For instance, when I viewing the code in the tvm/src/codegen，I am confused. I want to know what codegen does to make it match the corresponding backend.
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It translates the TVM IR into another IR that can be compiled into executable code. For example, code in src/codegen/llvm generates LLVM IR, which is then handed over to the LLVM’s optimizer/code generator to get the object file or assembly. codegen_c.cc will translate TVM IR into C code which then can be compiled using a C compiler, etc.
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Thank you for your answer,My current question is how to read the codegen code. because I can not reach the level of fast reading code.For example, I want to read the implementation of X86, but there are too many codegen files, such as codegen.c, codegen_c.cc and llvm folder corresponding codegen_cpu.cc, codegen_x86_64.cc.At the same time, there are more codes in the file. How can I read the key points?
X86-64 uses LLVM, so you could start with codegen_llvm.cc. codegen_cpu.cc is derived from codegen_llvm, and codegen_x86_64.cc is further derived from codegen_cpu. Most of the codegen happens in the first two files, whatever is not handled by codegen_cpu is dealt with in codegen_llvm. The last one, codegen_x86_64 is only there to generate some vector intrinsics.
They all work in a similar way: they have visitor functions that are called for each element (statement or expression) of the TVM/Halide IR, and generate the corresponding LLVM IR. For example,
llvm::Value* CodeGenLLVM::VisitExpr_(const And* op) {
return builder_->CreateAnd(MakeValue(op->a), MakeValue(op->b));
}
This is one of the simplest functions in there, but illustrates the concept: MakeValue takes an expression (in TVM/Halide IR) and returns the corresponding llvm::Value. Then the call to CreateAnd creates an And operator in LLVM IR, using the llvm::Value values returned by MakeValue.
Many of the translations are not so straightforward, so the “Visit” functions are more complicated, but the concept remains more or less the same.
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Thank you for your answer and I have helped a lot. I have been reading the code in these two days.I suddenly have a question about whether Halide IR was converted to LLVM IR in CodeGen. Is this step already using LLVM?
Yes, codegen converts Halide IR into LLVM IR.
Thank you very much for your help, but I still have some questions. If you have WeChat or other communication software, can you give me a convenient way to communicate?
No, sorry. Feel free to ask more questions on this forum, this way other people with the same questions can benefit from it.
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OK，Here is my current problem: my general purpose is to add a specific backend supported by LLVM in tvm, so I tried to use existing riscv, my idea is that riscv is a CPU architecture, and I don’t need to support intrin instructions now, what do I need to do?
It may be the case that you don’t need to do anything, although it’s somewhat unlikely. Are you using a RISC-V simulator? Does it simulate the whole system, or just the environment for running applications?
When you create an LLVM target, pass “-target riscv32” (or riscv64) to it, i.e. “llvm -target riscv32” and see what happens. You will need LLVM with RISC-V backend enabled, and one that supports JIT compilation (I’m not sure if that’s available for RISC-V, but you can try and see).
In general, the easiest approach is to pretend that RISC-V is just another CPU, and rely on the CPU code. When something fails, fix it and try again. You’ll get a better understanding of what pieces are missing, and that’s something that’s often difficult to fully predict ahead of time.
I think someone has already tried using TVM on RISC-V, but it didn’t work because LLVM didn’t support PIC for RISC-V at the time. The RISC-V backend is actively developed in LLVM, and so this may no longer be a problem now.
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Thank you very much for your help. I can ask you about the progress of the hexagon about the codegen part, because I also want to add a new dsp backend, but I am confused about what to do.
codegen_c.ccwill translate TVM IR into C code which then can be compiled using a C compiler, etc
I wanted to ask, can one get to access the C code which is being compiled, for any graph being un on TVM ? I wanted to see the C code output that codegen generates from TVM IR.
Hello! I would like to ask a favor:
In TVM generate code module, such as generating cuda code (/src/codegen/codegen_cuda. cc: 66):
void CodeGenCUDA::VisitStmt_(const ir::For* op) {
CHECK(is_const_int(op->min, 0));
if (op->for_type == ir::ForType::Unrolled) {
PrintIndent();
stream << “#pragma unroll\n”;
}
CodeGenC::VisitStmt_(op);
}
My understanding of this code is that the op->for_type type value was obtained in(/src/op/op_util.cc: 251):
ir::ForType IterVarTypeToForType(IterVarType iter_type) {
switch (iter_type) {
case kDataPar:
return ForType::Serial;
case kParallelized:
return ForType::Parallel;
case kVectorized:
return ForType::Vectorized;
case kUnrolled:
return ForType::Unrolled;
default:
return ForType::Serial;
}
}
But I use pdb and gdb joint tracking, and insert breakpoints here, I found that program did not perform to it, it makes me confused. Could this op->for_type value have been obtained elsewhere?
Thank you for your kind help!
For::make takes ForType as an argument, so For could be created directly with a given type. Set a breakpoint at For::make, and see if the type has already been set there.
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Thanks for your answer, I made it!
Sorry to bother you again!
I want to generate gpu-like code for DL operators with TVM and add guidance statements to the generated code. such as: #pragma unroll 4 . So I simulated the generation process of cuda code and found that cuda supports adding guidance statements (tvm/src/codegen/codegen_cuda.cc:66).
void CodeGenCUDA::VisitStmt_(const ir::For* op) {
CHECK(is_const_int(op->min, 0));
if (op->for_type == ir::ForType::Unrolled) {
PrintIndent();
stream << "#pragma unroll\n";
}
CodeGenC::VisitStmt_(op);
}
It is shown here that whether or not to add the guidance statement depends on the for_loop type in the operator: for_type . Just like yesterday I asked you, the value of for_type comes from For ::make , while thread_tag in For ::make comes from IterVarNode::make , as shown in the source code below(tvm/src/lang/expr.cc:129):
IterVar IterVarNode::make(Range dom,
Var var,
IterVarType t,
std::string thread_tag) {
NodePtr<IterVarNode> n = make_node<IterVarNode>();
n->dom = dom;
n->var = var;
n->iter_type = t;
n->thread_tag = thread_tag;
return IterVar(n);
}
tvm/src/api/api_lang.cc:315:
TVM_REGISTER_API("_IterVar")
.set_body_typed<IterVar(Range, Var, int, std::string)>([](
Range dom, Var var, int iter_type, std::string thread_tag ) {
return IterVarNode::make(
dom, var,
static_cast<IterVarType>(iter_type),
thread_tag);
});
I used gdb to trace the thread_tag here and found that tracing up to the C++ / python interface, but I still couldn’t find where the thread_tag value came from, which confused me.
Should the guidance statement be manually specified in the input operator? It’s not determined by TVM by the type of for_loop in the input operator, right?
The thread_tag comes from the function thread_axis (see python/tvm/api.py).
These guidance statements (like unrolling) come from the schedule for a given operator. It seems like TVM is doing some automatic unrolling, but in general scheduling directives should be added explicitly.
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Thank you!
The guidance language really should be specified by the user in the input operator
I am currently trying to generate C code. Do you have any research on this area? I saw a similar problem on github, but failed to generate C code, please see the following link picture:Please see the last question in this post