[Mesa-dev] [PATCH] r600g/llvm: don't export more colors than the number of CBs

[Vadim Girlin]

Currently llvm backend always exports at least one color in pixel
shader even if no color buffers are enabled. With depth/stencil exports
this can result in the following code:

EXPORT             PIXEL 0     R0.xyzw  VPM
EXPORT             PIXEL 61    R1.x___  VPM
EXPORT_DONE        PIXEL 61    R0._x__  VPM  EOP

AFAIU with zero color buffers no memory is reserved for colors in the export
ring and all exports in this example actually write to the same location.
The code above still works fine in this particular case, because correct
values are written last, but reordering can break it (especially with SB
which tends to reorder the exports).

Signed-off-by: Vadim Girlin <vadimgirlin at gmail.com>
---

This fixes regressions with LLVM+SB, so I consider it as a prerequisite
for enabling SB by default. Also it fixes some issues with LLVM backend alone.
Tested on evergreen only (I don't have other hw), needs testing on
pre-evergreen GPUs.

 src/gallium/drivers/r600/r600_llvm.c   | 2 +-
 src/gallium/drivers/r600/r600_shader.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/gallium/drivers/r600/r600_llvm.c b/src/gallium/drivers/r600/r600_llvm.c
index 03a68e4..d2f4aff 100644
--- a/src/gallium/drivers/r600/r600_llvm.c
+++ b/src/gallium/drivers/r600/r600_llvm.c
@@ -333,8 +333,8 @@ static void llvm_emit_epilogue(struct lp_build_tgsi_context * bld_base)
 		} else if (ctx->type == TGSI_PROCESSOR_FRAGMENT) {
 			switch (ctx->r600_outputs[i].name) {
 			case TGSI_SEMANTIC_COLOR:
-				has_color = true;
 				if ( color_count < ctx->color_buffer_count) {
+					has_color = true;
 					LLVMValueRef args[3];
 					args[0] = output;
 					if (ctx->fs_color_all) {
diff --git a/src/gallium/drivers/r600/r600_shader.c b/src/gallium/drivers/r600/r600_shader.c
index fb766c4..85f8469 100644
--- a/src/gallium/drivers/r600/r600_shader.c
+++ b/src/gallium/drivers/r600/r600_shader.c
@@ -1130,7 +1130,7 @@ static int r600_shader_from_tgsi(struct r600_screen *rscreen,
 		radeon_llvm_ctx.face_gpr = ctx.face_gpr;
 		radeon_llvm_ctx.r600_inputs = ctx.shader->input;
 		radeon_llvm_ctx.r600_outputs = ctx.shader->output;
-		radeon_llvm_ctx.color_buffer_count = MAX2(key.nr_cbufs , 1);
+		radeon_llvm_ctx.color_buffer_count = key.nr_cbufs;
 		radeon_llvm_ctx.chip_class = ctx.bc->chip_class;
 		radeon_llvm_ctx.fs_color_all = shader->fs_write_all && (rscreen->chip_class >= EVERGREEN);
 		radeon_llvm_ctx.stream_outputs = &so;
-- 
1.8.3.1