[Mesa-dev] [PATCH 2/2] radeonsi: hard-code pixel center for interpolateAtSample without multisample buffers

[Nicolai Hähnle]

From: Nicolai Hähnle <nicolai.haehnle at amd.com>

The GLSL rules for interpolateAtSample are unfortunate:

   "Returns the value of the input interpolant variable at
    the location of sample number sample. If
    multisample buffers are not available, the input
    variable will be evaluated at the center of the pixel.
    If sample sample does not exist, the position used to
    interpolate the input variable is undefined."

This fix will fallback to monolithic shader compilation when
interpolateAtSample is used without multisampling.

One alternative would be to always upload 16 sample positions,
filling the buffer up with repetition when the actual number of
samples is less, and then ANDing the sample ID with 0xf. However,
that punishes all well-behaving users of interpolateAtSample,
when in reality, only conformance tests should be affected by
the issue.

Fixes
dEQP-GLES31.functional.shaders.multisample_interpolation.interpolate_at_sample.non_multisample_buffer.*
---
 src/gallium/drivers/radeonsi/si_shader.c        | 28 ++++++++++++++++++++++++-
 src/gallium/drivers/radeonsi/si_shader.h        |  3 +++
 src/gallium/drivers/radeonsi/si_state_shaders.c |  3 +++
 3 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/src/gallium/drivers/radeonsi/si_shader.c b/src/gallium/drivers/radeonsi/si_shader.c
index 85de2e407b4..d0af60856b0 100644
--- a/src/gallium/drivers/radeonsi/si_shader.c
+++ b/src/gallium/drivers/radeonsi/si_shader.c
@@ -3665,21 +3665,47 @@ static void interp_fetch_args(
 		LLVMValueRef sample_id;
 		LLVMValueRef halfval = LLVMConstReal(ctx->f32, 0.5f);
 
 		/* fetch sample ID, then fetch its sample position,
 		 * and place into first two channels.
 		 */
 		sample_id = lp_build_emit_fetch(bld_base,
 						emit_data->inst, 1, TGSI_CHAN_X);
 		sample_id = LLVMBuildBitCast(gallivm->builder, sample_id,
 					     ctx->i32, "");
-		sample_position = load_sample_position(ctx, sample_id);
+
+		/* Section 8.13.2 (Interpolation Functions) of the OpenGL Shading
+		 * Language 4.50 spec says about interpolateAtSample:
+		 *
+		 *    "Returns the value of the input interpolant variable at
+		 *     the location of sample number sample. If multisample
+		 *     buffers are not available, the input variable will be
+		 *     evaluated at the center of the pixel. If sample sample
+		 *     does not exist, the position used to interpolate the
+		 *     input variable is undefined."
+		 *
+		 * This means that sample_id values outside of the valid are
+		 * in fact valid input, and the usual mechanism for loading the
+		 * sample position doesn't work.
+		 */
+		if (ctx->shader->key.mono.u.ps.interpolate_at_sample_force_center) {
+			LLVMValueRef center[4] = {
+				LLVMConstReal(ctx->f32, 0.5),
+				LLVMConstReal(ctx->f32, 0.5),
+				ctx->ac.f32_0,
+				ctx->ac.f32_0,
+			};
+
+			sample_position = lp_build_gather_values(gallivm, center, 4);
+		} else {
+			sample_position = load_sample_position(ctx, sample_id);
+		}
 
 		emit_data->args[0] = LLVMBuildExtractElement(gallivm->builder,
 							     sample_position,
 							     ctx->i32_0, "");
 
 		emit_data->args[0] = LLVMBuildFSub(gallivm->builder, emit_data->args[0], halfval, "");
 		emit_data->args[1] = LLVMBuildExtractElement(gallivm->builder,
 							     sample_position,
 							     ctx->i32_1, "");
 		emit_data->args[1] = LLVMBuildFSub(gallivm->builder, emit_data->args[1], halfval, "");
diff --git a/src/gallium/drivers/radeonsi/si_shader.h b/src/gallium/drivers/radeonsi/si_shader.h
index be17cf462be..22bb56b0ece 100644
--- a/src/gallium/drivers/radeonsi/si_shader.h
+++ b/src/gallium/drivers/radeonsi/si_shader.h
@@ -508,20 +508,23 @@ struct si_shader_key {
 
 	/* Flags for monolithic compilation only. */
 	struct {
 		/* One byte for every input: SI_FIX_FETCH_* enums. */
 		uint8_t		vs_fix_fetch[SI_MAX_ATTRIBS];
 
 		union {
 			uint64_t	ff_tcs_inputs_to_copy; /* for fixed-func TCS */
 			/* When PS needs PrimID and GS is disabled. */
 			unsigned	vs_export_prim_id:1;
+			struct {
+				unsigned interpolate_at_sample_force_center:1;
+			} ps;
 		} u;
 	} mono;
 
 	/* Optimization flags for asynchronous compilation only. */
 	struct {
 		/* For HW VS (it can be VS, TES, GS) */
 		uint64_t	kill_outputs; /* "get_unique_index" bits */
 		unsigned	clip_disable:1;
 
 		/* For shaders where monolithic variants have better code.
diff --git a/src/gallium/drivers/radeonsi/si_state_shaders.c b/src/gallium/drivers/radeonsi/si_state_shaders.c
index 6b63e69d91c..cbef8d8a646 100644
--- a/src/gallium/drivers/radeonsi/si_state_shaders.c
+++ b/src/gallium/drivers/radeonsi/si_state_shaders.c
@@ -1453,20 +1453,23 @@ static inline void si_shader_selector_key(struct pipe_context *ctx,
 				 * of (i,j), which is the optimization here. */
 				key->part.ps.prolog.force_persp_center_interp =
 					sel->info.uses_persp_center +
 					sel->info.uses_persp_centroid +
 					sel->info.uses_persp_sample > 1;
 
 				key->part.ps.prolog.force_linear_center_interp =
 					sel->info.uses_linear_center +
 					sel->info.uses_linear_centroid +
 					sel->info.uses_linear_sample > 1;
+
+				if (sel->info.opcode_count[TGSI_OPCODE_INTERP_SAMPLE])
+					key->mono.u.ps.interpolate_at_sample_force_center = 1;
 			}
 		}
 
 		key->part.ps.epilog.alpha_func = si_get_alpha_test_func(sctx);
 		break;
 	}
 	default:
 		assert(0);
 	}
 
-- 
2.11.0