[PATCH 2/3] newlib: riscv: Optimize memchr() and memrchr()

[Eric Salem]

On 5/7/25 8:44 AM, Kito Cheng wrote:
>> -      src = (unsigned char *) asrc;
>> +  if (src < end_addr)
>> +    {
>> +      uintxlen_t mask = __libc_splat_byte(d);
>> +      uintlslen_t val;
>> +
>> +      do
>> +        {
>> +#if __riscv_zilsd
>> +          asm volatile ("ld %0, 0(%1)"
>> +                        : "=R" (val)
>> +                        : "r" (src)
>> +          );
> 
> I strongly prefer not to use inline asm here, let the compiler do
> that, although I know upstream GCC doesn't implement that yet...

Here's an example trying to use Zilsd/Zclsd in C:

$ cat << EOF > test.c
#include <stdint.h>
#include <stdio.h>

int main(void) {

	uint64_t val;
	uint64_t *p = &val;

	p = &val;
	*p = 0x700000005;

	printf("%p %llu\n", p, val);
}
EOF
$ clang -march=rv32ic_zclsd_zilsd -O3 -c test.c
$ llvm-objdump --disassemble-symbols=main test.o

test.o:	file format elf32-littleriscv

Disassembly of section .text:

00000000 <main>:
       0: 1141         	addi	sp, sp, -0x10
       2: c606         	sw	ra, 0xc(sp)
       4: 451d         	li	a0, 0x7
       6: 4595         	li	a1, 0x5
       8: c02e         	sw	a1, 0x0(sp)
       a: c22a         	sw	a0, 0x4(sp)
       c: 00000537     	lui	a0, 0x0
      10: 00050513     	mv	a0, a0
      14: 858a         	mv	a1, sp
      16: 4615         	li	a2, 0x5
      18: 469d         	li	a3, 0x7
      1a: 00000097     	auipc	ra, 0x0
      1e: 000080e7     	jalr	ra <main+0x1a>
      22: 4501         	li	a0, 0x0
      24: 40b2         	lw	ra, 0xc(sp)
      26: 0141         	addi	sp, sp, 0x10
      28: 8082         	ret

Clang uses an even-odd register pair, but doesn't place the lower-order
bits in a0, and the higher in a1. If it did, it could use a single sd
instruction instead.

I think once support is better we'll be able to rely upon the compiler,
whether it requires the use of intrinsics or not. But for the time
being I don't see how you can use Zilsd/Zclsd in C without inline
assembly.