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

[Christoph Bumiller]

On 24.08.2013 11:44, Christian König wrote:
> Am 24.08.2013 03:30, schrieb 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>
>
> I briefly remember that we needed at least one color export otherwise
> the GPU might hang, but I'm not 100% sure of that.
>

If there are no color buffers bound but the original shader writes color
0, you still have to export it to keep the alpha test working ...

> Marek and Alex should probably also take a look on this before we
> commit it.
>
> Christian.
>
>> ---
>>
>> 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;
>
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