i965: Use SIMD16 instead of SIMD8 on Gen4 when possible.

Gen5+ systems allow you to specify multiple shader programs - both SIMD8
and SIMD16 - and the hardware will automatically dispatch to the most
appropriate one, given the number of subspans to be processed.

However, that is not the case on Gen4.  Instead, you program a single
shader.  If you enable multiple dispatch modes (SIMD8 and SIMD16), the
shader is supposed to contain a series of jump instructions at the
beginning.  The hardware will launch the shader at a small offset,
hitting one of the jumps.

We've always thought that sounds like a pain, and weren't clear how it
affected performance - is it worth having multiple shader types?  So,
we never bothered with SIMD16 until now.

This patch takes a simpler approach: try and compile a SIMD16 shader.
If possible, set the no_8 flag, telling the hardware to just use the
SIMD16 variant all the time.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -4030,8 +4030,7 @@ brw_wm_fs_emit(struct brw_context *brw,
 
    cfg_t *simd16_cfg = NULL;
    fs_visitor v2(brw, mem_ctx, key, prog_data, prog, fp, 16);
-   if (brw->gen >= 5 && likely(!(INTEL_DEBUG & DEBUG_NO16) ||
-                               brw->use_rep_send)) {
+   if (likely(!(INTEL_DEBUG & DEBUG_NO16) || brw->use_rep_send)) {
       if (!v.simd16_unsupported) {
          /* Try a SIMD16 compile */
          v2.import_uniforms(&v);
@@ -4049,7 +4048,7 @@ brw_wm_fs_emit(struct brw_context *brw,
 
    cfg_t *simd8_cfg;
    int no_simd8 = (INTEL_DEBUG & DEBUG_NO8) || brw->no_simd8;
-   if (no_simd8 && simd16_cfg) {
+   if ((no_simd8 || brw->gen < 5) && simd16_cfg) {
       simd8_cfg = NULL;
       prog_data->no_8 = true;
    } else {

src/mesa/drivers/dri/i965/brw_fs_visitor.cpp

@@ -1433,6 +1433,8 @@ fs_visitor::emit_texture_gen4(ir_texture_opcode op, fs_reg dst,
    bool simd16 = false;
    fs_reg orig_dst;
 
+   no16("SIMD16 texturing on Gen4 not supported yet.");
+
    /* g0 header. */
    mlen = 1;