[Mesa-dev] [PATCH] radv: try to select better export formats for chips without Rb+

[Samuel Pitoiset]

For some R8 formats, it's useless to export two channels
when no alpha blending is used. I assume the CB should
automatically clamps its inputs.

29077 shaders in 15096 tests
Totals:
SGPRS: 1321106 -> 1320970 (-0.01 %)
VGPRS: 935936 -> 935948 (0.00 %)
Spilled SGPRs: 25186 -> 25204 (0.07 %)
Code Size: 49813556 -> 49796944 (-0.03 %) bytes
Max Waves: 242091 -> 242088 (-0.00 %)

Totals from affected shaders:
SGPRS: 170608 -> 170472 (-0.08 %)
VGPRS: 99752 -> 99764 (0.01 %)
Spilled SGPRs: 10377 -> 10395 (0.17 %)
Code Size: 6332492 -> 6315880 (-0.26 %) bytes
Max Waves: 12317 -> 12314 (-0.02 %)

This helps some Rise Of Tomb Raider shaders, especially
when an expcnt instruction is added between two MRT exports.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
---
 src/amd/vulkan/radv_pipeline.c | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/src/amd/vulkan/radv_pipeline.c b/src/amd/vulkan/radv_pipeline.c
index 138e153f9a4..25281c3c6da 100644
--- a/src/amd/vulkan/radv_pipeline.c
+++ b/src/amd/vulkan/radv_pipeline.c
@@ -372,9 +372,10 @@ static bool is_dual_src(VkBlendFactor factor)
 	}
 }
 
-static unsigned si_choose_spi_color_format(VkFormat vk_format,
-					    bool blend_enable,
-					    bool blend_need_alpha)
+static unsigned si_choose_spi_color_format(struct radv_device *device,
+					   VkFormat vk_format,
+					   bool blend_enable,
+					   bool blend_need_alpha)
 {
 	const struct vk_format_description *desc = vk_format_description(vk_format);
 	unsigned format, ntype, swap;
@@ -486,6 +487,20 @@ static unsigned si_choose_spi_color_format(VkFormat vk_format,
 		unreachable("unhandled blend format");
 	}
 
+	if (device && /* NULL for internal meta formats */
+	    !device->physical_device->rbplus_allowed) {
+		/* Try to select better export formats for chips without Rb+. */
+		if (desc->nr_channels == 1 &&
+		    desc->channel[0].size == 8 &&
+		    swap == V_028C70_SWAP_STD && /* R */
+		    !vk_format_is_srgb(vk_format)) {
+			/* Do not need to export two channels for some R8
+			 * formats when alpha blending isn't used.
+			 */
+			blend = normal = V_028714_SPI_SHADER_32_R;
+		}
+	}
+
 	if (blend_enable && blend_need_alpha)
 		return blend_alpha;
 	else if(blend_need_alpha)
@@ -516,7 +531,8 @@ radv_pipeline_compute_spi_color_formats(struct radv_pipeline *pipeline,
 			bool blend_enable =
 				blend->blend_enable_4bit & (0xfu << (i * 4));
 
-			cf = si_choose_spi_color_format(attachment->format,
+			cf = si_choose_spi_color_format(pipeline->device,
+							attachment->format,
 			                                blend_enable,
 			                                blend->need_src_alpha & (1 << i));
 		}
@@ -586,7 +602,8 @@ const VkFormat radv_fs_key_format_exemplars[NUM_META_FS_KEYS] = {
 
 unsigned radv_format_meta_fs_key(VkFormat format)
 {
-	unsigned col_format = si_choose_spi_color_format(format, false, false);
+	unsigned col_format =
+		si_choose_spi_color_format(NULL, format, false, false);
 
 	assert(col_format != V_028714_SPI_SHADER_32_AR);
 	if (col_format >= V_028714_SPI_SHADER_32_AR)
-- 
2.20.1