nemanjai wrote:
> > > I have always been unconvinced that these are a good idea to have / add
> > > significant value over using inline assembly. IIRC Arm has them but
> > > nobody uses them?
...
> If it’s not a constant integer for inline assembly then how would it
> magically be a constant integer for an intrinsic? The IR’s going to be the
> same with an alloca/store/load, no?
To be completely clear with the use case:
```
$ cat csrread.c
// An obvious way a library writer tries to provide a
// convenience function for reading CSR's.
#ifdef _ASM
static unsigned __attribute__((always_inline)) read_csr(const unsigned CSR) {
unsigned Ret;
__asm__ volatile("csrr %0, %1" : "=r"(Ret) : "I"(CSR));
return Ret;
}
#else
#define read_csr __builtin_riscv_csrr
#endif
// Use the convenience function.
unsigned someFunc() {
return read_csr(0x300);
}
$ clang csrread.c -S --target=riscv32 -D_ASM -O1 && echo Success # No error
checking for no Zicsr but it compiles
Success
$ clang csrread.c -S --target=riscv32 -D_ASM && echo Success # Does not compile
at -O0
csrread.c:6:20: error: constraint 'I' expects an integer constant expression
6 | __asm__ volatile("csrr %0, %1" : "=r"(Ret) : "I"(CSR));
$ clang csrread.c -S --target=riscv32 -O1 && echo Success # The builtin
provides a nice error message for no Zicsr
csrread.c:15:10: error: builtin requires at least one of the following
extensions: 'Zicsr'
15 | return read_csr(0x300);
$ clang csrread.c -S --target=riscv32 -march=rv32i_zicsr && echo Success # The
builtin compiles fine at -O0
Success
```
In my view, the builtin solution is superior from a usability perspective.
https://github.com/llvm/llvm-project/pull/85091
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