[Mesa-dev] [PATCH] i965/gs: Avoid DW * DW mul

[Ben Widawsky]

The GS has an interesting use for mul. It's essentially used as a fancy mov (in
fact, I am not sure why a mov isn't used). The documentation in the function has
a very good explanation from Paul on the mechanics.

CHV has some quirks with regard to multiplication. While the documentation is
somewhat unclear, I've found that demoting the src1 operand in the GS mul solves
all the problems. I'd ask that any potential reviewer ignore the other instances
of mul for now (I have more patches), and simply make sure that what this patch
does is correct.

This fixes around 2000 piglit tests on BSW.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=84777 (with many dupes)
Cc: "10.4" <mesa-stable at lists.freedesktop.org>
Signed-off-by: Ben Widawsky <ben at bwidawsk.net>

---
 src/mesa/drivers/dri/i965/brw_vec4_generator.cpp  | 5 ++++-
 src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp | 3 ++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_vec4_generator.cpp b/src/mesa/drivers/dri/i965/brw_vec4_generator.cpp
index b353539..4f60797 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_generator.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_generator.cpp
@@ -534,8 +534,11 @@ vec4_generator::generate_gs_set_write_offset(struct brw_reg dst,
    brw_push_insn_state(p);
    brw_set_default_access_mode(p, BRW_ALIGN_1);
    brw_set_default_mask_control(p, BRW_MASK_DISABLE);
+   assert(src1.file == BRW_IMMEDIATE_VALUE &&
+          src1.type == BRW_REGISTER_TYPE_UD &&
+          src1.dw1.ud <= SHRT_MAX);
    brw_MUL(p, suboffset(stride(dst, 2, 2, 1), 3), stride(src0, 8, 2, 4),
-           src1);
+           retype(src1, BRW_REGISTER_TYPE_UW));
    brw_set_default_access_mode(p, BRW_ALIGN_16);
    brw_pop_insn_state(p);
 }
diff --git a/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
index db0e6cc..2d7ae5a 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_gs_visitor.cpp
@@ -388,7 +388,8 @@ vec4_gs_visitor::emit_control_data_bits()
           */
          src_reg per_slot_offset(this, glsl_type::uint_type);
          emit(SHR(dst_reg(per_slot_offset), dword_index, 2u));
-         emit(GS_OPCODE_SET_WRITE_OFFSET, mrf_reg, per_slot_offset, 1u);
+         emit(GS_OPCODE_SET_WRITE_OFFSET, mrf_reg, per_slot_offset,
+              src_reg(1u));
       }
 
       if (urb_write_flags & BRW_URB_WRITE_USE_CHANNEL_MASKS) {
-- 
2.1.3