Mesa (master): i965/fs: add a comment about how the rounding mode in fmul is set

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Module: Mesa
Branch: master
Commit: 6f1468c371c7ceebf4d70933d60f70ef119fedcc
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=6f1468c371c7ceebf4d70933d60f70ef119fedcc

Author: Andres Gomez <agomez at igalia.com>
Date:   Tue Sep 24 01:16:11 2019 +0300

i965/fs: add a comment about how the rounding mode in fmul is set

After
1711bf6cf2d ("intel/fs: Generate better code for fsign multiplied by a value"),
the conflicts resolution for setting the rounding mode after the
fused fmul and fsign optimization is non obvious.

Basically, the optimization doesn't really result in a MUL, or any
other operation which would need to have the rounding mode set. Hence,
we set it just before the actual MUL in the treatment of fmul.

Fixes: ba1e25e1aa6 ("i965/fs: set rounding mode when emitting fadd, fmul and ffma instructions")
Suggested-by: Caio Marcelo de Oliveira Filho <caio.oliveira at intel.com>
Signed-off-by: Andres Gomez <agomez at igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira at intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias at igalia.com>

---

 src/intel/compiler/brw_fs_nir.cpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/intel/compiler/brw_fs_nir.cpp b/src/intel/compiler/brw_fs_nir.cpp
index f9e81e14f3a..5b37f4dc382 100644
--- a/src/intel/compiler/brw_fs_nir.cpp
+++ b/src/intel/compiler/brw_fs_nir.cpp
@@ -1258,6 +1258,10 @@ fs_visitor::nir_emit_alu(const fs_builder &bld, nir_alu_instr *instr,
          }
       }
 
+      /* We emit the rounding mode after the previous fsign optimization since
+       * it won't result in a MUL, but will try to negate the value by other
+       * means.
+       */
       if (nir_has_any_rounding_mode_enabled(execution_mode)) {
          brw_rnd_mode rnd =
             brw_rnd_mode_from_execution_mode(execution_mode);