How can I validate a newly created instruction using LLVM?

Issue

How can I validate a newly created instruction using LLVM?

I am new to LLVM and computer architecture. Created a new instruction of bfloat16 type arithmetic targeting the RISCV-32 architecture. I was wondering if the output of this arithmetic instruction was correct. And I wanted to verify that the value stored in the float register is in IEEE754 bfloat16 format.

clang-14 -c -g -v --target=riscv32-unknown-elf -march=rv32izfh0p1 -menable-experimental-extensions -I/usr/include -o main.o main.c

riscv32-unknown-elf-gcc -g -o main main.o

I compiled it through the above command, and it was confirmed that the assembly code came out well as shown below.

enter image description here

Then, I tried to run compiled executable file with qemu-riscv32 and debug it using gdb. But an error illegal instruction occurred.

enter image description here

Question I think an illegal instruction error occurred because QEMU and gdb don't have information about the new instruction I created. Other than modifying QEMU and GDB, is there any way to validate the newly created instruction?

Solution

If you extend a CPU architecture by adding new instructions to it, you need to add support for those in not just the toolchain (which will then output code using that new instruction) but also in the CPU implementation itself (so that it can execute the code your compiler now generates). You can do that either on a real hardware CPU, which in practice will start with changes to its RTL which can be tested in simulation; or you can do it on an emulated version of the CPU such as that which QEMU provides. But if you don't have an implementation of a CPU at all that has your new instructions in it, there's no point in having the compiler emit them, because you have nothing that can run the code that compiler produces.

Answered By - Peter Maydell