesa/shader/slang/slang_codegen.c
index ecb2f6d..34784bb 100644
--- a/src/mesa/shader/slang/slang_codegen.c
+++ b/src/mesa/shader/slang/slang_codegen.c
@@ -5118,21 +5118,11 @@ _slang_codegen_global_variable(slang_assemble_ctx *A, slang_variable *var,
       }
 
       if (prog) {
-         /* user-defined varying */
-         GLbitfield flags;
-         GLint varyingLoc;
-         GLuint swizzle;
-
-         flags = 0x0;
-         if (var->type.centroid == SLANG_CENTROID)
-            flags |= PROG_PARAM_BIT_CENTROID;
-         if (var->type.variant == SLANG_INVARIANT)
-            flags |= PROG_PARAM_BIT_INVARIANT;
-
-         varyingLoc = _mesa_add_varying(prog->Varying, varName,
-                                        totalSize, flags);
-         swizzle = _slang_var_swizzle(size, 0);
-         store = _slang_new_ir_storage_swz(PROGRAM_VARYING, varyingLoc,
+         /* User-defined varying.  We'll allocate storage (a register slot)
+          * for it later when it's first used.  See emit_var_ref().
+          */
+         GLuint swizzle = _slang_var_swizzle(size, 0);
+         store = _slang_new_ir_storage_swz(PROGRAM_VARYING, -1,
                                            totalSize, swizzle);
       }
       else {
diff --git a/src/mesa/shader/slang/slang_emit.c b/src/mesa/shader/slang/slang_emit.c
index 7c0ea0c..efcb4c0 100644
--- a/src/mesa/shader/slang/slang_emit.c
+++ b/src/mesa/shader/slang/slang_emit.c
@@ -2286,6 +2286,24 @@ emit_var_decl(slang_emit_info *emitInfo, slang_ir_node *n)
 
 
 /**
+ * Examine the fully specified type to gather the optional flags for a
+ * varying variable.  CENTROID and INVARIANT are the only flags at this time.
+ */
+static GLbitfield
+compute_varying_flags(const slang_fully_specified_type *type)
+{
+   GLbitfield flags = 0x0;
+
+   if (type->centroid == SLANG_CENTROID)
+      flags |= PROG_PARAM_BIT_CENTROID;
+   if (type->variant == SLANG_INVARIANT)
+      flags |= PROG_PARAM_BIT_INVARIANT;
+
+   return flags;
+}
+
+
+/**
  * Emit code for a reference to a variable.
  * Actually, no code is generated but we may do some memory allocation.
  * In particular, state vars (uniforms) are allocated on an as-needed basis.
@@ -2293,6 +2311,8 @@ emit_var_decl(slang_emit_info *emitInfo, slang_ir_node *n)
 static struct prog_instruction *
 emit_var_ref(slang_emit_info *emitInfo, slang_ir_node *n)
 {
+   const char *varName = n->Var ? (const char *) n->Var->a_name : NULL;
+
    assert(n->Store);
    assert(n->Store->File != PROGRAM_UNDEFINED);
 
@@ -2303,18 +2323,23 @@ emit_var_ref(slang_emit_info *emitInfo, slang_ir_node *n)
          /* error */
          char s[100];
          /* XXX isn't this really an out of memory/resources error? */
-         _mesa_snprintf(s, sizeof(s), "Undefined variable '%s'",
-		  (char *) n->Var->a_name);
+         _mesa_snprintf(s, sizeof(s), "Undefined variable '%s'", varName);
          slang_info_log_error(emitInfo->log, s);
          return NULL;
       }
 
       n->Store->Index = index;
    }
+   else if (n->Store->File == PROGRAM_VARYING && n->Store->Index < 0) {
+      /* Allocate storage for a user-defined varying variable */
+      const GLbitfield flags = compute_varying_flags(&n->Var->type);
+      n->Store->Index = _mesa_add_varying(emitInfo->prog->Varying, varName,
+                                          n->Store->Size, flags);
+   }
    else if (n->Store->File == PROGRAM_UNIFORM ||
             n->Store->File == PROGRAM_SAMPLER) {
       /* mark var as used */
-      _mesa_use_uniform(emitInfo->prog->Parameters, (char *) n->Var->a_name);
+      _mesa_use_uniform(emitInfo->prog->Parameters, varName);
    }
    else if (n->Store->File == PROGRAM_INPUT) {
       assert(n->Store->Index >= 0);