[Mesa-dev] A question about TGSI->LLVM IR conversion

Christian König deathsimple at vodafone.de
Sun Feb 13 11:45:55 PST 2011


I'm currently playing around a bit with the TGSI->LLVM IR conversion
found in the llvmpipe/gallivm driver. While doing so I managed to hock
into the r600g shader generation a call to lp_build_tgsi_soa and dumping
the resulting LLVM IR additionally to the TGSI and R600 assembler
representation. The goal is to compare TGSI with R600 assembler and LLVM
IR for possible new ideas how to make better optimization or even
simplifying code generation. 

The problem is that the LLVM IR doesn't looks like what I have expected:

TGSI input:
  0: MOV OUT[0], IN[0]
  1: END

generated LLVM IR:
define void @test_shader(<4 x float>) {
  %output3 = alloca <4 x float>
  %output2 = alloca <4 x float>
  %output1 = alloca <4 x float>
  %output = alloca <4 x float>
  store <4 x float> zeroinitializer, <4 x float>* %output
  store <4 x float> zeroinitializer, <4 x float>* %output1
  store <4 x float> zeroinitializer, <4 x float>* %output2
  store <4 x float> zeroinitializer, <4 x float>* %output3
  store <4 x float> %0, <4 x float>* %output
  store <4 x float> %0, <4 x float>* %output1
  store <4 x float> %0, <4 x float>* %output2
  store <4 x float> %0, <4 x float>* %output3

It looks like that a vector type is used for each TGSI channel instead
of a scalar. So is there something wrong with my setup of gallivm, call
to lp_build_tgsi_soa or any other prerequisite not meet? Or is
lp_build_tgsi_soa just supposed to work like this because llvmpipe
always work with a 2x2 pixel block?

I also expected a pure SSA form, but the generated code uses stack
allocated memory for storing the temporary and output registers, but
this clearly seems to be a current limitation of lp_build_tgsi_soa.

Here is the code I'm using to call lp_build_tgsi_soa:

static void llvm_test(const struct pipe_shader_state *state)
        int i, j;

        struct gallivm_state *gallivm = gallivm_create();
        struct lp_type type = lp_type_float_vec(32);
        struct tgsi_shader_info info;

        LLVMTypeRef float_type = LLVMFloatTypeInContext(gallivm->context);
        LLVMTypeRef vec_type = LLVMVectorType(float_type, 4);
        LLVMTypeRef args[PIPE_MAX_SHADER_INPUTS] = { };
        LLVMValueRef inputs[PIPE_MAX_SHADER_INPUTS][4] = { };
        LLVMValueRef outputs[PIPE_MAX_SHADER_OUTPUTS][4] = { };
        LLVMValueRef consts_ptr;
        LLVMTypeRef func_type;
        LLVMValueRef function;
        LLVMBasicBlockRef block;
        LLVMBuilderRef builder;

        tgsi_scan_shader(state->tokens, &info);

        for (i=0; i<info.file_count[TGSI_FILE_INPUT]; ++i)
                args[i] = vec_type;

        func_type = LLVMFunctionType(LLVMVoidTypeInContext(gallivm->context),
                                        (LLVMTypeRef*)args, info.file_count[TGSI_FILE_INPUT], 0);
        function = LLVMAddFunction(gallivm->module, "test_shader", func_type);
        LLVMSetFunctionCallConv(function, LLVMCCallConv);

        for (i=0; i<info.file_count[TGSI_FILE_INPUT]; ++i)
                for (j=0; j<4; ++j)
                        inputs[i][j] = LLVMGetParam(function, i);

        consts_ptr = LLVMAddGlobal(gallivm->module, float_type, "consts_ptr");

        block = LLVMAppendBasicBlockInContext(gallivm->context, function, "entry");
        builder = gallivm->builder;
        LLVMPositionBuilderAtEnd(builder, block);

        lp_build_tgsi_soa(gallivm, state->tokens, type, NULL,
                          consts_ptr, NULL,
                          NULL, inputs,
                          outputs, NULL, &info);


Could somebody please explain this behaviour, is it a bug in my test
code, does it just works as designed?

Thanks for your time,

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