[Mesa-dev] [PATCH v3 25/42] intel/compiler: workaround for SIMD8 half-float MAD in gen8

[Iago Toral Quiroga]

Broadwell hardware has a bug that manifests in SIMD8 executions of
16-bit MAD instructions when any of the sources is a Y or W component.
We pack these components in the same SIMD register as components X and
Z respectively, but starting at offset 16B (so they live in the second
half of the register). The problem does not exist in SKL or later.

We work around this issue by moving any such sources to a temporary
starting at offset 0B. We want to do this after the main optimization loop
to prevent copy-propagation and friends to undo the fix.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen at intel.com>
---
 src/intel/compiler/brw_fs.cpp | 48 +++++++++++++++++++++++++++++++++++
 src/intel/compiler/brw_fs.h   |  1 +
 2 files changed, 49 insertions(+)

diff --git a/src/intel/compiler/brw_fs.cpp b/src/intel/compiler/brw_fs.cpp
index 0b3ec94e2d2..d6096cd667d 100644
--- a/src/intel/compiler/brw_fs.cpp
+++ b/src/intel/compiler/brw_fs.cpp
@@ -6540,6 +6540,48 @@ fs_visitor::optimize()
    validate();
 }
 
+/**
+ * Broadwell hardware has a bug that manifests in SIMD8 executions of 16-bit
+ * MAD instructions when any of the sources is a Y or W component. We pack
+ * these components in the same SIMD register as components X and Z
+ * respectively, but starting at offset 16B (so they live in the second half
+ * of the register).
+ *
+ * We work around this issue by moving any such sources to a temporary
+ * starting at offset 0B. We want to do this after the main optimization loop
+ * to prevent copy-propagation and friends to undo the fix.
+ */
+void
+fs_visitor::fixup_hf_mad()
+{
+   if (devinfo->gen != 8)
+      return;
+
+   bool progress = false;
+
+   foreach_block_and_inst_safe (block, fs_inst, inst, cfg) {
+      if (inst->opcode != BRW_OPCODE_MAD ||
+          inst->dst.type != BRW_REGISTER_TYPE_HF ||
+          inst->exec_size > 8)
+         continue;
+
+      for (int i = 0; i < 3; i++) {
+         if (inst->src[i].offset > 0) {
+            assert(inst->src[i].type == BRW_REGISTER_TYPE_HF);
+            const fs_builder ibld =
+               bld.at(block, inst).exec_all().group(inst->exec_size, 0);
+            fs_reg tmp = ibld.vgrf(inst->src[i].type);
+            ibld.MOV(tmp, inst->src[i]);
+            inst->src[i] = tmp;
+            progress = true;
+         }
+      }
+   }
+
+   if (progress)
+      invalidate_live_intervals();
+}
+
 /**
  * Three source instruction must have a GRF/MRF destination register.
  * ARF NULL is not allowed.  Fix that up by allocating a temporary GRF.
@@ -6698,6 +6740,7 @@ fs_visitor::run_vs()
    assign_curb_setup();
    assign_vs_urb_setup();
 
+   fixup_hf_mad();
    fixup_3src_null_dest();
    allocate_registers(8, true);
 
@@ -6782,6 +6825,7 @@ fs_visitor::run_tcs_single_patch()
    assign_curb_setup();
    assign_tcs_single_patch_urb_setup();
 
+   fixup_hf_mad();
    fixup_3src_null_dest();
    allocate_registers(8, true);
 
@@ -6816,6 +6860,7 @@ fs_visitor::run_tes()
    assign_curb_setup();
    assign_tes_urb_setup();
 
+   fixup_hf_mad();
    fixup_3src_null_dest();
    allocate_registers(8, true);
 
@@ -6865,6 +6910,7 @@ fs_visitor::run_gs()
    assign_curb_setup();
    assign_gs_urb_setup();
 
+   fixup_hf_mad();
    fixup_3src_null_dest();
    allocate_registers(8, true);
 
@@ -6965,6 +7011,7 @@ fs_visitor::run_fs(bool allow_spilling, bool do_rep_send)
 
       assign_urb_setup();
 
+      fixup_hf_mad();
       fixup_3src_null_dest();
       allocate_registers(8, allow_spilling);
 
@@ -7009,6 +7056,7 @@ fs_visitor::run_cs(unsigned min_dispatch_width)
 
    assign_curb_setup();
 
+   fixup_hf_mad();
    fixup_3src_null_dest();
    allocate_registers(min_dispatch_width, true);
 
diff --git a/src/intel/compiler/brw_fs.h b/src/intel/compiler/brw_fs.h
index 68287bcdcea..1879d4bc7f7 100644
--- a/src/intel/compiler/brw_fs.h
+++ b/src/intel/compiler/brw_fs.h
@@ -103,6 +103,7 @@ public:
    void setup_vs_payload();
    void setup_gs_payload();
    void setup_cs_payload();
+   void fixup_hf_mad();
    void fixup_3src_null_dest();
    void assign_curb_setup();
    void calculate_urb_setup();
-- 
2.17.1