ac/nir: do not emit unnecessary null exports in fragment shaders

Null exports should only be needed when no other exports are
emitted. This removes a bunch of 'exp null off, off, off, off done vm'.

Affected games are Dota 2 and Wolfenstein 2, not sure if that
really helps, but code size is decreasing there.

Polaris10:
Totals from affected shaders:
SGPRS: 8216 -> 8216 (0.00 %)
VGPRS: 7072 -> 7072 (0.00 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 454968 -> 453896 (-0.24 %) bytes
Max Waves: 772 -> 772 (0.00 %)

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

diff --git a/src/amd/common/ac_nir_to_llvm.c b/src/amd/common/ac_nir_to_llvm.c
index c785244dcc4438307b62398f98d004dd81ce7c67..9b8506986083f05bfc5855b51418e29a9f4bac1b 100644 (file)
--- a/src/amd/common/ac_nir_to_llvm.c
+++ b/src/amd/common/ac_nir_to_llvm.c
@@ -6539,17 +6539,13 @@ handle_tcs_outputs_post(struct radv_shader_context *ctx)
 
 static bool
 si_export_mrt_color(struct radv_shader_context *ctx,
-                   LLVMValueRef *color, unsigned index, bool is_last,
+                   LLVMValueRef *color, unsigned index,
                    struct ac_export_args *args)
 {
        /* Export */
        si_llvm_init_export_args(ctx, color, 0xf,
                                 V_008DFC_SQ_EXP_MRT + index, args);
-
-       if (is_last) {
-               args->valid_mask = 1; /* whether the EXEC mask is valid */
-               args->done = 1; /* DONE bit */
-       } else if (!args->enabled_channels)
+       if (!args->enabled_channels)
                return false; /* unnecessary NULL export */
 
        return true;
@@ -6576,7 +6572,6 @@ handle_fs_outputs_post(struct radv_shader_context *ctx)
 
        for (unsigned i = 0; i < AC_LLVM_MAX_OUTPUTS; ++i) {
                LLVMValueRef values[4];
-               bool last = false;
 
                if (!(ctx->output_mask & (1ull << i)))
                        continue;
@@ -6588,14 +6583,9 @@ handle_fs_outputs_post(struct radv_shader_context *ctx)
                        values[j] = ac_to_float(&ctx->ac,
                                                radv_load_output(ctx, i, j));
 
-               if (!ctx->shader_info->info.ps.writes_z &&
-                   !ctx->shader_info->info.ps.writes_stencil &&
-                   !ctx->shader_info->info.ps.writes_sample_mask)
-                       last = ctx->output_mask <= ((1ull << (i + 1)) - 1);
-
                bool ret = si_export_mrt_color(ctx, values,
                                               i - FRAG_RESULT_DATA0,
-                                              last, &color_args[index]);
+                                              &color_args[index]);
                if (ret)
                        index++;
        }
@@ -6614,6 +6604,19 @@ handle_fs_outputs_post(struct radv_shader_context *ctx)
                                         radv_load_output(ctx, FRAG_RESULT_SAMPLE_MASK, 0));
        }
 
+       /* Set the DONE bit on last non-null color export only if Z isn't
+        * exported.
+        */
+       if (index > 0 &&
+           !ctx->shader_info->info.ps.writes_z &&
+           !ctx->shader_info->info.ps.writes_stencil &&
+           !ctx->shader_info->info.ps.writes_sample_mask) {
+               unsigned last = index - 1;
+
+               color_args[last].valid_mask = 1; /* whether the EXEC mask is valid */
+               color_args[last].done = 1; /* DONE bit */
+       }
+
        /* Export PS outputs. */
        for (unsigned i = 0; i < index; i++)
                ac_build_export(&ctx->ac, &color_args[i]);