Mesa (master): radeonsi: clean up ffma handling

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Module: Mesa
Branch: master
Commit: 758ab39d25e10d585929b87a8a2891c5a68b7c55
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=758ab39d25e10d585929b87a8a2891c5a68b7c55

Author: Marek Olšák <marek.olsak at amd.com>
Date:   Fri Sep  4 06:25:47 2020 -0400

radeonsi: clean up ffma handling

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer at amd.com>
Reviewed-by: Connor Abbott <cwabbott0 at gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6596>

---

 src/amd/llvm/ac_nir_to_llvm.c         |  8 ++++----
 src/gallium/drivers/radeonsi/si_get.c | 11 ++++++++++-
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/src/amd/llvm/ac_nir_to_llvm.c b/src/amd/llvm/ac_nir_to_llvm.c
index d227358322d..b50314dec47 100644
--- a/src/amd/llvm/ac_nir_to_llvm.c
+++ b/src/amd/llvm/ac_nir_to_llvm.c
@@ -853,10 +853,10 @@ static void visit_alu(struct ac_nir_context *ctx, const nir_alu_instr *instr)
       }
       break;
    case nir_op_ffma:
-      /* FMA is better on GFX10, because it has FMA units instead of MUL-ADD units. */
-      result =
-         emit_intrin_3f_param(&ctx->ac, ctx->ac.chip_class >= GFX10 ? "llvm.fma" : "llvm.fmuladd",
-                              ac_to_float_type(&ctx->ac, def_type), src[0], src[1], src[2]);
+      /* FMA is slow on gfx6-8, so it shouldn't be used. */
+      assert(ctx->ac.chip_class >= GFX9);
+      result = emit_intrin_3f_param(&ctx->ac, "llvm.fma", ac_to_float_type(&ctx->ac, def_type),
+                                    src[0], src[1], src[2]);
       break;
    case nir_op_ldexp:
       src[0] = ac_to_float(&ctx->ac, src[0]);
diff --git a/src/gallium/drivers/radeonsi/si_get.c b/src/gallium/drivers/radeonsi/si_get.c
index 059bd8d00e8..e7479012ea9 100644
--- a/src/gallium/drivers/radeonsi/si_get.c
+++ b/src/gallium/drivers/radeonsi/si_get.c
@@ -937,7 +937,16 @@ void si_init_screen_get_functions(struct si_screen *sscreen)
       .lower_bitfield_insert_to_bitfield_select = true,
       .lower_bitfield_extract = true,
       .lower_sub = true,
-      .fuse_ffma = true,
+      /* gfx6-8: use MAD (FMA is 4x slower)
+       * gfx9-10: either is OK (MAD and FMA have the same performance)
+       * gfx10.3: use FMA (MAD doesn't exist, separate MUL+ADD are 2x slower)
+       *
+       * FMA has no advantage on gfx9-10 and MAD allows more algebraic optimizations.
+       * Keep FMA enabled on gfx10 to test it, which helps us validate correctness
+       * for gfx10.3 on gfx10.
+       */
+      .lower_ffma = sscreen->info.chip_class <= GFX9,
+      .fuse_ffma = sscreen->info.chip_class >= GFX10,
       .lower_fmod = true,
       .lower_pack_snorm_4x8 = true,
       .lower_pack_unorm_4x8 = true,