mesa: fix GLSL issue preventing use of all 16 generic vertex attributes

Only 15 actually worked before since we always reserved generic[0] as an
alias for vertex position.

The case of vertex attribute 0 is tricky.  The spec says that there is no
aliasing between generic vertex attributes 0..MAX_VERTEX_ATTRIBS-1 and the
conventional attributes.  But it also says that calls to glVertexAttrib(0, v)
are equivalent to glVertex(v).  The distinction seems to be in glVertex-mode
versus vertex array mode.

So update the VBO code so that if the shader uses generic[0] but not gl_Vertex,
route the attribute data set with glVertex() to go to shader input generic[0].

No change needed for the glDrawArrays/Elements() path.

This is a potentially risky change so regressions are possible.  All the usual
tests seem OK though.

1 files changed, 11 insertions, 2 deletions

diff --git a/src/mesa/shader/slang/slang_link.c b/src/mesa/shader/slang/slang_link.c
index c6d5cc0566..77bcda0871 100644
--- a/src/mesa/shader/slang/slang_link.c
+++ b/src/mesa/shader/slang/slang_link.c
@@ -318,7 +318,7 @@ _slang_resolve_attributes(struct gl_shader_program *shProg,
 {
    GLint attribMap[MAX_VERTEX_ATTRIBS];
    GLuint i, j;
-   GLbitfield usedAttributes;
+   GLbitfield usedAttributes; /* generics only, not legacy attributes */
 
    assert(origProg != linkedProg);
    assert(origProg->Target == GL_VERTEX_PROGRAM_ARB);
@@ -342,6 +342,15 @@ _slang_resolve_attributes(struct gl_shader_program *shProg,
       usedAttributes |= (1 << attr);
    }
 
+   /* If gl_Vertex is used, that actually counts against the limit
+    * on generic vertex attributes.  This avoids the ambiguity of
+    * whether glVertexAttrib4fv(0, v) sets legacy attribute 0 (vert pos)
+    * or generic attribute[0].  If gl_Vertex is used, we want the former.
+    */
+   if (origProg->InputsRead & VERT_BIT_POS) {
+      usedAttributes |= 0x1;
+   }
+
    /* initialize the generic attribute map entries to -1 */
    for (i = 0; i < MAX_VERTEX_ATTRIBS; i++) {
       attribMap[i] = -1;
@@ -384,7 +393,7 @@ _slang_resolve_attributes(struct gl_shader_program *shProg,
                    * Start at 1 since generic attribute 0 always aliases
                    * glVertex/position.
                    */
-                  for (attr = 1; attr < MAX_VERTEX_ATTRIBS; attr++) {
+                  for (attr = 0; attr < MAX_VERTEX_ATTRIBS; attr++) {
                      if (((1 << attr) & usedAttributes) == 0)
                         break;
                   }