Amit, it looks to me like you are talking at cross-purposes. Correct me
if I am wrong, but I am assuming you are writing code as a user, while
Georg-Johann is also giving advice for adding these new instructions to
the compiler, and that might be confusing you.
If you want the compiler to use your new custom instructions, you have
to tell it about them. Assuming you don't want to modify the compiler,
then inline assembly statements are the way to handle this. These are
compiler extensions, not macros. There is no magical way to get the
pre-compiled standard maths library to suddenly start using your new
custom instructions.
So you want to write something like this :
static inline float sin_fast(float x) {
if (__builtin_constant_p(sinf(x)) {
return sinf(x);
}
float y;
asm(" custom_sin %[src_reg], %[dst_reg]"
: [dst_reg] "=f" (y)
: [src_reg] "f" (x)
);
return y;
}
Replace "custom_sin" with the assembly opcode, as recognized by your
assembler. And I have no idea if the ordering of the source and
destination registers is correct for RISC-V.
Then use "sin_fast" instead of "sinf" in your code.
<https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html>
David
On 07/08/2024 12:04, Amit Hiremath wrote:
Hello Georg-Johann,
Thanks for the clarification. Actually, I do not want to use asm. If I
write sample c code like this:
#include <math.h>
#include <stdio.h>
int main()
{
float angle1 = 3.1415926;
printf("sin(3.14) = %.2lf\n", sin(angle1));
return 0;
}
The compiler should emit this instruction to the concerned FPU part in
RISC-V. Is it possible this way? Since RISC-V does not have instructions
for sine, cosine, exp, etc. they are taken care of by soft fp libraries.
I guess I do not need to rename instructions?
Thanks,
-Amit
On Wed, Aug 7, 2024 at 3:11 PM Georg-Johann Lay <avr@xxxxxxxx> wrote:
Am 07.08.24 um 08:24 schrieb Amit Hiremath:
Hello,
I want to add custom single precision floating point sine, cosine, exp
instructions to risc-v gnu tool chain, and I have designed hardware for
this. I was going through tutorials on how to add custom instructions at:
https://pcotret.gitlab.io/riscv-custom/sw_toolchain.html after adding
custom instructions, I think one has to use asm volatile macro to use
custom instructions in C. Is there anyway where one do not need to use
this
Hi, There is no need for asm volatile, asm is enough because there are
no side effects other than the computation. And for compile-time
constants, there is no need for asm, like in:
if (__builtin_constant_p (sinf (x))
y = sinf (x);
else
__asm ("..." : "=r" (y), ...);
macro? I would like the compiler to automatically map to custom
instructions in the risc-v processor, like how it will map to fadd.s,
fmul.s instructions, where one does not need to use asm volatile macro.
This would be a new insn named "sinsf2" for sinf. The "sf" stands for
SFmode (single float, e.g. IEEE single).
https://gcc.gnu.org/onlinedocs/gccint/Standard-Names.html
You can have a look at how other backends are doing it, for example
grep the GCC sources:
$ grep -rn '"sin' gcc/config | grep -v single
which yields a dozen or so hits, for example nvptx:
(define_insn "sinsf2"
[(set (match_operand:SF 0 "nvptx_register_operand" "=R")
(unspec:SF [(match_operand:SF 1 "nvptx_register_operand" "R")]
UNSPEC_SIN))]
"flag_unsafe_math_optimizations"
"%.\\tsin.approx%t0\\t%0, %1;")
(I don't know whether unspec is still required today. rtl.def
doesn't seen to have sin, so yes, you need an unspec.)
The insn condition is presumably because the nvptx sinf is not
fully IEEE compliant. When sinf is available on some devices
but not on others, the insn condition would express that, too.
The constraints would be the same like in an asm. When there
are no appropriate constraints and the asm is using local
register variables, then you'll have to add new register
constraints and register classes that describe which
hard registers are appropriate for the new instruction.
(But notice that there is an upcoming feature that allows
single hard register as a constraint like "{r42}".)
Finally, as you also have cosf, you would also add insns
for cossf2 and sincossf3.
HTH
Johann
I asked in riscv gnu tool chain forum about this issue:
https://github.com/riscv-collab/riscv-gnu-toolchain/issues/1526 they
suggested that I ask this query in the gcc forum.
Can you please guide me?
Many Thanks,
-Amith
